Senses

Answer 1

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Question 2

reversed prompt

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 3

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Question 4

Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium

Answer 5

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 6

Reverse.Prompt

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Answer 6

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Question 7

• Whereas the outer ear and the middle ear contain air,
• the inner ear is filled with fluid.
• The inner ear has three areas:

Answer 7

• concerned with equilibrium;

1. The semicircular canals and the
2. vestibule are the

• concerned with hearing: The cochlea
  
  • resembles the shell of a snail because it spirals.

Question 9

convert a signal from the environment, called a stimulus, into a nerve impulse.
This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons.
Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Answer 9

sensory receptor

Question 10

Reverse.Prompt

Answer 10

Sensation Visual

Question 11

15.1

Answer 11

Question 12

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 13

Reverse.Prompt

• • • The cornea, assisted by the
• lens and the
• humors,
• focuses images on the retina.
• Process/Steps:

1. Focusing starts with the cornea and
2. continues as the rays pass through the lens and the humors.
3. The image produced is much smaller than the object,
   
   • because light rays are bent (refracted) when they are brought into focus.
   • If the eye is too long or too short, the person may need corrective lenses to bring the image into focus.
4. The image on the retina is inverted (upside down) and reversed from left to right.
5. Visual accommodation occurs for close vision.
   
   • During visual accommodation,
     
     1. the lens changes its shape to bring the image into focus on the retina.
     2. The shape of the lens is controlled by the ciliary muscle, within the ciliary body.
        
        • When we view a distant object,
          
          1. the ciliary muscle is relaxed,
          2. causing the suspensory ligaments attached to the ciliary body to be taut.
          3. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a).
        • When we view a near object,
        1. the ciliary muscle contracts,
        2. releasing the tension on the suspensory ligaments.
        3. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b).
     • Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina.

Answer 13

Function of the Lens

Question 14

• Close work requires contraction of the ciliary muscle,
• so it often causes muscle fatigue, known as eyestrain.
• Eyestrain is more common after the age of 40,
  
  • because the lens loses some of its elasticity and is unable to accommodate.
• It is also common among those who work with computers,
  
  • because the intense focusing causes the person to blink less, allowing the eyes to dry out.
  • Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Answer 14

Eyestrain

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Question 15

Anatomy of the eye • 2 compartments 1. Anterior compartment: between the cornea and lens; filled with a clear fluid called aqueous humor – this liquid is continuously produced each day and drains through small ducts 2. Posterior compartment: most of the eye, behind the lens; contains a gelatinous material called vitreous humor that holds the retina in place and supports the lens – this liquid you are born with and remains; no more is produced

Question 16

reversed prompt

What are 3 types of receptors:

* Sensory receptors

* Exteroreceptors

* Interoreceptors

Chapter 15.1 slide 1.2

Answer 16

There are three of us:

dendrites specialized to detect certain types of stimuli –
We detect stimuli from outside the body

* (e.g., taste, hearing, vision)

We receive stimuli from inside the body

* (e.g., change in blood pressure)

* Directly involved in homeostasis and a part of a negative feedback loop

Question 17

15.2 Somatic SensesLEARNING OUTCOMES
Upon completion of this section, you should be able to
Distinguish between proprioceptors and cutaneous receptors with regard to function.
State the location and general function of each type of cutaneous receptor.
Explain the role of nociceptors and summarize the type of sensory input they detect.
Senses whose receptors are associated with the skin, muscles, joints, and viscera are termed the somatic senses. These receptors can be categorized into three types: proprioceptors, cutaneous Page 309receptors, and pain receptors. All of these send nerve impulses via the spinal cord to the primary somatosensory areas of the cerebral cortex (see Fig. 14.11).
Proprioceptors
Proprioceptors are mechanoreceptors involved in reflex actions that maintain muscle tone, and thereby the body’s equilibrium and posture. For example, proprioceptors called muscle spindles are embedded in muscle fibers (Fig. 15.2). If a muscle relaxes too much, the muscle spindle stretches, generating nerve impulses that cause the muscle to contract slightly. Conversely, when muscles are stretched too much, proprioceptors called Golgi tendon organs, buried in the tendons that attach muscles to bones, generate nerve impulses that cause the muscles to relax. Both types of receptors act together to maintain a functional degree of muscle tone. The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action. The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture. Proper balance and body position are maintained, despite the force of gravity always acting on the skeleton and muscles.

Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.
Cutaneous Receptors
The skin is composed of two layers: the epidermis and the dermis (see Section 4.6). The dermis contains cutaneous receptors (Fig. 15.3), which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a mosaic of these tiny receptors, as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure; at others, you will feel heat or cold (depending on the probe’s temperature).

Figure 15.3 Sensory receptors of the skin. The general function of each sensory receptor is shown here. However, receptors are not always this specialized. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations.

Several types of cutaneous receptors are sensitive to fine touch. These receptors give a person specific information, such as the location of the touch, as well as its shape, size, and texture. Meissner corpuscles and Krause end bulbs are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. Merkel discs are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.
Two types of cutaneous receptors sensitive to pressure are Pacinian corpuscles and Ruffini endings. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.
Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others respond to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.

Question 18

Structure/Function
Sclera: Protects and supports the eye
Cornea: Refracts light rays
Pupil: Admits light
Choroid: Absorbs stray light
Ciliary body: Holds lens in place, accommodation
Iris: Regulates light entrance
Retina: Contains photoreceptors for sight
Rod cells: Make black-and-white vision possible
Cone cells: Make color and acute vision possible
Fovea centralis: Contains mostly cones for acute vision
Other
Lens: Refracts and focuses light rays
Humors: Transmit light rays and support the eye
Optic nerve:
Transmits impulses to the visual cortex

Answer 18

See Table 15.2

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Question 19

reversed prompt

Communication - from peripheral nervous system deteching stimuli

Answer 19

Odor

Question 20

Reverse.Prompt

vestibular nerve

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Answer 20

1. originates in the semicircular canals, saccule, and utricle.
2. It takes nerve signals to the brain stem and cerebellum
3. Through its communication with the brain, helps us achieve equilibrium, but other structures in the body are also involved.
   
   1. proprioceptors
   2. Vision, if available, usually provides extremely helpful input the brain can act upon.
   3. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Question 21

Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Question 22

• 3. Layers of the eye: Retina •
• Sensory receptors from the retina form the optic nerve that takes impulses to the brain. •
• The blind spot is the optic disc, and is
  
  • where the optic nerve attaches;
  • it lacks photoreceptors, therefore consequently nothing can be visually detected at this location.

Question 23

reversed prompt

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 24

reversed prompt

Chemoreceptors – respond to nearby chemicals –

Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue

Photoreceptors – respond to light energy
Mechanoreceptors – respond to mechanical forces such as pressure
Thermoreceptors – stimulated by temperature changes

Answer 24

We are 4 types of sensory receptors and what we do!

15.1 Overview of Sensory Receptors and Sensations, 15.1 Lecture, 1.3

Question 25

Function of the Lens

Answer 25

• • • The cornea, assisted by the
• lens and the
• humors,
• focuses images on the retina.
• Process/Steps:

1. Focusing starts with the cornea and
2. continues as the rays pass through the lens and the humors.
3. The image produced is much smaller than the object,
   
   • because light rays are bent (refracted) when they are brought into focus.
   • If the eye is too long or too short, the person may need corrective lenses to bring the image into focus.
4. The image on the retina is inverted (upside down) and reversed from left to right.
5. Visual accommodation occurs for close vision.
   
   • During visual accommodation,
     
     1. the lens changes its shape to bring the image into focus on the retina.
     2. The shape of the lens is controlled by the ciliary muscle, within the ciliary body.
        
        • When we view a distant object,
          
          1. the ciliary muscle is relaxed,
          2. causing the suspensory ligaments attached to the ciliary body to be taut.
          3. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a).
        • When we view a near object,
        1. the ciliary muscle contracts,
        2. releasing the tension on the suspensory ligaments.
        3. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b).
     • Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina.

Question 26

Reverse.Prompt

Answer 26

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 27

Which parts function in balance and which parts function in hearing

Answer 28

Anatomy of the eye • Made of 3 layers/coats 1. Sclera: mostly white and fibrous except the cornea 2. Choroid: darkly-pigmented vascular layer 3. Retina: inner layer containing photoreceptors

Question 29

Section 14.2 describes the location and function of the reticular activating system (RAS).

Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.

Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Question 30

reversed prompt

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 31

focuses images on the retina:

* cornea: Focusing starts
lens and the humors: continues as the rays pass through

* The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus.

* If the eye is too long or too short,

* the person may need corrective lenses to bring the image into focus.

* The image on the retina is inverted (upside down) and reversed from left to right.

Answer 31

Vision: Focusing; Purpose of the Lens

Question 32

very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray.

Answer 32

rods

Question 33

Key Concepts to Focus On •

Answer 33

What are sensory receptors?

Question 34

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Question 35

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

Question 36

* Size/shape: 2.5 cm in diameter; elongated sphere
Anatomy: three layers, or coats:

the sclera:

white, fibrous outer layer is made up of the which is made of collagen fibers.

* ​The cornea: window of the eye.

the choroid

​thin, middle coat

* extensive blood supply, and its
dark pigment absorbs stray light rays that photoreceptors have not absorbed: helps visual acuity.​​
Toward the front becomes the doughnut-shaped iris.

* regulates the size of the pupil: a hole in the center of the iris through which light enters the eye.
The color of the iris (and therefore the color of the eyes) correlates with its pigmentation.

* Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue.

Behind the iris, the choroid thickens and forms the circular ciliary body: the ciliary muscle, which controls the shape of the lens for near and far vision.

Answer 36

Anatomy of the Eye (Structures)

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

Question 37

Reverse.Prompt

Question 38

Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Answer 38

Describe the functions of the four types of sensory receptors.

Question 39

Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.

Question 40

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Structure Function

Sclera Protects and supports the eye

Cornea Refracts light rays

Pupil Admits light

Choroid Absorbs stray light

Ciliary body Holds lens in place, accommodation

Iris Regulates light entrance

Retina Contains photoreceptors for sight

Rod cells Make black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

Lens Refracts and focuses light rays

Humors Transmit light rays and support the eye

Optic nerve Transmits impulses to the visual cortex

Answer 40

Chart functional eye structures

Answer 41

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Question 42

4,000 taste buds are located primarily on the tongue of adult humans. • We have 5 main types of taste receptors: sweet, sour, salty, bitter, and umami (savory). • Taste buds open at a taste pore; microvilli on cells where molecules bind; information sent to gustatory cortex in parietal lobe • 80-90% of what we perceive as taste is actually due to the sense of smell! 15.3 Senses of Taste and Smell T

Answer 43

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Question 44

Study all screen shots and slides over and over again

Question 45

We are able to convert a signal from the environment, called a______________, into a nerve impulse.

This conversion process is called _____________________.

Answer 45

* sensory receptor

* stimulus

* sensory transduction.

Question 46

Smell receptors

Answer 46

1. Depends on 10-20 million olfactory cells (modified neurons) in the roof of the nasal cavity •
2. Odor molecules activate specific combination of receptor proteins for recognition of specific smells and
3. information is sent directly to the olfactory cortex
   
   1. in the temporal lobe from the olfactory bulb.

Answer 47

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

Question 48

How the Brain Receives Taste Information

Taste Buds

Answer 48

1. open at a taste pore.
2. supporting cells and a number of elongated taste cells that end in microvilli.
3. When molecules bind to receptor proteins of the microvilli,
4. nerve signals are generated in sensory nerve fibers that go to the brain.
5. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.

Answer 49

Anatomy of the eye • 2 compartments 1. Anterior compartment: between the cornea and lens; filled with a clear fluid called aqueous humor – this liquid is continuously produced each day and drains through small ducts 2. Posterior compartment: most of the eye, behind the lens; contains a gelatinous material called vitreous humor that holds the retina in place and supports the lens – this liquid you are born with and remains; no more is produced

Question 50

During this process:

1. the lens changes its shape to bring the image into focus on the retina.
   
   • The shape of the lens is controlled by the ciliary muscle, within the ciliary body.
     
     • ​When we view a distant object, the ciliary muscle is relaxed
     • causing the suspensory ligaments attached to the ciliary body to be taut.
       
       • The ligaments put tension on the lens and cause it to remain relatively flat
       • When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments.
         
         • The lens becomes round and thick due to its natural elasticity ​​​
         • Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain.
           
           1. ​Eyestrain more common after 40 and for those who work with computers​
     • Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina.

Answer 50

visual accommodation

close vision.

See Figure (Fig. 15.7a).

(Fig. 15.7b).

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Question 51

reversed prompt

somatic senses.

Answer 51

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMES
Upon completion of this section, you should be able to
List the four categories of sensory receptors and describe what stimulus each responds to.
Distinguish between perception and sensation.
Explain the purpose of integration and sensory adaptation.
A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse. This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons. Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.
Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.
Tutorial: Negative Feedback

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.
Types of Sensory Receptors
Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.
Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.
Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.
The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.
Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).
How Sensation Occurs
Sensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.
As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.
Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.
Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.
All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!
Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.
CHECK YOUR PROGRESS 15.1
Describe the functions of the four types of sensory receptors.
Answer
Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Distinguish between sensation and integration.
Answer
Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.
Answer
Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

CONNECTING THE CONCEPTS
For more information on the regions of the brain associated with sensation, refer to the following discussions:
Section 14.2 describes the location and function of the reticular activating system (RAS).
Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.
Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Question 52

reversed prompt

Sensory receptors respond to environmental stimuli.
Nerve impulses travel to the cerebral cortex and this:
(conscious perception of stimuli) occurs.
Integration, the summing of signals occurs, and nerve signals can be initiated
Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound)

Answer 52

How does sensation occur?

What is sensation and where does it first occur?

Question 53

reversed prompt

We are Thermoreceptors and we play a role in homeostasis by:

(see Fig. 4.18)

Answer 53

1. hypothalamus and skin are stimulated by changes in temperature.
2. They respond to both heat and cold and play a major role in the regulation of internal body temperature .

Question 54

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Answer 55

15.3 Senses of Taste and Smell

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Compare and contrast the senses of taste and smell.

Identify the structures of the tongue and the olfactory areas of the nose.

Summarize how the brain receives taste and odor information.

Question 56

Reverse.Prompt

Visual Pathway to the Brain​

Answer 56

1. The pathway for vision begins once light has been focused on the photoreceptors in the retina.
2. Some integration occurs in the retina, where
   
   • nerve signals begin before
   • the optic nerve transmits them to the brain.

Question 57

15.2 Somatic Senses

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Distinguish between proprioceptors and cutaneous receptors with regard to function.

State the location and general function of each type of cutaneous receptor.

Explain the role of nociceptors and summarize the type of sensory input they detect.

Senses whose receptors are associated with the skin, muscles, joints, and viscera are termed the somatic senses. These receptors can be categorized into three types: proprioceptors, cutaneous Page 309receptors, and pain receptors. All of these send nerve impulses via the spinal cord to the primary somatosensory areas of the cerebral cortex (see Fig. 14.11).

Proprioceptors

Proprioceptors are mechanoreceptors involved in reflex actions that maintain muscle tone, and thereby the body’s equilibrium and posture. For example, proprioceptors called muscle spindles are embedded in muscle fibers (Fig. 15.2). If a muscle relaxes too much, the muscle spindle stretches, generating nerve impulses that cause the muscle to contract slightly. Conversely, when muscles are stretched too much, proprioceptors called Golgi tendon organs, buried in the tendons that attach muscles to bones, generate nerve impulses that cause the muscles to relax. Both types of receptors act together to maintain a functional degree of muscle tone. The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action. The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture. Proper balance and body position are maintained, despite the force of gravity always acting on the skeleton and muscles.

Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.

Cutaneous Receptors

The skin is composed of two layers: the epidermis and the dermis (see Section 4.6). The dermis contains cutaneous receptors (Fig. 15.3), which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a mosaic of these tiny receptors, as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure; at others, you will feel heat or cold (depending on the probe’s temperature).

Figure 15.3 Sensory receptors of the skin. The general function of each sensory receptor is shown here. However, receptors are not always this specialized. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations.

Several types of cutaneous receptors are sensitive to fine touch. These receptors give a person specific information, such as the location of the touch, as well as its shape, size, and texture. Meissner corpuscles and Krause end bulbs are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. Merkel discs are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.

Two types of cutaneous receptors sensitive to pressure are Pacinian corpuscles and Ruffini endings. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.

Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others respond to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.

Pain Receptors

Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.

Page 310Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

SCIENCE IN YOUR LIFE

What are phantom sensation and phantom pain?

Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.

Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.

CHECK YOUR PROGRESS 15.2

Describe how the body uses proprioceptors to indicate the position of the arms and legs.

Answer

By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.

Answer

Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.

Answer

Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Figure 4.9 provides a more detailed look at the structure of human skin.

Section 13.2 provides an overview of muscle fiber contraction.

Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 58

Reverse.Prompt

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Question 59

We are 4 types of sensory receptors and what we do!

15.1 Overview of Sensory Receptors and Sensations, 15.1 Lecture, 1.3

Answer 59

Chemoreceptors – respond to nearby chemicals –

Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue

Photoreceptors – respond to light energy
Mechanoreceptors – respond to mechanical forces such as pressure
Thermoreceptors – stimulated by temperature changes

Question 60

reversed prompt

thalamus in the brain - goes directly to cerebral cortex

Answer 60

olfaction doesn’t need to go through

Question 61

reversed prompt

where they are located- exteroreceptors- taste, hearing, vision interreceptors - detect internal stimuli

Answer 61

Sensory receptors

Question 62

Slides Photoreceptors

Answer 62

* respond to light energy

* Our eyes have these that are sensitive to light rays and thereby provide us with a sense of vision.

* Stimulation of the photoreceptors known as rod cells results in black-and-white vision.

* Stimulation of the photoreceptors known as cone cells results in color vision.

Question 63

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

Question 64

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 65

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Answer 66

Abnormalities of the eye • Myopia (nearsightedness) – eyeball is too long, making it hard to see far away objects • Hyperopia (farsightedness)– eyeball is too short, making it hard to see near objects • Astigmatism – cndition in which the cornea or lens is uneven, leading to a fuzzy image

Question 67

Reverse.Prompt

Anatomy of the ear Slides

What are the ear functions and structures?

15. 5 Sense of Hearing 2 1. Divisions of the ear:
16. 5 Sense of Hearing 4 Following the sound wave
17. 5 Sense of Hearing 6 3. Divisions of the ear: Inner ear •

Slides

See Figure 15.12 The three divisions of the human ear.

Answer 67

Primary Functions:

hearing
balance

Structure/Anatomy:
1. Outer Ear: functions in hearing; filled with air

Pinna: the external ear flap that catches sound waves
Auditory canal: directs sound waves to the tympanic membrane
Lined with fine hairs and modified sweat glands

​that secrete ear wax called ceruminous glands

2. Middle ear: functions in hearing; filled with air

 Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones – 
 Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves – 
 Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst 

* This tube has a 45 degree tilt in adults, but only
10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

3. Inner ear: functions in hearing and balance; filled with fluid

3 areas:

* cochlea

* semicircular canals

* vestibule

*

Stapes (middle ear bone) – vibrates and strikes the membrane of the oval window

*

* vestibular nerve

* vestibule oval window

* semicircular canals

* Middle ear

* Inner ear

* stapes incus malleus

* round window

* auditory tube

* temporal bone

* cochlea

* cochlear nerve

* auditory canal

* tympanic membrane

* pinna

* Outer ear

* http://www.health-reply.com/tags/opening/1/6/ Comparison of auditory tube angles between adults and infants

*

Question 68

Reverse.Prompt

As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain.

At that time, the brain integrates this information with other information received from other sensory receptors.

Consider what happens if you burn yourself and quickly remove your hand from a hot stove.

The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.

1. Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons.
2. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus.
3. chemoreceptors have receptor proteins that bind them to certain chemicals.
4. ion channels open, and ions flow across the plasma membrane.
5. stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber
6. The stronger the stimulus, the greater the frequency of nerve signals.
7. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts.
8. If nerve signals finally reach the cerebral cortex, sensation occurs.

Answer 68

How does sensation occur? Page 308

PNS to the CNS (Fig. 15.1).

Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.

Question 69

Proprioceptors •

Answer 69

Mechanoreceptors that are involved in
reflex actions
that maintain muscle tone, and
consequently equilibrium and posture

Answer 70

Key Concepts to Focus On • What are sensory receptors? • How do we detect the sense of taste and smell? • What is the anatomy of the eye? • How do we focus images? • What are some eye abnormalities? • What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing?

Answer 71

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

Question 72

Noise pollution • Loud noises (>85 decibels) or chronic noise can damage inner ear cells. • Environmental noise can cause mental health issues such as inability to concentrate, an increase in irritability, and anxiety. • Noise can cause loss of sleep and productivity, and can lead to anxiety.

Answer 72

15.14

Question 73

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Question 74

hypothalamus and
skin are
stimulated by changes in temperature.
They respond to both heat and cold and
play a major role in the regulation of internal body temperature (see Fig. 4.18).

Answer 74

We are Thermoreceptors and we play a role in homeostasis by:

Answer 75

15.4 Sense of VisionLEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera,and the transparentcornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid** is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped **iris.The iris regulates the size of thepupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFEWhat is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens** is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the **aqueous humor.A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person hasglaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina,is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called thevitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis** where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the **optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

StructureFunction

Sclera

Protects and supports the eye CorneaRefracts light rays PupilAdmits light

Choroid

Absorbs stray light Ciliary bodyHolds lens in place, accommodation IrisRegulates light entrance

Retina

Contains photoreceptors for sight Rod cellsMake black-and-white vision possible Cone cellsMake color and acute vision possible Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays HumorsTransmit light rays and support the eye Optic nerveTransmits impulses to the visual cortexTable 15.2Structures of the EyeTable Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8aillustrates the structure of the photoreceptors calledrod cells**and**cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b).Rhodopsin**is a complex molecule made up of the protein opsin and a light-absorbing molecule called**retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFEWhy does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma** has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as **optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted.These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism,can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY HealthCorrecting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 76

Mechanoreceptors that are involved in reflex actions that maintain muscle tone, and consequently equilibrium and posture

Answer 76

Proprioreceptors

Answer 77

The inner ear: Semicircular canals and vestibule • Detects angular movement (rotational equilibrium) – Depends on hair cells at the base of each semicircular canal (ampulla) • Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) – Depends on hair cells in the utricle and saccule • Signals sent to cerebellum

Question 78

• • elongated sphere about 2.5 cm in diameter. It has
• three layers, or coats:
  
  1. the sclera: outer layer is made up of the white, fibrous and transparent cornea made of collagen fibers: window of the eye.
  2. the choroid, and the thin middle coat
     
     • extensive blood supply,
     • dark pigment absorbs stray light rays that photoreceptors have not absorbed.
     • helps visual acuity
     • Toward the front: doughnut-shaped iris
       
       1. regulates the size of the pupil, a hole in the center of the iris through which light enters the eye.
          
          • The color of the iris (and therefore the color of the eyes) correlates with its pigmentation.
            
            1. Heavily pigmented eyes are brown, and
            2. lightly pigmented eyes are green or blue.
       2. Behind the iris, the choroid thickens and forms the circular ciliary body.
          
          • contains the ciliary muscle, which controls the shape of the lens for near and far vision.
  3. retina The sclera, and the , which is . The cornea is known as the
• • *

Answer 78

What is a chloroid? Locate it on a diagram

Anatomy and Physiology of the Eye

(Fig. 15.6).

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

Question 79

convert stimulus (both external and internal environments) ————————-> nerve impulse: sensory transduction

* Types:

* modified neurons, and

* others: specialized cells closely associated with neurons.

Answer 79

What is a sensory receptor?

Question 80

Reverse.Prompt

Pain receptors • Sensitive to chemicals released by damaged tissues • In inflammation, cells release chemicals to stimulate pain receptors • Referred pain- stimulation of pain receptors is felt as pain from the skin – Thought to be due to nerve impulses from pain receptors of internal organs travel to spinal cord and synapse with neurons also receiving impulses from skin

Answer 80

Companies, Inc. Permission required for reproduction or display. odor molecules nasal cavity frontal lobe of cerebral hemisphere olfactory bulb neuron olfactory tract bone of skull sensory nerve fibers olfactory epithelium olfactory cell odor molecules olfactory cilia of b. olfactory cell a. supporting cell olfactory epithelium olfactory bulb Figure 15.5 The sense of smell. S

Question 81

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

Question 82

Reverse.Prompt

Nervous system and movement

In addition to the cerebellum, in movement:

Answer 82

• In addition, the motor cortex in the frontal lobe of the brain signals
  
  • where the limbs should be located at any particular moment

After integrating all these nerve inputs, the cerebellum coordinates skeletal

1. muscle contraction to correct our position in space if necessary.

*

Question 83

Reverse.Prompt

Hearing Path

From the Cochlea to the Auditory Cortex

Page 322

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

Answer 83

• cochlea
  
  • three canals.
    
    1. The sensory organ for hearing, called the spiral organ (or the organ of Corti),
       
       • cochlear canal. The spiral organ
       • little hair cells and a gelatinous material called the tectorial membrane.
         
         • _​_The hair cells sit on the basilar membrane,
           
           • and their stereocilia are embedded in the tectorial membrane.
  • Steps/Process/Pathway
    
    1. When the stapes strikes the membrane of the oval window,
    2. pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane.
    3. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend.
    4. Then, nerve signals begin in the cochlear nerve
    5. and travel to the brain.
    6. When they reach the auditory cortex in the temporal lobe,
    7. they are interpreted as a sound.

Question 84

reversed prompt

• respond to light energy.
• Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision.
• Stimulation of the photoreceptors known as rod cells results in black-and-white vision.
• Stimulation of the photoreceptors known as cone cells results in color vision.

Answer 84

Photoreceptors

Question 85

Reverse.Prompt

1. sound waves enter the auditory canal.
2. when a large number of waves strike the tympanic membrane, it (vibrates) ever so slightly
   
   • auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes.
3. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles; about 20 times.
4. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

Answer 85

Through the Auditory Canal and Middle Ear

Steps of the hearing process

Question 86

Reverse.Prompt

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

Question 87

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Question 88

Key Concepts to Focus On • What are sensory receptors? • How do we detect the sense of taste and smell? • What is the anatomy of the eye? • How do we focus images? • What are some eye abnormalities? • What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing

Question 89

reversed prompt

interreceptors

Answer 89

Blood pressure, homeostasis, negative feedback loop

Question 90

reversed prompt

Answer 90

SUMMARIZE15.1Overview of Sensory Receptors and Sensations
Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:
Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.
Photoreceptors detect light stimuli.
Mechanoreceptors detect stimuli generated by mechanical forces.
Thermoreceptors detect stimuli caused by changes in temperature.
All of these classes function as follows:
Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.
Sensation occurs when nerve signals reach the cerebral cortex.
Perception is an interpretation of sensations.
15.2Somatic Senses
Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:
Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.
Cutaneous receptors in the skin sense touch, pressure, and temperature.
Nociceptors detect pain by responding to chemical signals from damaged tissues.
Page 325
15.3Senses of Taste and Smell
Taste and smell are due to chemoreceptors stimulated by molecules in the environment.
Sense of Taste
Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.
Sense of Smell
The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.
15.4Sense of Vision
Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.
Anatomy and Physiology of the Eye
The eye has three layers:
The sclera (outer layer) protects and supports the eye.
The choroid (middle, pigmented layer) absorbs stray light rays.
The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).
Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.
Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.
Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.
Abnormalities of the Eye
Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).
15.5Sense of Hearing
Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.
Anatomy and Physiology of the Ear
The ear has three parts:
In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.
In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.
In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.
The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.
Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.
The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.
Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.
15.6Sense of Equilibrium
The ear also contains mechanoreceptors for equilibrium.
Rotational Equilibrium Pathway
Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.
Gravitational Equilibrium Pathway
Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.
ASSESSTESTING YOURSELF
Choose the correct answer for each question.
15.1Overview of Sensory Receptors and Sensations
Which receptors detect stimuli within the body?
interoceptors
exteroceptors
homeoreceptors
reflex receptors
A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a
mechanoreceptor.
photoreceptor.
chemoreceptor.
thermoreceptor.
None of these are correct.
Where does the process of sensation occur in the body?
at the sensory receptor
in the spinal cord
within the synapses between neurons of the PNS
in the cerebral cortex
All of these are correct.Page 326
15.2Somatic Senses
Which type of receptor detects the chemicals released by damaged tissues?
nociceptors
proprioceptors
Meissner corpuscles
Ruffini endings
None of these are correct.
Which type of receptor assists in the maintenance of muscle tone?
nociceptors
proprioceptors
Pacinian corpuscles
Krause end bulbs
All of these are correct.
15.3Senses of Taste and Smell
The senses of taste and smell rely primarily on which type of receptor?
mechanoreceptors
nociceptors
protoreceptors
proprioceptors
chemoreceptors
Olfactory bulbs are located
on the tongue.
in the nasal cavity.
in the brain stem.
in the aorta.
None of these are correct.
15.4Sense of Vision
Label this diagram of a human eye.
Which structure of the eye is incorrectly matched with its function?
lens—focusing
cones—color vision
iris—regulation of amount of light
choroid—location of cones
sclera—protection
Adjustment of the lens to focus on objects close to the viewer is called
convergence.
visual accommodation.
focusing.
constriction.
To focus on objects that are close to the viewer, the
suspensory ligaments must be pulled tight.
lens needs to become more rounded.
ciliary muscle will be relaxed.
image must focus on the area of the optic nerve.
15.5Sense of Hearing
Label this diagram of a human ear.
Which of the following is not involved in the sense of hearing?
auditory canal
tympanic membrane
ossicles
semicircular canals
cochlea
Which one of these correctly describes the location of the spiral organ?
between the tympanic membrane and the oval window in the inner ear
in the utricle and saccule within the vestibule
between the tectorial membrane and the basilar membrane in the cochlear canal
between the nasal cavities and the throat
between the outer and inner ear within the semicircular canalsPage 327
15.6Sense of Equilibrium
Which of the following structures would allow you to know you were upside down, even if you were in total darkness?
utricle and saccule
cochlea
semicircular canals
tectorial membrane
Moving your head forward would be detected by which of the following structures?
the semicircular canals
the utricle and saccule
the cochlea
the auditory canal
None of these are correct.
ENGAGETHINKING CRITICALLY
Which receptors are activated when you enjoy supper in a pizza restaurant?
Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?
Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?
Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?
The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?
Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?
Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC
ANSWER KEYTesting Yourself
Click here for the answers to the Testing Yourself questions.
Answer
Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically
Click here for the answers to the Thinking Critically questions.
Answer
Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Question 91

reversed prompt

Answer 91

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 92

reversed prompt

What receptors are in the dermis and make the skin sensitive to touch, pressure, pain, and temperature?

Answer 92

Cutaneous receptors

Answer 93

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

Answer 94

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Question 95

Reverse.Prompt

What role does the cerebellum play in movement?

Answer 95

cerebellum

• data reaches from
  
  • inner ear (the semicircular canals, utricle, and saccule),
  • proprioceptive inputs.
• determines direction of the movement of the head at that moment.
  
  • vital to maintaining balance and gravitational equilibrium.
• processes information visual

Answer 96

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

Question 97

Reverse.Prompt

What is a chloroid? Locate it on a diagram

Anatomy and Physiology of the Eye

(Fig. 15.6).

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

Answer 97

• • elongated sphere about 2.5 cm in diameter. It has
• three layers, or coats:
  
  1. the sclera: outer layer is made up of the white, fibrous and transparent cornea made of collagen fibers: window of the eye.
  2. the choroid, and the thin middle coat
     
     • extensive blood supply,
     • dark pigment absorbs stray light rays that photoreceptors have not absorbed.
     • helps visual acuity
     • Toward the front: doughnut-shaped iris
       
       1. regulates the size of the pupil, a hole in the center of the iris through which light enters the eye.
          
          • The color of the iris (and therefore the color of the eyes) correlates with its pigmentation.
            
            1. Heavily pigmented eyes are brown, and
            2. lightly pigmented eyes are green or blue.
       2. Behind the iris, the choroid thickens and forms the circular ciliary body.
          
          • contains the ciliary muscle, which controls the shape of the lens for near and far vision.
  3. retina The sclera, and the , which is . The cornea is known as the
• • *

Question 98

SUMMARIZE15.1Overview of Sensory Receptors and Sensations
Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:
Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.
Photoreceptors detect light stimuli.
Mechanoreceptors detect stimuli generated by mechanical forces.
Thermoreceptors detect stimuli caused by changes in temperature.
All of these classes function as follows:
Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.
Sensation occurs when nerve signals reach the cerebral cortex.
Perception is an interpretation of sensations.
15.2Somatic Senses
Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:
Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.
Cutaneous receptors in the skin sense touch, pressure, and temperature.
Nociceptors detect pain by responding to chemical signals from damaged tissues.
Page 325
15.3Senses of Taste and Smell
Taste and smell are due to chemoreceptors stimulated by molecules in the environment.
Sense of Taste
Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.
Sense of Smell
The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.
15.4Sense of Vision
Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.
Anatomy and Physiology of the Eye
The eye has three layers:
The sclera (outer layer) protects and supports the eye.
The choroid (middle, pigmented layer) absorbs stray light rays.
The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).
Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.
Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.
Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.
Abnormalities of the Eye
Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).
15.5Sense of Hearing
Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.
Anatomy and Physiology of the Ear
The ear has three parts:
In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.
In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.
In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.
The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.
Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.
The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.
Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.
15.6Sense of Equilibrium
The ear also contains mechanoreceptors for equilibrium.
Rotational Equilibrium Pathway
Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.
Gravitational Equilibrium Pathway
Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.
ASSESSTESTING YOURSELF
Choose the correct answer for each question.
15.1Overview of Sensory Receptors and Sensations
Which receptors detect stimuli within the body?
interoceptors
exteroceptors
homeoreceptors
reflex receptors
A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a
mechanoreceptor.
photoreceptor.
chemoreceptor.
thermoreceptor.
None of these are correct.
Where does the process of sensation occur in the body?
at the sensory receptor
in the spinal cord
within the synapses between neurons of the PNS
in the cerebral cortex
All of these are correct.Page 326
15.2Somatic Senses
Which type of receptor detects the chemicals released by damaged tissues?
nociceptors
proprioceptors
Meissner corpuscles
Ruffini endings
None of these are correct.
Which type of receptor assists in the maintenance of muscle tone?
nociceptors
proprioceptors
Pacinian corpuscles
Krause end bulbs
All of these are correct.
15.3Senses of Taste and Smell
The senses of taste and smell rely primarily on which type of receptor?
mechanoreceptors
nociceptors
protoreceptors
proprioceptors
chemoreceptors
Olfactory bulbs are located
on the tongue.
in the nasal cavity.
in the brain stem.
in the aorta.
None of these are correct.
15.4Sense of Vision
Label this diagram of a human eye.
Which structure of the eye is incorrectly matched with its function?
lens—focusing
cones—color vision
iris—regulation of amount of light
choroid—location of cones
sclera—protection
Adjustment of the lens to focus on objects close to the viewer is called
convergence.
visual accommodation.
focusing.
constriction.
To focus on objects that are close to the viewer, the
suspensory ligaments must be pulled tight.
lens needs to become more rounded.
ciliary muscle will be relaxed.
image must focus on the area of the optic nerve.
15.5Sense of Hearing
Label this diagram of a human ear.
Which of the following is not involved in the sense of hearing?
auditory canal
tympanic membrane
ossicles
semicircular canals
cochlea
Which one of these correctly describes the location of the spiral organ?
between the tympanic membrane and the oval window in the inner ear
in the utricle and saccule within the vestibule
between the tectorial membrane and the basilar membrane in the cochlear canal
between the nasal cavities and the throat
between the outer and inner ear within the semicircular canalsPage 327
15.6Sense of Equilibrium
Which of the following structures would allow you to know you were upside down, even if you were in total darkness?
utricle and saccule
cochlea
semicircular canals
tectorial membrane
Moving your head forward would be detected by which of the following structures?
the semicircular canals
the utricle and saccule
the cochlea
the auditory canal
None of these are correct.
ENGAGETHINKING CRITICALLY
Which receptors are activated when you enjoy supper in a pizza restaurant?
Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?
Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?
Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?
The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?
Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?
Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC
ANSWER KEYTesting Yourself
Click here for the answers to the Testing Yourself questions.
Answer
Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically
Click here for the answers to the Thinking Critically questions.
Answer
Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Question 99

Chapter Review

SUMMARIZE

15.1Overview of Sensory Receptors and Sensations

Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:

Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.

Photoreceptors detect light stimuli.

Mechanoreceptors detect stimuli generated by mechanical forces.

Thermoreceptors detect stimuli caused by changes in temperature.

All of these classes function as follows:

Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.

Sensation occurs when nerve signals reach the cerebral cortex.

Perception is an interpretation of sensations.

15.2Somatic Senses

Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:

Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.

Cutaneous receptors in the skin sense touch, pressure, and temperature.

Nociceptors detect pain by responding to chemical signals from damaged tissues.

Page 325

15.3Senses of Taste and Smell

Taste and smell are due to chemoreceptors stimulated by molecules in the environment.

Sense of Taste

Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.

Sense of Smell

The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.

15.4Sense of Vision

Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.

Anatomy and Physiology of the Eye

The eye has three layers:

The sclera (outer layer) protects and supports the eye.

The choroid (middle, pigmented layer) absorbs stray light rays.

The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).

Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.

Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.

Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.

Abnormalities of the Eye

Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).

15.5Sense of Hearing

Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.

Anatomy and Physiology of the Ear

The ear has three parts:

In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.

In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.

In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.

The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.

Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.

The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.

Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.

15.6Sense of Equilibrium

The ear also contains mechanoreceptors for equilibrium.

Rotational Equilibrium Pathway

Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.

Gravitational Equilibrium Pathway

Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.

ASSESS

TESTING YOURSELF

Choose the correct answer for each question.

15.1Overview of Sensory Receptors and Sensations

Which receptors detect stimuli within the body?

interoceptors

exteroceptors

homeoreceptors

reflex receptors

A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a

mechanoreceptor.

photoreceptor.

chemoreceptor.

thermoreceptor.

None of these are correct.

Where does the process of sensation occur in the body?

at the sensory receptor

in the spinal cord

within the synapses between neurons of the PNS

in the cerebral cortex

All of these are correct.Page 326

15.2Somatic Senses

Which type of receptor detects the chemicals released by damaged tissues?

nociceptors

proprioceptors

Meissner corpuscles

Ruffini endings

None of these are correct.

Which type of receptor assists in the maintenance of muscle tone?

nociceptors

proprioceptors

Pacinian corpuscles

Krause end bulbs

All of these are correct.

15.3Senses of Taste and Smell

The senses of taste and smell rely primarily on which type of receptor?

mechanoreceptors

nociceptors

protoreceptors

proprioceptors

chemoreceptors

Olfactory bulbs are located

on the tongue.

in the nasal cavity.

in the brain stem.

in the aorta.

None of these are correct.

15.4Sense of Vision

Label this diagram of a human eye.

Which structure of the eye is incorrectly matched with its function?

lens—focusing

cones—color vision

iris—regulation of amount of light

choroid—location of cones

sclera—protection

Adjustment of the lens to focus on objects close to the viewer is called

convergence.

visual accommodation.

focusing.

constriction.

To focus on objects that are close to the viewer, the

suspensory ligaments must be pulled tight.

lens needs to become more rounded.

ciliary muscle will be relaxed.

image must focus on the area of the optic nerve.

15.5Sense of Hearing

Label this diagram of a human ear.

Which of the following is not involved in the sense of hearing?

auditory canal

tympanic membrane

ossicles

semicircular canals

cochlea

Which one of these correctly describes the location of the spiral organ?

between the tympanic membrane and the oval window in the inner ear

in the utricle and saccule within the vestibule

between the tectorial membrane and the basilar membrane in the cochlear canal

between the nasal cavities and the throat

between the outer and inner ear within the semicircular canalsPage 327

15.6Sense of Equilibrium

Which of the following structures would allow you to know you were upside down, even if you were in total darkness?

utricle and saccule

cochlea

semicircular canals

tectorial membrane

Moving your head forward would be detected by which of the following structures?

the semicircular canals

the utricle and saccule

the cochlea

the auditory canal

None of these are correct.

ENGAGE

THINKING CRITICALLY

Which receptors are activated when you enjoy supper in a pizza restaurant?

Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?

Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?

Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?

The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?

Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Question 100

Important for both hearing and balance • 3 areas: cochlea, semicircular canals, vestibule • Stapes (middle ear bone) – vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea • Vestibule – functions in gravitational equilibrium • Semicircular canals – functions in rotational equilibrium

Question 101

Laser-assisted in situ karatomileusis •

• Need cornea to be thick enough
• • Conjunctiva is cut away from front of eye and folded back to expose cornea •
• A defined amount of corneal tissue is removed to either flatten or increase steepness of cornea curvature •
• Conjunctiva is then put back into place

Answer 102

SCIENCE IN YOUR LIFEWhy does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Question 103

Reverse.Prompt

Answer 103

Proprioceptors • Mechanoreceptors that are involved in reflex actions that maintain muscle tone, and consequently equilibrium

Question 104

movement of the head in the vertical or horizontal planes,

*

Answer 104

Gravitational Equilibrium Pathway

Question 105

1. If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision.
2. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted.
   
   • Nearsighted people can see close objects better than they can see objects at a distance.
   • The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object,
   • the image is brought to focus in front of the retina.
   • They can see close objects because their lens can compensate for the elongated shape of the eye.
   • To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.
3. Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted.
   
   1. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina. When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.
4. When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea.

Answer 105

Distance Vision

(Fig. 15.11a)

(Fig. 15.11b)

(Fig. 15.11c)

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Question 106

reversed prompt

Answer 106

Divisions of the ear: Outer ear • Includes – Pinna: the external ear flap that catches sound waves – Auditory canal: directs sound waves to the tympanic membrane • Lined with fine hairs and modified sweat glands that secrete ear wax called ceruminous glands

Question 107

reversed prompt

sensory transduction

(Table 15.1)- Exteroreceptors

Answer 107

1. A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse.
2. This conversion is commonly referred to as
3. Some sensory receptors are modified neurons,
4. and others are specialized cells closely associated with neurons.
5. Sensory receptors may detect stimuli originating from both the internal and external environments.

• Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium
• Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Answer 108

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

Question 109

3) Eye Anatomy: the retina,

posterior compartment: filled with a clear, gelatinous material called the vitreous humor.

function:

* holds the retina in place and

* supports the lens.

photoreceptors

rod cells:

* very sensitive to light, but
they do not detect color.

* at night or in a darkened room, we see only shades of gray.

cone cells.

* require bright light,

* are sensitive to different wavelengths of light

* This sensitivity gives us the ability to distinguish colors.

fovea centralis: cone cells are densely packed.

* Light is normally focused on the fovea when we look directly at an object.

* This is helpful because the sharpest images are produced by the fovea centralis.

Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Answer 109

Anatomy of the Eye (cont)

Layer 3: Retina

Question 110

chemoreceptors

Answer 110

chemicals, nosiceptors- pain-

Question 111

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Question 112

Primary Functions:

1. hearing
2. balance

Structure/Anatomy:

1. Outer Ear: functions in hearing; filled with air

• Pinna: the external ear flap that catches sound waves
• Auditory canal: directs sound waves to the tympanic membrane
  
  • Lined with fine hairs and modified sweat glands that secrete ear wax called ceruminous glands

2. Middle ear: functions in hearing; filled with air

• Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones –
• Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves –
• Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to
  
  • equalize pressure so the eardrum does not burst
  • This tube has a 45 degree tilt in adults, but only
  • 10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

3. Inner ear: functions in hearing and balance; filled with fluid ; 3 areas:

• cochlea
• semicircular canals
• vestibule

Stapes (middle ear bone) – vibrates and strikes the membrane of the oval window

* vestibular nerve
vestibule oval window
semicircular canals
Middle ear
Inner ear
stapes incus malleus
round window
auditory tube
temporal bone
cochlea
cochlear nerve
auditory canal
tympanic membrane
pinna
Outer ear

* http://www.health-reply.com/tags/opening/1/6/ Comparison of auditory tube angles between adults and infants

*

*

Answer 112

Anatomy of the ear Slides

What are the ear functions and structures?

15. 5 Sense of Hearing 2 1. Divisions of the ear:
16. 5 Sense of Hearing 4 Following the sound wave
17. 5 Sense of Hearing 6 3. Divisions of the ear: Inner ear •

Slides

See Figure 15.12 The three divisions of the human ear.

Question 113

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Answer 114

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

Question 115

The lens is a flexible, transparent, and concave structure. • Visual accommodation occurs when the lens changes shape to focus light on the retina and form an image. • As we age, the lens loses elasticity, and we use glasses to correct for this. 15.4 Sense of Vision T

Question 116

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

Answer 116

Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

Question 117

Reverse.Prompt

BIOLOGY TODAY HealthCorrecting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

has an extensive blood supply

absorbs stray light

contains a dark pigment

Question 118

Adaptation

Answer 118

decrease in stimulus response

Question 119

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Question 120

reversed prompt

Answer 120

Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.Tutorial: Negative Feedback
*
* Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.Types of Sensory Receptors
* Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.
* The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).How Sensation OccursSensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.
* Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.
*
* All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.CHECK YOUR PROGRESS 15.1Describe the functions of the four types of sensory receptors. AnswerChemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.
* Distinguish between sensation and integration. AnswerSensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.
* Summarize the importance of sensory receptors in the maintenance of homeostasis in the body. AnswerSensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.
* CONNECTING THE CONCEPTS
* For more information on the regions of the brain associated with sensation, refer to the following discussions:Section 14.2 describes the location and function of the reticular activating system (RAS).Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.
*
*
*
*
*

Question 121

Reverse.Prompt

Answer 121

Cutaneous receptors • Receptors in the dermis that make the skin sensitive to touch, pressure, pain, and temperature. Figure 15.3 Sensory receptors of the skin. 15.2 Somatic Senses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Question 122

Reverse.Prompt

* Sensory Receptors

* chemoreceptors,

* photoreceptors,

* mechanoreceptors, and

* thermoreceptors.

Chemoreceptors

* If the pH lowers, the breathing rate increases.

* As more carbon dioxide is exhaled, the blood pH rises.

* pain receptors

Answer 122

There are 4 categories of us:

They are:

We respond to chemical substances in the immediate vicinity, just check out Table 15.1. Taste and Smell, which detect external stimuli

Other organs sensitive to internal stimuli use us: For example, we help monitor blood pH and are located in the carotid arteries and aorta— this is how we work:
Nociceptors (aka ______________________) are also a type of us.
are a type of chemoreceptor.
They are naked dendrites that respond to chemicals released by damaged tissues.
Nociceptors are protective, because they alert us to possible danger.

For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Answer 123

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 124

reversed prompt

rods- black and white vision
cones-

Answer 124

Table 15.1

Question 125

Reverse.Prompt

Mechanical Receptors

Taste

Answer 125

4,000 taste buds are located primarily on the tongue of adult humans. •
We have 5 main types of taste receptors:

Chapter Review

SUMMARIZE

16.1Endocrine Glands

The endocrine system works with the nervous system to regulate the activities of the other body systems. Endocrine glands secrete hormones into the bloodstream. From there, they are distributed to target organs or tissues. This differs from exocrine glands, which secrete products into ducts.

Hormones, a type of chemical signal, usually act at a distance between body parts. Hormones are either peptides or steroids.

Pheromones are chemical signals that influence the behavior of another individual.

Reception of a peptide hormone at the plasma membrane activates an enzyme cascade inside the cell. Peptide hormones typically use second messenger systems, such as cyclic adenosine monophosphate (cAMP).

Steroid hormones combine with a receptor, and the complex attaches to and activates DNA. Protein synthesis follows.

16.2Hypothalamus and Pituitary Gland

The endocrine system is controlled by the hypothalamus, which regulates the secretions of the pituitary gland. Neurosecretory cells in the hypothalamus produce antidiuretic hormone (ADH) and oxytocin, which are stored in axon endings in the posterior pituitary until released.

The hypothalamus produces hypothalamic-releasing and hypothalamic-inhibiting hormones, which pass to the anterior pituitary by way of a portal system.

The anterior pituitary produces several types of hormones, including thyroid-stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), gonadotropic hormones, follicle-stimulating hormone (FSH), luteinizing hormone (LH), prolactin, melanocyte-stimulating hormone, and growth hormone (GH). Some of these stimulate other hormonal glands to secrete hormones.

Endocrine disorders associated with growth hormones include pituitary dwarfism, gigantism, and acromegaly.

16.3Thyroid and Parathyroid Glands

The thyroid gland requires iodine to produce triiodothyronine (T3) and thyroxine (T4), which increase the metabolic rate.

If iodine is available in limited quantities, endemic goiter develops. Congenital hypothyroidism occurs if the thyroid does not develop correctly.

In adults, hypothyroidism leads to myxedema, while hyperthyroidism results in an exophthalmic goiter.

The thyroid gland produces calcitonin, which helps lower the blood calcium level.

The parathyroid glands secrete parathyroid hormone (PTH), which raises the blood calcium level.

16.4Adrenal Glands

The adrenal glands respond to stress.

Adrenal Medulla

The adrenal medulla immediately secretes epinephrine and norepinephrine. Heartbeat and blood pressure increase, blood glucose level rises, and muscles become energized.

Adrenal Cortex

The adrenal cortex produces the glucocorticoids (cortisol), the mineralocorticoids (aldosterone), and gonadocorticoids (dehydroepiandrosterone [DHEA]), androgens, and estradiol (estrogen). The glucocorticoids regulate carbohydrate, protein, and fat metabolism and suppress the inflammatory response. Mineralocorticoids are influenced by renin from the kidneys and regulate water and salt balance, leading to increases in blood volume and blood pressure.

Problems with the adrenal cortex may result in Addison disease or Cushing syndrome.

Page 352

16.5Pancreas

The pancreas contains both endocrine and exocrine cells. The pancreatic islets secrete the hormones insulin and glucagon.

Insulin lowers the blood glucose level.

Glucagon raises the blood glucose level.

Diabetes mellitus is due to the failure of the pancreas to produce insulin or the failure of the cells to take it up.

16.6Other Endocrine Glands

Other endocrine glands also produce hormones:

The testes and ovaries (gonads) produce the sex hormones. Male sex hormones are the androgens (testosterone); female sex hormones are the estrogens and progesterone. Anabolic steroids mimic the action of testosterone.

The thymus secretes thymosins, which stimulate T-lymphocyte production and maturation.

The pineal gland produces melatonin, which may be involved in circadian rhythms and the development of the reproductive organs.

Some organs and tissues also produce hormones:

Kidneys produce erythropoietin (EPO).

Adipose tissue produces leptin, which acts on the hypothalamus.

Prostaglandins are produced within cells and act locally.

16.7Hormones and Homeostasis

The nervous and endocrine systems exert control over the other systems and thereby maintain homeostasis.

The nervous system is able to respond to the external environment after receiving data from the sensory receptors. Sensory receptors are present in such organs as the eyes and ears.

The nervous and endocrine systems work together to govern the subconscious control of internal organs. This control often depends on reflex actions involving the hypothalamus and medulla oblongata.

The nervous and endocrine systems work so closely together that they form what is sometimes called the neuroendocrine system.

ASSESS

TESTING YOURSELF

Choose the best answer for each question.

16.1Endocrine Glands

Identify each of the endocrine organs in the figure.

Page 353Peptide hormones interact with which structures on the surface of a cell?

receptor proteins

second messenger systems

pheromones

neurotransmitters

digestive enzymes

A steroid hormone requires the use of a second messenger system to enter a cell.

true

false

16.2Hypothalamus and Pituitary Gland

Which of the following acts as the link between the nervous system and the endocrine system?

posterior pituitary gland

anterior pituitary gland

hypothalamus

parathyroid

Growth hormone (GH) is released by which endocrine gland?

posterior pituitary gland

anterior pituitary gland

hypothalamus

adrenal gland

Which of the following hormones is regulated by positive feedback mechanisms?

thyroid-stimulating hormone (TSH)

gonadotropic hormone

oxytocin

growth hormone

None of these are correct.

16.3Thyroid and Parathyroid Glands

Thyroid hormones directly regulate which aspect of human physiology?

circadian rhythm

sex hormone production

metabolic rate

stress response

None of these are correct.

Which of the following hormones increase(s) blood calcium levels?

calcitonin

parathyroid hormone

thyroxine (T4)

mineralocorticoids

16.4Adrenal Glands

Which of the following hormones is (are) not produced by the adrenal cortex?

glucocorticoids

mineralocorticoids

gonadocorticoids

norepinephrine

Which of the following is not correct regarding aldosterone?

It is produced by the adrenal cortex.

It is inhibited by the action of epinephrine.

Its release is regulated by renin from the kidneys.

It causes the kidneys to reabsorb sodium +(Na+) ions.

All of these are correct.

Which of the following hormones help(s) regulate the electrolyte balance of body fluids?

mineralocorticoids

glucocorticoids

androgens

epinephrine

None of these are correct.

16.5Pancreas

Which of the following correctly describes the hormone insulin?

It is produced by B cells in the pancreas.

It increases glucose uptake by liver and muscle cells.

It is a peptide hormone.

It lowers blood glucose levels.

All of these are correct.

The hormone antagonistic to insulin is

epinephrine.

parathyroid hormone.

glucagon.

cortisol.

progesterone.

The disease that is believed to be caused by an autoimmune response that destroys the pancreatic islets is called

Addison disease.

diabetes insipidus.

type 1 diabetes mellitus.

type 2 diabetes mellitus.

gestational diabetes.

16.6Other Endocrine Glands

The hormone produced by the pineal gland to regulate the circadian rhythm is called

estradiol.

renin.

leptin.

melatonin.

None of these are correct.

This hormone is involved with providing a feeling of fullness after a meal and thus has a role in weight regulation.

erythropoietin (EPO)

melatonin

cortisol

prostaglandin

leptinPage 354

16.7Hormones and Homeostasis

The nervous system is primarily responsible for responses to the ______ environment, while the endocrine system responds to the ______ environment.

external; internal

external; external

internal; external

internal; internal

Which of the following would not be a response of the endocrine system?

release of ADH to prevent water loss

use of cortisol to control the stress response

movement of your fingers away from a hot surface

regulation of blood glucose levels

production of sex hormones

ENGAGE

BioNOW

Want to know how this science is relevant to your life? Check out the BioNOW video below:

BioNOW: Quail Hormones

From your understanding of the endocrine system in humans, what endocrine glands and hormones are most likely involved in this response of the quails to the changes in their environment?

THINKING CRITICALLY

Blood tests are a way to diagnose any number of endocrine disorders because hormones are transported by the circulatory system. GH and IGF-1 can be checked to determine if deficiencies are the reason for a child’s slow growth. Blood levels of TSH, T3, and T4 provide information about thyroid function. Some tests, such as the glucose tolerance test from the chapter opener, do not directly measure the level of the glucose-regulating hormones (in this case, insulin). Instead, they indirectly monitor whether an endocrine gland is performing correctly by measuring specific compounds in the blood.

How is follicle-stimulating hormone similar to growth hormone with regard to how their target cells respond to their signals?

It is possible to diagnose hypothyroidism by high levels of TSH in the blood. Explain what would cause a high TSH level. (Hint: You may want to consider what happens to TSH when the activity of the thyroid is normal.)

Why would a diabetic urinate frequently and always be thirsty?

Many diets advertise that they are specifically designed for diabetics. How would these diets be different from a “normal” diet?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. a. hypothalamus; b. pituitary gland; c. thyroid; d. adrenal gland; e. parathyroid glands; f. thymus; g. pancreas; h. testes; i. ovary; 2. a; 3. b; 4. c; 5. b; 6. c; 7. c; 8. b; 9. d; 10. b; 11. a; 12. e; 13. c; 14. c; 15. d; 16. e; 17. a; 18. c

BioNOW

Click here for the answer to the BioNOW question.

Answer

BioNOW: The quail responded to increased temperature and light levels by beginning breeding behaviors. Melatonin released by the pineal gland in response to light regulates sexual development in the birds. The hypothalamus becomes active in stimulating the release of GH, LH, and FSH, which are involved in sexual development and reproduction. With increased levels of testosterone and estrogens, secondary sex characteristics develop, and sperm and egg production is stimulated.

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Follicle-stimulating hormone and growth hormone are both protein hormones, thus they both bind to a receptor on the plasma membrane and activate a second messenger system (in this case, both use cAMP). 2. When thyroxine is produced, negative feedback occurs to stop TSH, but when T3 and T4 are low, the anterior pituitary produces more TSH than normal. 3. When blood glucose is too high, the excess glucose cannot be reabsorbed from the glomerular filtrate in the kidneys. By osmotic pressure, water follows the glucose into the filtrate and an excessive amount of urine is produced, resulting in dehydration and thirst. 4. The diet would regulate the intake of glucose by favoring foods with a lower glycemic index (see Chapter 9). Because type 2 diabetes is associated with obesity, any diet that reduces overall caloric intake might be helpful.

Question 126

reversed prompt

Divisions of the ear:

Inner ear

Answer 126

Important for both hearing and balance

3 areas:

cochlea: (middle ear bone) – vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea
semicircular canals
vestibule: functions in gravitational equilibrium

Stapes

Vestibule –

Question 127

Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.Tutorial: Negative Feedback
*
* Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.Types of Sensory Receptors
* Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.
* The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).How Sensation OccursSensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.
* Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.
*
* All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.CHECK YOUR PROGRESS 15.1Describe the functions of the four types of sensory receptors. AnswerChemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.
* Distinguish between sensation and integration. AnswerSensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.
* Summarize the importance of sensory receptors in the maintenance of homeostasis in the body. AnswerSensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.
* CONNECTING THE CONCEPTS
* For more information on the regions of the brain associated with sensation, refer to the following discussions:Section 14.2 describes the location and function of the reticular activating system (RAS).Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.
*
*
*
*
*

Question 128

• How do we detect the sense of taste and smell? •

Question 129

Check out this table:

Table 15.1Exteroceptors

Table Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Answer 129

Make a few notes

Question 130

reversed prompt

* Some sensory receptors are

* modified neurons, and

* others are specialized cells closely associated with neurons.

* Sensory receptors may detect stimuli originating from both the internal and external environments.

2. Exteroceptors are

sensory receptors that
detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium

3. Interoceptors

receive stimuli from inside the body.
Examples of interoceptors are the

baroreceptors (also called pressoreceptors) that respond to changes in blood pressure
osmoreceptors that monitor the body’s water-salt balance, and
chemoreceptors that monitor the pH of the blood.

Answer 130

See download and understand (Table 15.1)

1) Receptors and what do they do?

Question 131

Sensory receptors role in homeostasis

Answer 131

The functioning of our sensory receptors makes a significant contribution to homeostasis: ​​to keep the internal environment constant.

* Without sensory input, we would not receive information about our internal and external environments.

* This information leads to

appropriate reflex and
voluntary actions

*

Question 133

Reverse.Prompt

1. Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.

Answer 133

Sense of Smell

1. Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell.
   
   • This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose.
     
     • Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity
       
       1. Olfactory cells are
          
          • modified neurons.
          • Each cell ends in a tuft of about five olfactory cilia,
            
            • which bear receptor proteins for odor molecules.

Question 134

A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse. This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons. Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Question 135

Reverse.Prompt

Question 136

Reverse.Prompt

• When the body is still, the
  
  • otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells.
• When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced.
• The otolithic membrane sags, bending the stereocilia of the hair cells beneath.
• If the stereocilia move toward the largest stereocilium, called the kinocilium,
• nerve impulses increase in the vestibular nerve.
• If the stereocilia
• move away from the kinocilium, nerve impulses decrease in the vestibular nerve.
• The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

Answer 136

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

Question 137

Reverse.Prompt

rods

Answer 137

very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray.

Question 138

reversed prompt

Answer 138

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 139

Chapter Review

SUMMARIZE

15.1Overview of Sensory Receptors and Sensations

Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:

Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.

Photoreceptors detect light stimuli.

Mechanoreceptors detect stimuli generated by mechanical forces.

Thermoreceptors detect stimuli caused by changes in temperature.

All of these classes function as follows:

Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.

Sensation occurs when nerve signals reach the cerebral cortex.

Perception is an interpretation of sensations.

15.2Somatic Senses

Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:

Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.

Cutaneous receptors in the skin sense touch, pressure, and temperature.

Nociceptors detect pain by responding to chemical signals from damaged tissues.

Page 325

15.3Senses of Taste and Smell

Taste and smell are due to chemoreceptors stimulated by molecules in the environment.

Sense of Taste

Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.

Sense of Smell

The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.

15.4Sense of Vision

Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.

Anatomy and Physiology of the Eye

The eye has three layers:

The sclera (outer layer) protects and supports the eye.

The choroid (middle, pigmented layer) absorbs stray light rays.

The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).

Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.

Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.

Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.

Abnormalities of the Eye

Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).

15.5Sense of Hearing

Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.

Anatomy and Physiology of the Ear

The ear has three parts:

In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.

In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.

In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.

The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.

Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.

The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.

Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.

15.6Sense of Equilibrium

The ear also contains mechanoreceptors for equilibrium.

Rotational Equilibrium Pathway

Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.

Gravitational Equilibrium Pathway

Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.

ASSESS

TESTING YOURSELF

Choose the correct answer for each question.

15.1Overview of Sensory Receptors and Sensations

Which receptors detect stimuli within the body?

interoceptors

exteroceptors

homeoreceptors

reflex receptors

A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a

mechanoreceptor.

photoreceptor.

chemoreceptor.

thermoreceptor.

None of these are correct.

Where does the process of sensation occur in the body?

at the sensory receptor

in the spinal cord

within the synapses between neurons of the PNS

in the cerebral cortex

All of these are correct.Page 326

15.2Somatic Senses

Which type of receptor detects the chemicals released by damaged tissues?

nociceptors

proprioceptors

Meissner corpuscles

Ruffini endings

None of these are correct.

Which type of receptor assists in the maintenance of muscle tone?

nociceptors

proprioceptors

Pacinian corpuscles

Krause end bulbs

All of these are correct.

15.3Senses of Taste and Smell

The senses of taste and smell rely primarily on which type of receptor?

mechanoreceptors

nociceptors

protoreceptors

proprioceptors

chemoreceptors

Olfactory bulbs are located

on the tongue.

in the nasal cavity.

in the brain stem.

in the aorta.

None of these are correct.

15.4Sense of Vision

Label this diagram of a human eye.

Which structure of the eye is incorrectly matched with its function?

lens—focusing

cones—color vision

iris—regulation of amount of light

choroid—location of cones

sclera—protection

Adjustment of the lens to focus on objects close to the viewer is called

convergence.

visual accommodation.

focusing.

constriction.

To focus on objects that are close to the viewer, the

suspensory ligaments must be pulled tight.

lens needs to become more rounded.

ciliary muscle will be relaxed.

image must focus on the area of the optic nerve.

15.5Sense of Hearing

Label this diagram of a human ear.

Which of the following is not involved in the sense of hearing?

auditory canal

tympanic membrane

ossicles

semicircular canals

cochlea

Which one of these correctly describes the location of the spiral organ?

between the tympanic membrane and the oval window in the inner ear

in the utricle and saccule within the vestibule

between the tectorial membrane and the basilar membrane in the cochlear canal

between the nasal cavities and the throat

between the outer and inner ear within the semicircular canalsPage 327

15.6Sense of Equilibrium

Which of the following structures would allow you to know you were upside down, even if you were in total darkness?

utricle and saccule

cochlea

semicircular canals

tectorial membrane

Moving your head forward would be detected by which of the following structures?

the semicircular canals

the utricle and saccule

the cochlea

the auditory canal

None of these are correct.

ENGAGE

THINKING CRITICALLY

Which receptors are activated when you enjoy supper in a pizza restaurant?

Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?

Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?

Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?

The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?

Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Answer 140

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Question 141

©Amelie Benoist/Science Source
CHAPTER OUTLINE
15.1Overview of Sensory Receptors and Sensations
15.2Somatic Senses
15.3Senses of Taste and Smell
15.4Sense of Vision
15.5Sense of Hearing
15.6Sense of Equilibrium
BEFORE YOU BEGIN
Before beginning this chapter, take a few moments to review the following discussions:
Section 14.1What are the roles of the central and peripheral nervous systems in the body?
Section 14.2What is the role of the primary somatosensory area of the cerebral cortex?
Section 14.2How does the cerebellum help maintain balance?
Improving your Eyesight
John had always worn glasses. As a computer programmer, he often spent 6–10 hours a day looking at a computer screen. Initially, he tried using bifocal and progressive lenses, but they never seemed to adjust his vision correctly. After a visit to his eye doctor, John decided he would like to permanently correct his vision, and remove his need for glasses, by undergoing LASIK surgery.
LASIK, which stands for laser-assisted in situ keratomileusis, is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. During the procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own.
John was an ideal candidate for this type of surgery—he was over 18 years of age and his last two eye exams had indicated that his vision was relatively stable. He knew there would be a small amount of discomfort from the procedure, but his doctor informed him that the procedure was generally safe and that almost 80% of patients improved their vision to the point that they no longer needed glasses.
In this chapter, we will explore the structure and function of not only the eye but also of the other sensory organs that contribute information to our central nervous system for interpretation.
As you read through the chapter, think about the following questions:
What is the role of the cornea in vision?
How does the eye normally adjust to looking at objects at different distances?

Question 142

Reverse.Prompt

From the Retina to the Visual Cortex

(Fig. 15.10)

317

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

Answer 142

• Process/Steps

1. To reach the _________, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma .
   
   • The optic chiasma
     
     • has an X shape,
     • formed by a crossing-over of optic nerve fibers.
2. After exiting the optic chiasma, the optic nerves continue as optic tracts.
3. Fibers from the right half of each retina converge and continue on together in the right optic tract.
4. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.
5. The optic tracts sweep around the hypothalamus, and
6. most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus.
7. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe.
8. The image is split in the visual cortex.
   
   • This division of incoming information happens because the right visual cortex receives information from the right optic tract,
   • and the left visual cortex receives information from the left optic tract.
9. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Answer 143

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 144

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Question 145

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

Question 146

Why does spinning around cause you to become dizzy?

Answer 146

• When we spin, the cupula slowly begins
  
  • to move in the same direction we are spinning, and
  • bending of the stereocilia causes
  • hair cells to send messages to the brain.
  • As time goes by, the cupula catches up to the rate we are spinning,
  • and the hair cells no longer send messages to the brain.
  • When we stop spinning, the slow-moving cupula continues to
  • move in the direction of the
  • spin and the stereocilia bend again,
  • indicating we are moving. Yet the eyes know we have stopped.
  • The mixed messages sent to the brain cause us to feel dizzy.Page 323

Question 147

What is the anatomy of the ear? •

Answer 148

Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.

Question 149

Reverse.Prompt

Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Answer 149

Distinguish between sensation and integration.

Answer 150

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

Question 151

Reverse.Prompt

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

Question 152

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.

Question 153

reversed prompt

hearing and balance

outer ear-

Answer 153

Ear

Question 154

• 2 compartments 1. Anterior compartment: between the cornea and lens; filled with a clear fluid called aqueous humor – this liquid is continuously produced each day and drains through small ducts 2. Posterior compartment: most of the eye, behind the lens; contains a gelatinous material called vitreous humor that holds the retina in place and supports the lens – this liquid you are born with and remains; no more is produced

Answer 154

Anatomy of the eye

Question 155

cerebellum

• data reaches from
  
  • inner ear (the semicircular canals, utricle, and saccule),
  • proprioceptive inputs.
• determines direction of the movement of the head at that moment.
  
  • vital to maintaining balance and gravitational equilibrium.
• processes information visual

Answer 155

What role does the cerebellum play in movement?

Question 156

reversed prompt

How does sensation occur? •

Answer 156

1. Sensory receptors respond to environmental stimuli.
2. Nerve impulses travel to the cerebral cortex and sensation (conscious perception of stimuli) occurs.
3. Integration, the summing of signals occurs, and
4. nerve signals can be initiated
5. Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound).

Answer 157

The lens is a flexible, transparent, and concave structure. • Visual accommodation occurs when the lens changes shape to focus light on the retina and form an image. • As we age, the lens loses elasticity, and we use glasses to correct for this. 15.4 Sense of Vision The lens a. Focusing on distant object b

Answer 158

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Question 159

15.4 Sense of Vision

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera, and the transparent cornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped iris. The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 160

1. direction of the movement of head at moment.
2. vital to maintaining balance and gravitational equilibrium.
3. from the inner ear
   
   • ​​(the semicircular canals, utricle, and saccule), as well as
   • visual and proprioceptive inputs.
4. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment.
5. After integrating all these nerve inputs,
6. coordinates skeletal muscle contraction to correct our position in space if necessary.

Answer 160

cerebellum in gravitational equilibrium with the inner ear and visual and proprioceptive inputs

Answer 161

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes 15.1 Overview of Sensory Receptors and Sensations 5 Senses and the receptors involved 15.1 Overview of Sensory R

Question 162

Reverse.Prompt

What are nerve signals work and who initiates them? Who receives them and how do they influence resulting sensation?

Answer 162

* All sensory receptors initiate _________________

* The sensation that results depends on the part of the brain receiving the nerve signals.

* _____________ that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects.

* Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated.

* If it were possible to switch these nerves, stimulation of the eyes would result in hearing!

*

Answer 163

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

Question 164

How the Brain Receives Odor Information

Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.

Answer 164

The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.

Question 165

skin, muscles, joints, and viscera are termed the

three types of _____________________________________

1)
2)
3)

via the spinal cord to the primary somatosensory areas of the cerebral cortex

Check out Fig. 14.11

Answer 165

Book

We are the proprioceptors, cutaneous receptors, and pain receptors and we send nerve impulses via the _______________________ to the _________________________ or the ____________________.

Page 309

Question 166

Sensation

Answer 166

Conscious perception of stimuli

Question 167

Reverse.Prompt

Answer 167

Section 14.2 describes the location and function of the reticular activating system (RAS).

Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.

Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Question 168

What are sensory receptors? • How do we detect the sense of taste and smell? • What is the anatomy of the eye? • How do we focus images? • What are some eye abnormalities? • What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing?

Question 169

• To reach the visual cortex,
• nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10).
  
  • X shape: crossing-over of optic nerve fibers.
• post optic chiasma exit, the optic nerves continue as optic tracts.
• Fibers from the right half of each retina converge and continue on together in the right optic tract.
• Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.
• The optic tracts sweep around the hypothalamus, and
  
  • most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus.
• Axons from the thalamic nuclei form optic radiations
• that take nerve impulses to the visual cortex within the occipital lobe.
• The image is split in the visual cortex.
• This division of incoming information happens
  
  • because the right visual cortex receives information from the right optic tract, and the
  • left visual cortex receives information from the left optic tract.
• For good depth perception, the right and left visual cortices communicate with each other.
• Also, because the image is inverted and reversed,
• it must be righted in the brain for us to correctly perceive the visual field.

Answer 169

From the Retina to the Visual Cortex

Final step of Visual processing

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

Question 170

Distinguish between sensation and integration.

Answer 170

Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Question 171

Cataracts and Glaucoma

Cataracts

• develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily.
• A cloudy lens allows less light to reach the retina and slowly causes vision loss.
• Fortunately, a doctor can surgically remove the cloudy lens and replace it with a
• clear plastic lens, which often
• restores the light level passing through the lens and improves the patient’s vision.

Glaucoma

• is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision.
• The condition may eventually cause blindness.
• Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option.
• During glaucoma surgery, the doctor uses a
• laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

Answer 171

BIOLOGY TODAY HealthCorrecting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

has an extensive blood supply

absorbs stray light

contains a dark pigment

Answer 172

1. Layers of the eye: Sclera • Sclera – the white of the eye that maintains eye shape – Cornea: transparent portion of the sclera that is important in refracting light – Pupil: a hole that allows light into the eyeball

Question 173

Depends on 10-20 million olfactory cells (modified neurons) in the roof of the nasal cavity • Odor molecules activate specific combination of receptor proteins for recognition of specific smells and information is sent directly to the olfactory cortex in the temporal lobe from the olfactory bulb

Question 174

What are sensory receptors?

Answer 174

Dendrites,

Question 175

Chapter 15 Senses 2 Key Concepts to Focus On

* What are sensory receptors?

* How do we detect the sense of taste and smell?

* What is the anatomy of the eye?

* How do we focus images?

* What are some eye abnormalities?

* What is the anatomy of the ear?

* Which parts function in balance and which parts function in hearing?

Question 176

Reverse.Prompt

15.3 Senses of Taste and Smell

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Compare and contrast the senses of taste and smell.

Identify the structures of the tongue and the olfactory areas of the nose.

Summarize how the brain receives taste and odor information.

Question 177

How does sensation occur?

(Slides)

Answer 177

Sensory receptors respond to environmental stimuli.
Nerve impulses travel to the cerebral cortex and sensation (conscious perception of stimuli) occurs. •
Integration, the summing of signals occurs, and nerve signals can be initiated •
Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound).

Question 178

Reverse.Prompt

Answer 178

15.4 Sense of Vision

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera, and the transparent cornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped iris. The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 179

SCIENCE IN YOUR LIFEWhat is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

Question 180

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

Question 181

Reverse.Prompt

The structures involved in gravitational equilibrium

Answer 181

The mechanoreceptors in the

utricle and

saccule

detect or vertical and horizontal plane movement or gravitational equilibrium

• two membranous sacs located in the inner ear
• near the semicircular canals.
• Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b).
• Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane.
• The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the
• saccule responds best to vertical (up-and-down) movements.

Question 182

1. When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells.
2. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced.
   
   • The otolithic membrane sags, bending the stereocilia of the hair cells beneath.
3. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve.
4. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve.
5. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

Answer 182

How do the utricles and saccules detect rotational vertical/horizontal plane movement?

Question 183

CHECK YOUR PROGRESS 15.3

Identify the structures of the tongue and nose involved in the senses of taste and smell.

Answer

Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.

Answer

They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.

Answer

Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS

For more information on chemoreceptors, refer to the following discussions:

Section 10.5 describes the function of the respiratory center in the medulla oblongata.

Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 184

Blood pressure, homeostasis, negative feedback loop

Answer 184

interreceptors

Question 185

• In addition, the motor cortex in the frontal lobe of the brain signals
  
  • where the limbs should be located at any particular moment

After integrating all these nerve inputs, the cerebellum coordinates skeletal

1. muscle contraction to correct our position in space if necessary.

*

Answer 185

Nervous system and movement

In addition to the cerebellum, in movement:

Question 186

reversed prompt

Answer 186

Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

Question 187

Types of Sensory Receptors

Answer 187

1. chemoreceptors,
2. photoreceptors,
3. mechanoreceptors, and
4. thermoreceptors.

Question 188

reversed prompt

Answer 188

15.6 Sense of EquilibriumLEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain how mechanoreceptors are involved in the sense of equilibrium.
Identify the structures of the ear involved in the sense of equilibrium.
Distinguish between rotational and gravitational equilibrium.
The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Rotational Equilibrium Pathway
Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.
Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323
Gravitational Equilibrium Pathway
The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.
Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.
These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.
Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.
CHECK YOUR PROGRESS 15.6
State the location and function of the structures involved in maintaining balance.
Answer
All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.
Answer
Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.
Answer
Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS
For more information on the sense of equilibrium, refer to the following discussions:
Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.
Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.
CONCLUSION
Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.
John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.
His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

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Question 189

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMESUpon completion of this section, you should be able toList the four categories of sensory receptors and describe what stimulus each responds to.Distinguish between perception and sensation.Explain the purpose of integration and sensory adaptation.

Question 190

15.4 Sense of VisionLEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera,and the transparentcornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid** is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped **iris.The iris regulates the size of thepupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFEWhat is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens** is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the **aqueous humor.A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person hasglaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina,is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called thevitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis** where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the **optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

StructureFunction

Sclera

Protects and supports the eye CorneaRefracts light rays PupilAdmits light

Choroid

Absorbs stray light Ciliary bodyHolds lens in place, accommodation IrisRegulates light entrance

Retina

Contains photoreceptors for sight Rod cellsMake black-and-white vision possible Cone cellsMake color and acute vision possible Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays HumorsTransmit light rays and support the eye Optic nerveTransmits impulses to the visual cortexTable 15.2Structures of the EyeTable Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8aillustrates the structure of the photoreceptors calledrod cells**and**cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b).Rhodopsin**is a complex molecule made up of the protein opsin and a light-absorbing molecule called**retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFEWhy does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma** has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as **optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted.These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism,can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY HealthCorrecting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 191

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 192

Reverse.Prompt

Answer 193

What is the anatomy of the eye? •

Answer 194

Pain Receptors
Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.
Page 310Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

Answer 195

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Answer 197

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

Question 198

Reverse.Prompt

* Baroreceptors:

* pressure receptors

* strong or slight pressure

2. Proprietoreceptors

Answer 198

We are a type of mechano receptors and your sense of touch depends on ______________________ , sensitive to either ____________________.

* We are a specific type of receptor located in certain arteries detecting changes in blood pressure

* stretch receptors in the lungs detect the degree of lung inflation.

* We another kind and respond to the stretching of muscle fibers, tendons, joints, and ligaments; Our signals make you aware of the position of your limbs.

Question 199

The inner ear: Semicircular canals and vestibule • Detects angular movement (rotational equilibrium) – Depends on hair cells at the base of each semicircular canal (ampulla) • Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) – Depends on hair cells in the utricle and saccule • Signals sent to cerebellum

Question 200

reversed prompt

Answer 200

mechanoreceptors- motion detection, rotational equilibrium, vestibule

Question 201

Reverse.Prompt

Anatomy of the ear

Answer 201

The ear functions in hearing and balance.

* 3 divisions

* Outer ear: functions in hearing; filled with air

* Middle ear: functions in hearing; filled with air

* Inner ear: functions in hearing and balance; filled with fluid

Question 202

Sense of Smell

1. Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell.
   
   • This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose.
     
     • Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity
       
       1. Olfactory cells are
          
          • modified neurons.
          • Each cell ends in a tuft of about five olfactory cilia,
            
            • which bear receptor proteins for odor molecules.

Answer 202

1. Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.

Question 203

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 205

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Question 206

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMES
Upon completion of this section, you should be able to
List the four categories of sensory receptors and describe what stimulus each responds to.
Distinguish between perception and sensation.
Explain the purpose of integration and sensory adaptation.
A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse. This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons. Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.
Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.
Tutorial: Negative Feedback

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.
Types of Sensory Receptors
Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.
Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.
Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.
The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.
Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).
How Sensation Occurs
Sensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.
As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.
Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.
Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.
All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!
Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.
CHECK YOUR PROGRESS 15.1
Describe the functions of the four types of sensory receptors.
Answer
Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Distinguish between sensation and integration.
Answer
Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.
Answer
Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

CONNECTING THE CONCEPTS
For more information on the regions of the brain associated with sensation, refer to the following discussions:
Section 14.2 describes the location and function of the reticular activating system (RAS).
Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.
Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Answer 206

somatic senses.

Question 207

• Process/Steps

1. To reach the _________, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma .
   
   • The optic chiasma
     
     • has an X shape,
     • formed by a crossing-over of optic nerve fibers.
2. After exiting the optic chiasma, the optic nerves continue as optic tracts.
3. Fibers from the right half of each retina converge and continue on together in the right optic tract.
4. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.
5. The optic tracts sweep around the hypothalamus, and
6. most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus.
7. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe.
8. The image is split in the visual cortex.
   
   • This division of incoming information happens because the right visual cortex receives information from the right optic tract,
   • and the left visual cortex receives information from the left optic tract.
9. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Answer 207

From the Retina to the Visual Cortex

(Fig. 15.10)

317

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

Question 208

Reverse.Prompt

Integration: the summing up of signals.

sensory adaptation, which is a decrease in response to a stimulus over time.

* We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it.

* When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased.

Answer 208

This is the step before sensory receptors initiate nerve signals.

Question 209

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Question 210

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Question 211

How does sensation occur? • Sensory receptors respond to environmental stimuli. • Nerve impulses travel to the cerebral cortex and sensation (conscious perception of stimuli) occurs. • Integration, the summing of signals occurs, and nerve signals can be initiated • Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound).

Question 212

1. Layers of the eye: Sclera • Sclera – the white of the eye that maintains eye shape – Cornea: transparent portion of the sclera that is important in refracting light – Pupil: a hole that allows light into the eyeball

Question 213

3. Layers of the eye: Retina • The retina contains photoreceptors called rods and cones. • Rods are sensitive to light. • Cones require bright light and respond to wavelengths of light (color). • The macula lutea is an area of the retina densely packed with cones where images are focused. – The fovea centralis is the center of this area, and is the area of highest visual acuity (sharpness of vision)

Question 214

Reverse.Prompt

Motion sicknes

Answer 214

• Continuous stimulation of the stereocilia can contribute to motion sickness,
  
  • especially when messages reaching the brain conflict with visual information from the eyes.
  • Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side.
    
    • If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving.
    • Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus
      
      1. reducing the impulses received by the cerebellum, and alleviating motion sickness.

Question 215

Reverse.Prompt

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

Answer 216

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?

Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.

Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.

CHECK YOUR PROGRESS 15.2

Describe how the body uses proprioceptors to indicate the position of the arms and legs.

Answer

By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.

Answer

Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.

Answer

Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Figure 4.9 provides a more detailed look at the structure of human skin.

Section 13.2 provides an overview of muscle fiber contraction.

Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Answer 217

Abnormalities of the eye • Glaucoma – fluid pressure builds up in the eye • Pinkeye (conjunctivitis)- inflammation of conjunctiva, mucous membrane that covers eyeball and inner part of eyelid

Question 218

spiral organ

• differentiated part: pitch; is sensitive to different wave frequencies, or pitch.
  
  1. Near the tip, the spiral organ responds to low pitches, such as those of a tuba.
  2. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle.
• The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex.
• The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.
• BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Answer 218

Effect of Sound Waves

Pitch

Question 219

Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.

Answer 220

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Question 221

reversed prompt

Answer 221

Sensory ReceptorStimulusCategorySenseSensory OrganTaste cellsChemicalsChemoreceptorTasteTaste budOlfactory cellsChemicalsChemoreceptorSmellOlfactory epitheliumRod cells and cone cells in retinaLight raysPhotoreceptorVisionEyeHair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEarHair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEarHair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEarTable 15.1ExteroceptorsTable Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Question 222

Reverse.Prompt

• no rods and cones
• located where optic nerve exits retina
• vision impossible in this area
• The two eyes together provide complete vision because the right eye is right of center, and the for the left eye is left of center.

Answer 222

Blind Spot

Figure 15.9

Question 223

There are 4 categories of us:

They are:

We respond to chemical substances in the immediate vicinity, just check out Table 15.1. Taste and Smell, which detect external stimuli

Other organs sensitive to internal stimuli use us: For example, we help monitor blood pH and are located in the carotid arteries and aorta— this is how we work:
Nociceptors (aka ______________________) are also a type of us.
are a type of chemoreceptor.
They are naked dendrites that respond to chemicals released by damaged tissues.
Nociceptors are protective, because they alert us to possible danger.

For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Answer 223

* Sensory Receptors

* chemoreceptors,

* photoreceptors,

* mechanoreceptors, and

* thermoreceptors.

Chemoreceptors

* If the pH lowers, the breathing rate increases.

* As more carbon dioxide is exhaled, the blood pH rises.

* pain receptors

Question 224

reversed prompt

Answer 224

Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.

Question 225

• cochlea
  
  • three canals.
    
    1. The sensory organ for hearing, called the spiral organ (or the organ of Corti),
       
       • cochlear canal. The spiral organ
       • little hair cells and a gelatinous material called the tectorial membrane.
         
         • _​_The hair cells sit on the basilar membrane,
           
           • and their stereocilia are embedded in the tectorial membrane.
  • Steps/Process/Pathway
    
    1. When the stapes strikes the membrane of the oval window,
    2. pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane.
    3. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend.
    4. Then, nerve signals begin in the cochlear nerve
    5. and travel to the brain.
    6. When they reach the auditory cortex in the temporal lobe,
    7. they are interpreted as a sound.

Answer 225

Hearing Path

From the Cochlea to the Auditory Cortex

Page 322

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

Question 226

15.4 Sense of Vision

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera, and the transparent cornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped iris. The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Answer 227

• 2. Layers of the eye: Choroid •
• Choroid – middle layer that absorbs light rays not absorbed by the retina –
• Iris: donut-shaped, colored structure that regulates the size of the pupil –
• Ciliary body: structure behind the iris that contains a muscle that controls the shape of the lens
• • Lens – attached to the ciliary body; refracts and focuses light rays

Question 228

• Continuous stimulation of the stereocilia can contribute to motion sickness,
  
  • especially when messages reaching the brain conflict with visual information from the eyes.
  • Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side.
    
    • If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving.
    • Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus
      
      1. reducing the impulses received by the cerebellum, and alleviating motion sickness.

Answer 228

Motion sicknes

Question 229

Reverse.Prompt

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

Question 230

Reverse.Prompt

Page 312

Answer 230

• Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place?
• For example, a smell of a certain food may remind you of a favorite vacation.
• This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3).
  
  • One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.
• The number of olfactory cells declines with age.
• This can be dangerous if an older person can’t smell smoke or a gas leak.
• Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.

Answer 231

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

Question 232

• Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place?
• For example, a smell of a certain food may remind you of a favorite vacation.
• This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3).
  
  • One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.
• The number of olfactory cells declines with age.
• This can be dangerous if an older person can’t smell smoke or a gas leak.
• Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.

Answer 232

Page 312

Question 233

How does sensation occur?

What is sensation and where does it first occur?

Answer 233

Sensory receptors respond to environmental stimuli.
Nerve impulses travel to the cerebral cortex and this:
(conscious perception of stimuli) occurs.
Integration, the summing of signals occurs, and nerve signals can be initiated
Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound)

Answer 234

How do we focus images? •

Answer 235

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Question 236

Reverse.Prompt

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Question 237

Function of the Retina

Fig. 15.9)

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

Figure 15.9

Answer 237

• has 3 layers of neurons
  
  1. The layer closest to the choroid contains the rod cells and cone cells.
  2. A layer of bipolar cells covers the rods and cones.
  3. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve.
• Process: Steps
  
  1. Because ______________-Before rod and cone cells are stimulated, light must penetrate to the back
  2. The rod cells and cone cells synapse with the bipolar cells.
  3. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve.
     
     1. Many more rod cells and cone cells than ganglion cells.
        
        • About 150 million rod cells, 6.5 million cone cells, but only, 1million ganglion cells.
     2. The sensitivity of cones versus rods
        
        • mirrored by how directly they connect to ganglion cells.
          
          • 150 rods activate the same ganglion cell.
            
            1. stimulation of rods results in vision that is blurred and indistinct.
          • some cone cells in the fovea centralis activate only one ganglion cell.
            
            1. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.
     3. As signals pass to bipolar cells and ganglion cells, integration occurs.
        
        • Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals.
     4. Ganglion cells converge to form the optic nerve,
        
        • which transmits information to the visual cortex.
     5. Additional integration occurs in the visual cortex.

Question 238

These mechanical receptors are involved in blood pressure negative feedback homeostasis

** process question** short answer

Answer 238

Interoceptors are directly involved in homeostasis
and are regulated by a negative feedback mechanism (see Fig. 4.16).

* For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain.

* The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax.

* The blood pressure then falls.

* Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

Question 239

Odor

Answer 239

Communication - from peripheral nervous system deteching stimuli

Answer 240

The inner ear: Semicircular canals and vestibule • Detects angular movement (rotational equilibrium) – Depends on hair cells at the base of each semicircular canal (ampulla) • Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) – Depends on hair cells in the utricle and saccule • Signals sent to cerebellum

Question 241

reversed prompt

decrease in stimulus response

Answer 241

Adaptation

Question 242

Reverse.Prompt

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

Answer 243

Important for both hearing and balance • 3 areas: cochlea, semicircular canals, vestibule • Stapes (middle ear bone) – vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea • Vestibule – functions in gravitational equilibrium • Semicircular canals – functions in rotational equilibrium

Question 244

reversed prompt

Mechanoreceptors that are involved in reflex actions that maintain muscle tone, and consequently equilibrium and posture

Answer 244

Proprioceptors •

Question 245

How does sensation occur? Page 308

PNS to the CNS (Fig. 15.1).

Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.

Answer 245

* Sensory receptors respond to environmental stimuli by generating nerve signals.
nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.

As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain.
At that time, the brain integrates this information with other information received from other sensory receptors.

Consider what happens if you burn yourself and quickly remove your hand from a hot stove.

The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.

Some sensory receptors are free nerve endings or encapsulated nerve endings,
and others are specialized cells closely associated with neurons.
Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus.

chemoreceptors have receptor proteins that bind them to certain chemicals.

* ion channels open, and ions flow across the plasma membrane.
stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber

* The stronger the stimulus, the greater the frequency of nerve signals.
Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts.
If nerve signals finally reach the cerebral cortex, sensation occurs.

Question 246

* mechanoreceptors
reflex actions that maintain muscle tone, and thereby the body’s equilibrium and posture.

For example,_______________ called muscle spindles are embedded in muscle fibers

* relaxes too much, the muscle spindle stretches, generating nerve impulses that cause the muscle to contract slightly.

* The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action.
The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture. Proper balance and body position are maintained, despite the force of gravity always acting on the skeleton and muscles.

Conversely, stretched too much,_____________ called Golgi tendon organs, buried in the tendons that attach muscles to bones: generate nerve impulses that cause the muscles to relax.
Both types of receptors act together to maintain a functional degree of muscle tone.

Answer 246

Proprioceptors

See Figure 15.2

Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.

Question 247

When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

Question 248

chemical senses

Answer 248

Taste and smell

Question 249

Reverse.Prompt

Question 250

Anatomy of the eye • Made of 3 layers/coats 1. Sclera: mostly white and fibrous except the cornea 2. Choroid: darkly-pigmented vascular layer 3. Retina: inner layer containing photoreceptors

Question 251

15.1 Overview of Sensory Receptors and Sensations

LEARNING OUTCOMES

Upon completion of this section, you should be able to

List the four categories of sensory receptors and describe what stimulus each responds to.

Distinguish between perception and sensation.

Explain the purpose of integration and sensory adaptation.

Question 252

• located primarily in the fovea
• activated by bright light. They allow us to
• detect the
  
  1. fine detail and the
  2. color of an object.
     
     1. Color vision depends on three types of cones, which contain pigments
        
        • called the B (blue),
        • G (green), and
        • R (red) pigments.
     2. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each.
        
        • This accounts for their individual absorption patterns.
        • Various combinations of cones are believed to be stimulated by in-between shades of color.

Answer 252

Functions of Photorecepts

Cones

Question 253

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

Answer 254

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Question 255

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Question 256

Reverse.Prompt

visual accommodation

close vision.

See Figure (Fig. 15.7a).

(Fig. 15.7b).

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Answer 256

During this process:

1. the lens changes its shape to bring the image into focus on the retina.
   
   • The shape of the lens is controlled by the ciliary muscle, within the ciliary body.
     
     • ​When we view a distant object, the ciliary muscle is relaxed
     • causing the suspensory ligaments attached to the ciliary body to be taut.
       
       • The ligaments put tension on the lens and cause it to remain relatively flat
       • When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments.
         
         • The lens becomes round and thick due to its natural elasticity ​​​
         • Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain.
           
           1. ​Eyestrain more common after 40 and for those who work with computers​
     • Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina.

Question 257

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 258

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?
Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.
Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.
CHECK YOUR PROGRESS 15.2
Describe how the body uses proprioceptors to indicate the position of the arms and legs.
Answer
By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.
Answer
Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.
Answer
Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS
For more information on the material in this section, refer to the following discussions:
Figure 4.9 provides a more detailed look at the structure of human skin.
Section 13.2 provides an overview of muscle fiber contraction.
Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 259

reversed prompt

* sensory receptor

* stimulus

* sensory transduction.

Answer 259

We are able to convert a signal from the environment, called a______________, into a nerve impulse.

This conversion process is called _____________________.

Question 261

reversed prompt

Answer 261

(Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.
*

Question 262

Reverse.Prompt

The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.

Answer 262

How the Brain Receives Odor Information

Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.

Question 263

reversed prompt

Receptor cells

Mechanoreceptors Types

Proprioceptors

(Fig. 15.2).

* Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.

Answer 263

mechanoreceptors
involved in reflex actions
that maintain muscle tone,
and thereby the body’s equilibrium and posture.
Specific types

muscle spindles are embedded in muscle fibers
Triggered Responses/Homeostasis:

If a muscle relaxes too much, the

* muscle spindle stretches,

* generating nerve impulses that cause

* the muscle to contract slightly.

Golgi tendon organs: buried in the tendons that attach muscles to bones

​​Conversely, when muscles are stretched too much, , ,

* generate nerve impulses that cause

* the muscles to relax.

Homeostasis: both muscle spindles and Golgi tendon organs act together
Goal: to maintain a functional degree of muscle tone.

The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action.

The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture.

* despite the force of gravity always acting on the skeleton and muscles.

Question 264

Companies, Inc. Permission required for reproduction or display. odor molecules nasal cavity frontal lobe of cerebral hemisphere olfactory bulb neuron olfactory tract bone of skull sensory nerve fibers olfactory epithelium olfactory cell odor molecules olfactory cilia of b. olfactory cell a. supporting cell olfactory epithelium olfactory bulb Figure 15.5 The sense of smell. S

Answer 264

Pain receptors • Sensitive to chemicals released by damaged tissues • In inflammation, cells release chemicals to stimulate pain receptors • Referred pain- stimulation of pain receptors is felt as pain from the skin – Thought to be due to nerve impulses from pain receptors of internal organs travel to spinal cord and synapse with neurons also receiving impulses from skin

Question 265

• The third layer of the eye, the retina, is located in the
• posterior compartment. This compartment is filled with
• a clear, gelatinous material called the vitreous humor.
  
  • The vitreous humor holds the retina in place and supports the lens.** The retina contains photoreceptors called **rod cells and cone cells.
  • The retina has a very special region called the fovea centralis where
    
    • cone cells are densely packed.
    • Light is normally focused on the fovea when we look directly at an object.
    • This is helpful because the sharpest images are produced by the fovea centralis.
• Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.
  *

Answer 265

Table 15.2 summarizes the major structures of the eye and their functions.

Question 266

reversed prompt

Answer 266

Chapter 15 Senses 2 Key Concepts to Focus On

* What are sensory receptors?

* How do we detect the sense of taste and smell?

* What is the anatomy of the eye?

* How do we focus images?

* What are some eye abnormalities?

* What is the anatomy of the ear?

* Which parts function in balance and which parts function in hearing?

Question 267

reversed prompt

Answer 267

15.2 Somatic Senses

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Distinguish between proprioceptors and cutaneous receptors with regard to function.

State the location and general function of each type of cutaneous receptor.

Explain the role of nociceptors and summarize the type of sensory input they detect.

Senses whose receptors are associated with the skin, muscles, joints, and viscera are termed the somatic senses. These receptors can be categorized into three types: proprioceptors, cutaneous Page 309receptors, and pain receptors. All of these send nerve impulses via the spinal cord to the primary somatosensory areas of the cerebral cortex (see Fig. 14.11).

Proprioceptors

Proprioceptors are mechanoreceptors involved in reflex actions that maintain muscle tone, and thereby the body’s equilibrium and posture. For example, proprioceptors called muscle spindles are embedded in muscle fibers (Fig. 15.2). If a muscle relaxes too much, the muscle spindle stretches, generating nerve impulses that cause the muscle to contract slightly. Conversely, when muscles are stretched too much, proprioceptors called Golgi tendon organs, buried in the tendons that attach muscles to bones, generate nerve impulses that cause the muscles to relax. Both types of receptors act together to maintain a functional degree of muscle tone. The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action. The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture. Proper balance and body position are maintained, despite the force of gravity always acting on the skeleton and muscles.

Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.

Cutaneous Receptors

The skin is composed of two layers: the epidermis and the dermis (see Section 4.6). The dermis contains cutaneous receptors (Fig. 15.3), which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a mosaic of these tiny receptors, as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure; at others, you will feel heat or cold (depending on the probe’s temperature).

Figure 15.3 Sensory receptors of the skin. The general function of each sensory receptor is shown here. However, receptors are not always this specialized. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations.

Several types of cutaneous receptors are sensitive to fine touch. These receptors give a person specific information, such as the location of the touch, as well as its shape, size, and texture. Meissner corpuscles and Krause end bulbs are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. Merkel discs are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.

Two types of cutaneous receptors sensitive to pressure are Pacinian corpuscles and Ruffini endings. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.

Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others respond to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.

Pain Receptors

Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.

Page 310Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

SCIENCE IN YOUR LIFE

What are phantom sensation and phantom pain?

Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.

Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.

CHECK YOUR PROGRESS 15.2

Describe how the body uses proprioceptors to indicate the position of the arms and legs.

Answer

By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.

Answer

Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.

Answer

Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Figure 4.9 provides a more detailed look at the structure of human skin.

Section 13.2 provides an overview of muscle fiber contraction.

Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Answer 268

The lens is a flexible, transparent, and concave structure. • Visual accommodation occurs when the lens changes shape to focus light on the retina and form an image. • As we age, the lens loses elasticity, and we use glasses to correct for this. 15.4 Sense of Vision T

Question 269

reversed prompt

What is a sensory receptor?

Answer 269

convert stimulus (both external and internal environments) ————————-> nerve impulse: sensory transduction

* Types:

* modified neurons, and

* others: specialized cells closely associated with neurons.

Question 270

Reverse.Prompt

What are photoreceptors?

Answer 270

We respond to light energy!

our eyes contain____________ sensitive to light rays, thereby provide us with a sense of vision.
Two types of us and when stimulated :

rod cells : black-and-white vision.
cone cells: results in color vision.

Question 271

Sensory receptors

Answer 271

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 272

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Question 273

See download and understand (Table 15.1)

1) Receptors and what do they do?

Answer 273

* Some sensory receptors are

* modified neurons, and

* others are specialized cells closely associated with neurons.

* Sensory receptors may detect stimuli originating from both the internal and external environments.

2. Exteroceptors are

sensory receptors that
detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium

3. Interoceptors

receive stimuli from inside the body.
Examples of interoceptors are the

baroreceptors (also called pressoreceptors) that respond to changes in blood pressure
osmoreceptors that monitor the body’s water-salt balance, and
chemoreceptors that monitor the pH of the blood.

Question 274

reversed prompt

convert a signal from the environment, called a stimulus, into a nerve impulse.
This conversion is commonly referred to as sensory transduction.
modified neurons, and others are specialized cells closely associated with neurons.
Sensory receptors may detect stimuli originating from both the internal and external environments.
Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1).
Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Answer 274

sensory receptor

Question 275

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Question 276

reversed prompt

Book

We are the proprioceptors, cutaneous receptors, and pain receptors and we send nerve impulses via the _______________________ to the _________________________ or the ____________________.

Page 309

Answer 276

skin, muscles, joints, and viscera are termed the

three types of _____________________________________

1)
2)
3)

via the spinal cord to the primary somatosensory areas of the cerebral cortex

Check out Fig. 14.11

Question 277

reversed prompt

Answer 277

Divisions of the ear: Middle ear • Includes – Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones – Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves – Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst • This tube has a 45 degree tilt in adults, but only 10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

Question 278

Proprioceptors

Answer 278

Mechanoreceptors that are involved in
reflex actions that maintain muscle tone, and

consequently equilibrium and posture

Question 279

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

Question 280

This is the step before sensory receptors initiate nerve signals.

Answer 280

Integration: the summing up of signals.

sensory adaptation, which is a decrease in response to a stimulus over time.

* We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it.

* When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased.

Question 281

Cutaneous receptors • Receptors in the dermis that make the skin sensitive to touch, pressure, pain, and temperature. Figure 15.3 Sensory receptors of the skin. 15.2 Somatic Senses Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Question 282

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

Question 283

reversed prompt

Proprioreceptors

Answer 283

Mechanoreceptors that are involved in reflex actions that maintain muscle tone, and consequently equilibrium and posture

Answer 284

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?
Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.
Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.
CHECK YOUR PROGRESS 15.2
Describe how the body uses proprioceptors to indicate the position of the arms and legs.
Answer
By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.
Answer
Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.
Answer
Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS
For more information on the material in this section, refer to the following discussions:
Figure 4.9 provides a more detailed look at the structure of human skin.
Section 13.2 provides an overview of muscle fiber contraction.
Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 285

Reverse.Prompt

Answer 285

15.4 Sense of VisionLEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Question 286

Reverse.Prompt

How the Brain Receives Taste Information

Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. When molecules bind to receptor proteins of the microvilli, nerve signals are generated in sensory nerve fibers that go to the brain. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.

Question 287

1. auditory canal.
2. Thereafter, hearing requires the other parts of the ear,
3. the cochlear nerve, and the
4. brain.

Answer 287

Auditory Pathway to the Brain

Question 288

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMES

Upon completion of this section, you should be able to

List the four categories of sensory receptors and describe what stimulus each responds to.

Distinguish between perception and sensation.

Explain the purpose of integration and sensory adaptation.

Question 289

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

Anatomy and Physiology of the Ear

The Middle Ear Structures

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Answer 289

• middle ear structure

1. begins at the tympanic membrane (eardrum) and
2. ends at the oval window and the round window
   
   • a bony wall containing two small openings covered by membranes.
3. ossicles
   
   • Three small bones are found between the tympanic membrane and the oval window.
   • malleus (hammer), the incus (anvil), and the stapes (stirrup)
     
     • because their shapes resemble these objects.
     • The malleus adheres to the tympanic membrane, and the
     • stapes touches the oval window.
4. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx.
   
   • Its purpose is to equalize air pressure across the tympanic membrane.
     
     • When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider.
       
       • As this occurs, we often feel the ears “pop.”
         *

Question 290

Reverse.Prompt

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Answer 290

From the Retina to the Visual Cortex

Answer 291

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Question 292

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes 15.1 Overview of Sensory Receptors and Sensations 5 Senses and the receptors involved 15.1 Overview of Sensory Receptor

Answer 293

3. Layers of the eye: Retina • Sensory receptors from the retina form the optic nerve that takes impulses to the brain. • The blind spot is the optic disc, and is where the optic nerve attaches; it lacks photoreceptors, therefore consequently nothing can be visually detected at this location.

Question 294

Reverse.Prompt

Distance Vision

(Fig. 15.11a)

(Fig. 15.11b)

(Fig. 15.11c)

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Answer 294

1. If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision.
2. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted.
   
   • Nearsighted people can see close objects better than they can see objects at a distance.
   • The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object,
   • the image is brought to focus in front of the retina.
   • They can see close objects because their lens can compensate for the elongated shape of the eye.
   • To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.
3. Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted.
   
   1. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina. When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.
4. When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea.

Question 295

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Answer 296

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Answer 297

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

Question 298

Anatomy of the eye • Made of 3 layers/coats 1. Sclera: mostly white and fibrous except the cornea 2. Choroid: darkly-pigmented vascular layer 3. Retina: inner layer containing photoreceptors

Question 299

Reverse.Prompt

Rotational Equilibrium Pathway

(Fig. 15.14a)

Answer 299

• Mechanoreceptors in the
  
  • semicircular canals
  • detect rotational and/or angular movement of the head—rotational equilibrium .
• The three semicircular canals are arranged so there is one in each dimension of space.

1. The base, or ampulla, of each of the three canals is slightly enlarged.
   
   • Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae.
   • Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged.

• As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend.
• This causes a change in the pattern of signals carried by the vestibular nerve to the brain.
• The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium.
• Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Question 300

Reverse.Prompt

How do the utricles and saccules detect rotational vertical/horizontal plane movement?

Answer 300

1. When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells.
2. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced.
   
   • The otolithic membrane sags, bending the stereocilia of the hair cells beneath.
3. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve.
4. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve.
5. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

Answer 301

• How do we detect the sense of taste and smell? •

Question 302

reversed prompt

Answer 302

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes

Answer 303

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

Question 304

reversed prompt

Answer 304

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes 15.1 Overview of Sensory Receptors and Sensations 5 Senses and the receptors involved 15.1 Overview of Sensory Receptor

Question 305

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

Question 306

reversed prompt

Conscious perception of stimuli

Answer 306

Sensation

Question 307

Nociceptors • Pain receptors • Sensitive to chemicals released by damaged tissues • In inflammation, cells release chemicals to stimulate pain receptors • Referred pain- stimulation of pain receptors is felt as pain from the skin – Thought to be due to nerve impulses from pain

Question 308

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

Question 309

reversed prompt

sensory receptor

Answer 309

convert a signal from the environment, called a stimulus, into a nerve impulse.
This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons.
Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Answer 310

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Question 311

Reverse.Prompt

• concerned with equilibrium;

1. The semicircular canals and the
2. vestibule are the

• concerned with hearing: The cochlea
  
  • resembles the shell of a snail because it spirals.

Answer 311

• Whereas the outer ear and the middle ear contain air,
• the inner ear is filled with fluid.
• The inner ear has three areas:

Question 312

reversed prompt

Answer 312

15.2 Somatic SensesLEARNING OUTCOMES
Upon completion of this section, you should be able to
Distinguish between proprioceptors and cutaneous receptors with regard to function.
State the location and general function of each type of cutaneous receptor.
Explain the role of nociceptors and summarize the type of sensory input they detect.
Senses whose receptors are associated with the skin, muscles, joints, and viscera are termed the somatic senses. These receptors can be categorized into three types: proprioceptors, cutaneous Page 309receptors, and pain receptors. All of these send nerve impulses via the spinal cord to the primary somatosensory areas of the cerebral cortex (see Fig. 14.11).
Proprioceptors
Proprioceptors are mechanoreceptors involved in reflex actions that maintain muscle tone, and thereby the body’s equilibrium and posture. For example, proprioceptors called muscle spindles are embedded in muscle fibers (Fig. 15.2). If a muscle relaxes too much, the muscle spindle stretches, generating nerve impulses that cause the muscle to contract slightly. Conversely, when muscles are stretched too much, proprioceptors called Golgi tendon organs, buried in the tendons that attach muscles to bones, generate nerve impulses that cause the muscles to relax. Both types of receptors act together to maintain a functional degree of muscle tone. The knee-jerk reflex, which involves muscle spindles, offers an opportunity for physicians to test a reflex action. The information sent by muscle spindles to the CNS is used to maintain the body’s equilibrium and posture. Proper balance and body position are maintained, despite the force of gravity always acting on the skeleton and muscles.

Figure 15.2 The action of proprioceptors. 1. When a muscle is stretched, muscle spindles send sensory nerve impulses to the spinal cord. 2. Motor nerve impulses from the spinal cord cause slight muscle contraction. 3. When tendons are stretched excessively, Golgi tendon organs cause muscle relaxation.
Cutaneous Receptors
The skin is composed of two layers: the epidermis and the dermis (see Section 4.6). The dermis contains cutaneous receptors (Fig. 15.3), which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a mosaic of these tiny receptors, as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure; at others, you will feel heat or cold (depending on the probe’s temperature).

Figure 15.3 Sensory receptors of the skin. The general function of each sensory receptor is shown here. However, receptors are not always this specialized. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations.

Several types of cutaneous receptors are sensitive to fine touch. These receptors give a person specific information, such as the location of the touch, as well as its shape, size, and texture. Meissner corpuscles and Krause end bulbs are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. Merkel discs are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.
Two types of cutaneous receptors sensitive to pressure are Pacinian corpuscles and Ruffini endings. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.
Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others respond to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.
Pain Receptors
Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.
Page 310Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.
SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?
Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.
Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.
CHECK YOUR PROGRESS 15.2
Describe how the body uses proprioceptors to indicate the position of the arms and legs.
Answer
By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.
Answer
Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.
Answer
Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS
For more information on the material in this section, refer to the following discussions:
Figure 4.9 provides a more detailed look at the structure of human skin.
Section 13.2 provides an overview of muscle fiber contraction.
Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

15.3 Senses of Taste and SmellLEARNING OUTCOMES
Upon completion of this section, you should be able to
Compare and contrast the senses of taste and smell.
Identify the structures of the tongue and the olfactory areas of the nose.
Summarize how the brain receives taste and odor information.
Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.
Sense of Taste
In adult humans, approximately 4,000 taste buds are located primarily on the tongue (Fig. 15.4). Many taste buds lie along the walls of the papillae. These small elevations on the tongue are visible to the naked eye. Isolated taste buds are also present on the hard palate, the pharynx, and the epiglottis. Researchers have identified chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter. These receptors are not clustered in buds, and they do not send taste signals to the brain. Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.
Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.
(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images
Humans have five main types of taste receptors: sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes. A food can stimulate more than one of these types of taste buds. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.
How the Brain Receives Taste Information
Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. When molecules bind to receptor proteins of the microvilli, nerve signals are generated in sensory nerve fibers that go to the brain. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.
Sense of Smell
Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell. This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose. Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity (Fig. 15.5). Olfactory cells are modified neurons. Each cell ends in a tuft of about five olfactory cilia, which bear receptor proteins for odor molecules.
Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.
How the Brain Receives Odor Information
Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.
The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.
Page 312Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place? For example, a smell of a certain food may remind you of a favorite vacation. This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3). One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.
The number of olfactory cells declines with age. This can be dangerous if an older person can’t smell smoke or a gas leak. Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.
CHECK YOUR PROGRESS 15.3
Identify the structures of the tongue and nose involved in the senses of taste and smell.
Answer
Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.
Answer
They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.
Answer
Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS
For more information on chemoreceptors, refer to the following discussions:
Section 10.5 describes the function of the respiratory center in the medulla oblongata.
Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 313

Reverse.Prompt

homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16).

Answer 313

Interoceptors are directly involved in this process whose steps are listed below:

blood pressure rises, baroreceptors signal a regulatory center in the brain.
The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax.
The blood pressure then falls.

* Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

* Tutorial: Negative Feedback

Question 314

reversed prompt

Answer 315

Photoreceptors of the retina • Rods – They contain a visual pigment called rhodopsin. – Rods are important for peripheral and night vision (black and white vision). – Vitamin A is important for proper functioning of the rods. • Cones – They are located mostly in the fovea centralis. – Cones allow us to detect fine detail and color. – 3 different kinds of cones containing red, green, and blue pigments.

Question 316

Chemoreceptors

Answer 316

• 1. respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Question 317

There are three of us:

dendrites specialized to detect certain types of stimuli –
We detect stimuli from outside the body

* (e.g., taste, hearing, vision)

We receive stimuli from inside the body

* (e.g., change in blood pressure)

* Directly involved in homeostasis and a part of a negative feedback loop

Answer 317

What are 3 types of receptors:

* Sensory receptors

* Exteroreceptors

* Interoreceptors

Chapter 15.1 slide 1.2

Question 318

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 319

These are the:

 Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones – 
 Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves – 
 Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst 

This tube has a 45 degree tilt in adults, but only 10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

Answer 319

What are the divisions of the Middle Ear, in order, and their functions?

Divisions of the ear

Chapter 15 slides: Ear

Question 320

We are a type of mechano receptors and your sense of touch depends on ______________________ , sensitive to either ____________________.

* We are a specific type of receptor located in certain arteries detecting changes in blood pressure

* stretch receptors in the lungs detect the degree of lung inflation.

* We another kind and respond to the stretching of muscle fibers, tendons, joints, and ligaments; Our signals make you aware of the position of your limbs.

Answer 320

* Baroreceptors:

* pressure receptors

* strong or slight pressure

2. Proprietoreceptors

Question 321

reversed prompt

Answer 321

Types of Sensory Receptors

Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.

Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Question 322

reversed prompt

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes 15.1 Overview of Sensory Receptors and Sensations 5 Senses and the receptors involved 15.1 Overview of Sensory R

Answer 323

15.3 Senses of Taste and Smell

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Compare and contrast the senses of taste and smell.

Identify the structures of the tongue and the olfactory areas of the nose.

Summarize how the brain receives taste and odor information.

Question 324

1. originates in the semicircular canals, saccule, and utricle.
2. It takes nerve signals to the brain stem and cerebellum
3. Through its communication with the brain, helps us achieve equilibrium, but other structures in the body are also involved.
   
   1. proprioceptors
   2. Vision, if available, usually provides extremely helpful input the brain can act upon.
   3. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Answer 324

vestibular nerve

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Question 325

How do we focus images? •

Question 326

Odorants

Answer 326

activate receptors proteins recognizing specific smells, olfactory cortex- temporal lobe

Question 327

What are some eye abnormalities? •

Answer 328

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Question 329

reversed prompt

Answer 329

Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.

Question 330

Table 15.1

Answer 330

rods- black and white vision
cones-

Question 331

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

Answer 332

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 333

StructureFunction

Sclera

Protects and supports the eye Cornea Refracts light rays Pupil Admits light

Choroid

Absorbs stray light Ciliary body Holds lens in place, accommodation Iris Regulates light entrance

Retina

Contains photoreceptors for sight Rod cells Make black-and-white vision possible Cone cells Make color and acute vision possible Fovea centralis Contains mostly cones for acute vision

Other

Lens Refracts and focuses light rays Humors Transmit light rays and support the eye Optic nerve Transmits impulses to the visual cortex Table 15.2Structures of the EyeTable Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Question 334

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Answer 335

Anatomy of the eye • Made of 3 layers/coats 1. Sclera: mostly white and fibrous except the cornea 2. Choroid: darkly-pigmented vascular layer 3. Retina: inner layer containing photoreceptors

Question 336

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

Answer 337

CHECK YOUR PROGRESS 15.3

Identify the structures of the tongue and nose involved in the senses of taste and smell.

Answer

Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.

Answer

They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.

Answer

Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS

For more information on chemoreceptors, refer to the following discussions:

Section 10.5 describes the function of the respiratory center in the medulla oblongata.

Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 338

15.3 Senses of Taste and Smell

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Compare and contrast the senses of taste and smell.

Identify the structures of the tongue and the olfactory areas of the nose.

Summarize how the brain receives taste and odor information.

Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.

Sense of Taste

In adult humans, approximately 4,000 taste buds are located primarily on the tongue (Fig. 15.4). Many taste buds lie along the walls of the papillae. These small elevations on the tongue are visible to the naked eye. Isolated taste buds are also present on the hard palate, the pharynx, and the epiglottis. Researchers have identified chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter. These receptors are not clustered in buds, and they do not send taste signals to the brain. Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.

Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.

(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images

Humans have five main types of taste receptors: sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes. A food can stimulate more than one of these types of taste buds. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.

How the Brain Receives Taste Information

Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. When molecules bind to receptor proteins of the microvilli, nerve signals are generated in sensory nerve fibers that go to the brain. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.

Sense of Smell

Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell. This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose. Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity (Fig. 15.5). Olfactory cells are modified neurons. Each cell ends in a tuft of about five olfactory cilia, which bear receptor proteins for odor molecules.

Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.

How the Brain Receives Odor Information

Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.

The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.

Page 312Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place? For example, a smell of a certain food may remind you of a favorite vacation. This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3). One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.

The number of olfactory cells declines with age. This can be dangerous if an older person can’t smell smoke or a gas leak. Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.

CHECK YOUR PROGRESS 15.3

Identify the structures of the tongue and nose involved in the senses of taste and smell.

Answer

Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.

Answer

They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.

Answer

Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS

For more information on chemoreceptors, refer to the following discussions:

Section 10.5 describes the function of the respiratory center in the medulla oblongata.

Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 340

• The cochlea converts vibrations into nerve impulses. •
  
  • It contains the organ of Corti (spiral organ)
    
    • the sense organ containing hairs for hearing. –
    • Bending of embedded hairs causes vibrations that
    • initiate nerve impulses which travel to the cochlear nerve
    • and then to the brain. –
    • Pitch** is determined by varying wave **frequencies** that are detected by different parts of the organ of **Corti/spiral organ. –
    • Volume is determined by the amplitude of sound waves.

Answer 340

Cochlea —> Cochlear Nerve——> Brain

Question 341

From the Retina to the Visual Cortex

Answer 341

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Question 342

Sensation Visual

Answer 343

Abnormalities of the eye • Color blindness – genetic abnormality in which colors, usually red and green, cannot be distinguished; most common in males (if you are color blind you cannot see the “7” in the picture on the left below) • Cataracts – lens of the eye is cloudy

Question 344

Reverse.Prompt

Functions of Photorecepts

Cones

Answer 344

• located primarily in the fovea
• activated by bright light. They allow us to
• detect the
  
  1. fine detail and the
  2. color of an object.
     
     1. Color vision depends on three types of cones, which contain pigments
        
        • called the B (blue),
        • G (green), and
        • R (red) pigments.
     2. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each.
        
        • This accounts for their individual absorption patterns.
        • Various combinations of cones are believed to be stimulated by in-between shades of color.

Answer 345

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

Question 346

Humans have five main types of taste receptors: sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes. A food can stimulate more than one of these types of taste buds. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.

Question 347

15.6 Sense of EquilibriumLEARNING OUTCOMES
Upon completion of this section, you should be able to
Explain how mechanoreceptors are involved in the sense of equilibrium.
Identify the structures of the ear involved in the sense of equilibrium.
Distinguish between rotational and gravitational equilibrium.
The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Rotational Equilibrium Pathway
Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.
Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323
Gravitational Equilibrium Pathway
The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.
Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.
These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.
Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.
CHECK YOUR PROGRESS 15.6
State the location and function of the structures involved in maintaining balance.
Answer
All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.
Answer
Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.
Answer
Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS
For more information on the sense of equilibrium, refer to the following discussions:
Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.
Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.
CONCLUSION
Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.
John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.
His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

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Question 348

Reverse.Prompt

Eyestrain

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Answer 348

• Close work requires contraction of the ciliary muscle,
• so it often causes muscle fatigue, known as eyestrain.
• Eyestrain is more common after the age of 40,
  
  • because the lens loses some of its elasticity and is unable to accommodate.
• It is also common among those who work with computers,
  
  • because the intense focusing causes the person to blink less, allowing the eyes to dry out.
  • Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Question 349

reversed prompt

Dendrites,

Answer 349

What are sensory receptors?

Question 350

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 351

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

Answer 352

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 353

Reverse.Prompt

Anatomy of the Eye (cont)

Layer 3: Retina

Answer 353

3) Eye Anatomy: the retina,

posterior compartment: filled with a clear, gelatinous material called the vitreous humor.

function:

* holds the retina in place and

* supports the lens.

photoreceptors

rod cells:

* very sensitive to light, but
they do not detect color.

* at night or in a darkened room, we see only shades of gray.

cone cells.

* require bright light,

* are sensitive to different wavelengths of light

* This sensitivity gives us the ability to distinguish colors.

fovea centralis: cone cells are densely packed.

* Light is normally focused on the fovea when we look directly at an object.

* This is helpful because the sharpest images are produced by the fovea centralis.

Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Question 354

reversed prompt

1. Divisions of the ear: Outer ear • Includes – Pinna: the external ear flap that catches sound waves – Auditory canal: directs sound waves to the tympanic membrane • Lined with fine hairs and modified sweat glands that secrete ear wax called ceruminous glands

Answer 354

Outer ear

Question 355

reversed prompt

Answer 355

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 356

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

Question 357

• • Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
• Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Answer 357

Types of Sensory Receptors

Question 358

reversed prompt

Answer 358

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.

Answer 359

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

Answer 360

SCIENCE IN YOUR LIFEWhat is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

Question 361

Reverse.Prompt

Chart functional eye structures

Answer 361

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Structure Function

Sclera Protects and supports the eye

Cornea Refracts light rays

Pupil Admits light

Choroid Absorbs stray light

Ciliary body Holds lens in place, accommodation

Iris Regulates light entrance

Retina Contains photoreceptors for sight

Rod cells Make black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

Lens Refracts and focuses light rays

Humors Transmit light rays and support the eye

Optic nerve Transmits impulses to the visual cortex

Question 362

Tutorial: Negative Feedback

Those of us, for example, in the eyes, ears, and skin, continuously send messages to the central nervous system.

In this way, they keep us informed regarding the conditions of the external environment.

Question 363

How does sensation occur? • Sensory receptors respond to environmental stimuli. • Nerve impulses travel to the cerebral cortex and sensation (conscious perception of stimuli) occurs. • Integration, the summing of signals occurs, and nerve signals can be initiated • Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound)

Question 364

reversed prompt

Taste and smell

Answer 364

chemical senses

Answer 365

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Question 366

Reverse.Prompt

Cochlear implant •

Answer 366

Cochlear implants

• bypass damaged portions of the ear •
• Directly stimulate the auditory nerve •
• Signals generated by the implant are sent by way of the
• auditory nerve to the brain, which recognizes the signals as sound •
• Hearing through a cochlear implant is different from normal hearing and takes time to learn or relearn htt

Question 367

* All sensory receptors initiate _________________

* The sensation that results depends on the part of the brain receiving the nerve signals.

* _____________ that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects.

* Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated.

* If it were possible to switch these nerves, stimulation of the eyes would result in hearing!

*

Answer 367

What are nerve signals work and who initiates them? Who receives them and how do they influence resulting sensation?

Question 368

• Divisions of the Outer Ear

Answer 368

1. Pinna: the external ear flap that catches sound waves
2. Auditory canal: directs sound waves to the tympanic membrane

• Lined with fine hairs and modified sweat glands that secrete ear wax called ceruminous glands

Question 369

Reverse.Prompt

See Table 15.2

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Answer 369

Structure/Function
Sclera: Protects and supports the eye
Cornea: Refracts light rays
Pupil: Admits light
Choroid: Absorbs stray light
Ciliary body: Holds lens in place, accommodation
Iris: Regulates light entrance
Retina: Contains photoreceptors for sight
Rod cells: Make black-and-white vision possible
Cone cells: Make color and acute vision possible
Fovea centralis: Contains mostly cones for acute vision
Other
Lens: Refracts and focuses light rays
Humors: Transmit light rays and support the eye
Optic nerve:
Transmits impulses to the visual cortex

Question 370

Reverse.Prompt

Cochlea —> Cochlear Nerve——> Brain

Answer 370

• The cochlea converts vibrations into nerve impulses. •
  
  • It contains the organ of Corti (spiral organ)
    
    • the sense organ containing hairs for hearing. –
    • Bending of embedded hairs causes vibrations that
    • initiate nerve impulses which travel to the cochlear nerve
    • and then to the brain. –
    • Pitch** is determined by varying wave **frequencies** that are detected by different parts of the organ of **Corti/spiral organ. –
    • Volume is determined by the amplitude of sound waves.

Question 371

reversed prompt

activate receptors proteins recognizing specific smells, olfactory cortex- temporal lobe

Answer 371

Odorants

Question 372

Anatomy and Physiology of the Ear

Answer 372

Figure 15.12

• 1. three divisions: outer, middle, and inner.
• The outer ear consists of the pinna (external flap) and the auditory canal. ​​
  
  • The opening of the auditory canal is lined with fine hairs and sweat glands.
  • Modified sweat glands are located in the upper wall of the canal.
    
    • They secrete earwax, protective

Question 373

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Answer 374

Chapter Review

SUMMARIZE

15.1Overview of Sensory Receptors and Sensations

Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:

Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.

Photoreceptors detect light stimuli.

Mechanoreceptors detect stimuli generated by mechanical forces.

Thermoreceptors detect stimuli caused by changes in temperature.

All of these classes function as follows:

Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.

Sensation occurs when nerve signals reach the cerebral cortex.

Perception is an interpretation of sensations.

15.2Somatic Senses

Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:

Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.

Cutaneous receptors in the skin sense touch, pressure, and temperature.

Nociceptors detect pain by responding to chemical signals from damaged tissues.

Page 325

15.3Senses of Taste and Smell

Taste and smell are due to chemoreceptors stimulated by molecules in the environment.

Sense of Taste

Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.

Sense of Smell

The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.

15.4Sense of Vision

Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.

Anatomy and Physiology of the Eye

The eye has three layers:

The sclera (outer layer) protects and supports the eye.

The choroid (middle, pigmented layer) absorbs stray light rays.

The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).

Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.

Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.

Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.

Abnormalities of the Eye

Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).

15.5Sense of Hearing

Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.

Anatomy and Physiology of the Ear

The ear has three parts:

In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.

In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.

In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.

The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.

Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.

The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.

Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.

15.6Sense of Equilibrium

The ear also contains mechanoreceptors for equilibrium.

Rotational Equilibrium Pathway

Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.

Gravitational Equilibrium Pathway

Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.

ASSESS

TESTING YOURSELF

Choose the correct answer for each question.

15.1Overview of Sensory Receptors and Sensations

Which receptors detect stimuli within the body?

interoceptors

exteroceptors

homeoreceptors

reflex receptors

A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a

mechanoreceptor.

photoreceptor.

chemoreceptor.

thermoreceptor.

None of these are correct.

Where does the process of sensation occur in the body?

at the sensory receptor

in the spinal cord

within the synapses between neurons of the PNS

in the cerebral cortex

All of these are correct.Page 326

15.2Somatic Senses

Which type of receptor detects the chemicals released by damaged tissues?

nociceptors

proprioceptors

Meissner corpuscles

Ruffini endings

None of these are correct.

Which type of receptor assists in the maintenance of muscle tone?

nociceptors

proprioceptors

Pacinian corpuscles

Krause end bulbs

All of these are correct.

15.3Senses of Taste and Smell

The senses of taste and smell rely primarily on which type of receptor?

mechanoreceptors

nociceptors

protoreceptors

proprioceptors

chemoreceptors

Olfactory bulbs are located

on the tongue.

in the nasal cavity.

in the brain stem.

in the aorta.

None of these are correct.

15.4Sense of Vision

Label this diagram of a human eye.

Which structure of the eye is incorrectly matched with its function?

lens—focusing

cones—color vision

iris—regulation of amount of light

choroid—location of cones

sclera—protection

Adjustment of the lens to focus on objects close to the viewer is called

convergence.

visual accommodation.

focusing.

constriction.

To focus on objects that are close to the viewer, the

suspensory ligaments must be pulled tight.

lens needs to become more rounded.

ciliary muscle will be relaxed.

image must focus on the area of the optic nerve.

15.5Sense of Hearing

Label this diagram of a human ear.

Which of the following is not involved in the sense of hearing?

auditory canal

tympanic membrane

ossicles

semicircular canals

cochlea

Which one of these correctly describes the location of the spiral organ?

between the tympanic membrane and the oval window in the inner ear

in the utricle and saccule within the vestibule

between the tectorial membrane and the basilar membrane in the cochlear canal

between the nasal cavities and the throat

between the outer and inner ear within the semicircular canalsPage 327

15.6Sense of Equilibrium

Which of the following structures would allow you to know you were upside down, even if you were in total darkness?

utricle and saccule

cochlea

semicircular canals

tectorial membrane

Moving your head forward would be detected by which of the following structures?

the semicircular canals

the utricle and saccule

the cochlea

the auditory canal

None of these are correct.

ENGAGE

THINKING CRITICALLY

Which receptors are activated when you enjoy supper in a pizza restaurant?

Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?

Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?

Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?

The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?

Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Question 375

StructureFunction

Sclera

Protects and supports the eye CorneaRefracts light rays PupilAdmits light

Choroid

Absorbs stray light Ciliary bodyHolds lens in place, accommodation IrisRegulates light entrance

Retina

Contains photoreceptors for sight Rod cellsMake black-and-white vision possible Cone cellsMake color and acute vision possible Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays HumorsTransmit light rays and support the eye Optic nerveTransmits impulses to the visual cortexTable 15.2Structures of the EyeTable Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

Question 376

The mechanoreceptors in the

utricle and

saccule

detect or vertical and horizontal plane movement or gravitational equilibrium

• two membranous sacs located in the inner ear
• near the semicircular canals.
• Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b).
• Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane.
• The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the
• saccule responds best to vertical (up-and-down) movements.

Answer 376

The structures involved in gravitational equilibrium

Question 377

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.

Answer 377

Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

Answer 378

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 379

reversed prompt

Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 379

Sensory receptors

Question 380

Reverse.Prompt

Mechanical Receptors

Smell

Answer 380

Smell receptors •
Depends on 10-20 million olfactory cells (modified neurons) in the roof of the nasal cavity •
Odor molecules activate specific combination of receptor proteins for recognition of specific smells and information is sent directly to the
olfactory cortex in the temporal lobe from the olfactory bulb.

Question 381

Reverse.Prompt

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Answer 382

3. Divisions of the ear: Inner ear • Important for both hearing and balance • 3 areas: cochlea, semicircular canals, vestibule • Stapes (middle ear bone) – vibrates and strikes the membrane of the oval window causing fluid waves in the cochlea • Vestibule – functions in gravitational equilibrium

Question 383

Cochlear implant • Cochlear implants bypass damaged portions of the ear • Directly stimulate the auditory nerve • Signals generated by the implant are sent by way of the auditory nerve to the brain, which recognizes the signals as sound • Hearing through a cochlear implant is different from normal hearing and takes time to learn or relearn htt

Question 384

Reverse.Prompt

Vision

Answer 384

Vision requires

* eyes: 1st) integration of stimuli in eyes: ————> 2nd) nerve signals are sent to

* the brain. cerebral cortex 1/3 of processing visual information.

Question 385

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Question 386

Reverse.Prompt

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Question 387

The skin is composed of two layers:

1. the epidermis and the
2. dermis (see Section 4.6).
   
   • contains cutaneous receptors
     
     • which make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold).
     • mosaic of these tiny receptors,
       
       • as you can determine by slowly passing a metal probe over your skin. At certain points, you will feel touch or pressure; at others, you will feel heat or cold (depending on the probe’s temperature).

Figure 15.3 Sensory receptors of the skin. The general function of each sensory receptor is shown here. However, receptors are not always this specialized. For example, microscopic examination of the skin of the ear shows only free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations.

Several types of cutaneous receptors are sensitive to fine touch. These receptors give a person specific information, such as the location of the touch, as well as its shape, size, and texture. Meissner corpuscles and Krause end bulbs are concentrated in the fingertips, palms, lips, tongue, nipples, penis, and clitoris. Merkel discs are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.
Two types of cutaneous receptors sensitive to pressure are Pacinian corpuscles and Ruffini endings. Pacinian corpuscles are onion-shaped sensory receptors that lie deep inside the dermis. Ruffini endings are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.
Temperature receptors are simply free nerve endings in the epidermis. Some free nerve endings are responsive to cold; others respond to warmth. Cold receptors are far more numerous than warmth receptors, but the two types have no known structural differences.

Answer 387

Cutaneous Receptors(Fig. 15.3),

Answer 388

2. Divisions of the ear: Middle ear • Includes – Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones – Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves – Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst • This tube has a 45 degree tilt in adults, but only 10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

Question 389

Reverse.Prompt

How does sensation occur?

Slides

Short Answer Question

Answer 389

Sensory receptors respond to environmental stimuli.
• Nerve impulses travel to the cerebral cortex
and sensation (conscious perception of stimuli) occurs.
Integration, the summing of signals occurs, and nerve signals can be initiated
Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound)

Question 390

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Gravitational Equilibrium Pathway

The mechanoreceptors in the utricle and saccule detect movement of the head in the vertical or horizontal planes, or gravitational equilibrium. The utricle and saccule are two membranous sacs located in the inner ear near the semicircular canals. Both of these sacs contain little hair cells whose stereocilia are embedded within a gelatinous material called an otolithic membrane (Fig. 15.14b). Calcium carbonate (CaCO3) granules, or otoliths, rest on this membrane. The utricle is especially sensitive to horizontal (back-and-forth) movements and the bending of the head, and the saccule responds best to vertical (up-and-down) movements.

Page 324When the body is still, the otoliths in the utricle and the saccule rest on the otolithic membrane above the hair cells. When the head bends or the body moves in the horizontal and vertical planes, the otoliths are displaced. The otolithic membrane sags, bending the stereocilia of the hair cells beneath. If the stereocilia move toward the largest stereocilium, called the kinocilium, nerve impulses increase in the vestibular nerve. If the stereocilia move away from the kinocilium, nerve impulses decrease in the vestibular nerve. The frequency of nerve impulses in the vestibular nerve indicates whether you are moving up or down.

These data reach the cerebellum, which uses them to determine the direction of the movement of the head at that moment. Remember that the cerebellum (see Section 14.2) is vital to maintaining balance and gravitational equilibrium. The cerebellum processes information from the inner ear (the semicircular canals, utricle, and saccule), as well as visual and proprioceptive inputs. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment. After integrating all these nerve inputs, the cerebellum coordinates skeletal muscle contraction to correct our position in space if necessary.

Continuous stimulation of the stereocilia can contribute to motion sickness, especially when messages reaching the brain conflict with visual information from the eyes. Imagine you are standing inside a ship that is tossing up and down on the waves. Your visual inputs signal that you are standing still, because you can see the wall in front of you and that wall isn’t moving. However, the inputs from all three sensory areas of the inner ear tell your brain you are moving up and down and from side to side. If you can match the two sets of information coming into the brain, you will begin to feel better. Thus, it makes sense to stand on deck if possible, so that visual signals and inner-ear signals both tell your brain that you’re moving. Some antihistamine drugs, such as dimenhydrinate (Dramamine), reduce the excitability of the receptors in the inner ear, thus reducing the impulses received by the cerebellum, and alleviating motion sickness.

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Question 391

reversed prompt

Answer 391

Types of Sensory Receptors
Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.
Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Question 392

Pain Receptors
Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.
Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

Question 393

Reverse.Prompt

Mechanoreceptors that are involved in
reflex actions
that maintain muscle tone, and
consequently equilibrium and posture

Answer 393

Proprioceptors •

Question 394

1. rhodopsin in this type of photoreceptor
   
   • The visual pigment in them
   • deep purple pigment Rhodopsin is a
   • complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A.
2. Process:
   
   • When a _____________absorbs light splits into opsin and retinal.
   • This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane.
   • The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases.
   • Thereafter, signals go to other neurons in the retina.
3. Characteristics/Classification/Features
   
   • very sensitive to light and, therefore, are suited to
   • night vision.
   • Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision.
   • plentiful throughout the retina, except the fovea
   • Therefore these also provide us with
     
     • peripheral vision
     • perception of motion.

Answer 394

Function of Photoreceptors

Rods

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

(Fig. 15.8b)

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

Question 395

What is the anatomy of the eye? •

Question 396

Reverse.Prompt

cerebellum in gravitational equilibrium with the inner ear and visual and proprioceptive inputs

Answer 396

1. direction of the movement of head at moment.
2. vital to maintaining balance and gravitational equilibrium.
3. from the inner ear
   
   • ​​(the semicircular canals, utricle, and saccule), as well as
   • visual and proprioceptive inputs.
4. In addition, the motor cortex in the frontal lobe of the brain signals where the limbs should be located at any particular moment.
5. After integrating all these nerve inputs,
6. coordinates skeletal muscle contraction to correct our position in space if necessary.

Question 398

Photoreceptors of the retina • Rods – They contain a visual pigment called rhodopsin. – Rods are important for peripheral and night vision (black and white vision). – Vitamin A is important for proper functioning of the rods. • Cones – They are located mostly in the fovea centralis. – Cones allow us to detect fine detail and color. – 3 different kinds of cones containing red, green, and blue pigments.

Question 399

reversed prompt

Answer 399

What are sensory receptors? • How do we detect the sense of taste and smell? • What is the anatomy of the eye? • How do we focus images? • What are some eye abnormalities? • What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing?

Question 400

(Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.
*

Question 401

The ear functions in hearing and balance.The ear functions in hearing and balance.

Question 402

Reverse.Prompt

Answer 402

Nociceptors • Pain receptors •
Sensitive to chemicals released by damaged tissues •
In inflammation, cells release chemicals to stimulate pain receptors •
Referred pain- stimulation of pain receptors is felt as pain from the skin – Thought to be due to nerve impulses from pain

Question 405

Anatomy of the ear •

Answer 405

The ear functions in hearing and balance. •

3 divisions

1. Outer ear: functions in hearing; filled with air
2. Middle ear: functions in hearing; filled with air
3. Inner ear: functions in hearing and balance; filled with fluid

Question 406

The inner ear: Semicircular canals and vestibule •

1. Detects angular movement (rotational equilibrium) –

• Depends on hair cells at the base of each semicircular canal (ampulla)
• 1. Detects movement of the head in the vertical and horizontal planes (gravitational equilibrium) –
  1. Depends on hair cells in the utricle and saccule • Signals sent to cerebellum

Question 407

Reverse.Prompt

Answer 407

The ear functions in hearing and balance.The ear functions in hearing and balance.

Question 408

reversed prompt

Answer 408

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 409

A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse. This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons. Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Table 15.1Exteroceptors

Table Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Sensory ReceptorStimulusCategorySenseSensory Organ

Taste cellsChemicalsChemoreceptorTasteTaste bud

Olfactory cellsChemicalsChemoreceptorSmellOlfactory epithelium

Rod cells and cone cells in retinaLight raysPhotoreceptorVisionEye

Hair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEar

Hair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEar

Hair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEar

Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

Tutorial: Negative Feedback

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.

Types of Sensory Receptors

Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.

Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.

Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.

The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.

Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).

How Sensation Occurs

Sensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.

As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.

Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.

Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.

All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!

Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.

CHECK YOUR PROGRESS 15.1

Describe the functions of the four types of sensory receptors.

Answer

Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Distinguish between sensation and integration.

Answer

Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.

Answer

Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

CONNECTING THE CONCEPTS

For more information on the regions of the brain associated with sensation, refer to the following discussions:

Section 14.2 describes the location and function of the reticular activating system (RAS).

Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.

Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Question 410

Reverse.Prompt

Thermoreceptors

Answer 410

We are the last type of receptors

* found in the hypothalamus and skin are

* stimulated by changes in temperature.

* They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).

Question 411

Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.

Question 412

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?

Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.

Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.

CHECK YOUR PROGRESS 15.2

Describe how the body uses proprioceptors to indicate the position of the arms and legs.

Answer

By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.

Answer

Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.

Answer

Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Figure 4.9 provides a more detailed look at the structure of human skin.

Section 13.2 provides an overview of muscle fiber contraction.

Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 413

15.3 Senses of Taste and SmellLEARNING OUTCOMES
Upon completion of this section, you should be able to
Compare and contrast the senses of taste and smell.
Identify the structures of the tongue and the olfactory areas of the nose.
Summarize how the brain receives taste and odor information.
Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.
Sense of Taste
In adult humans, approximately 4,000 taste buds are located primarily on the tongue (Fig. 15.4). Many taste buds lie along the walls of the papillae. These small elevations on the tongue are visible to the naked eye. Isolated taste buds are also present on the hard palate, the pharynx, and the epiglottis. Researchers have identified chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter. These receptors are not clustered in buds, and they do not send taste signals to the brain. Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.
Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.
(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images
Humans have five main types of taste receptors: sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes. A food can stimulate more than one of these types of taste buds. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.
How the Brain Receives Taste Information
Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. When molecules bind to receptor proteins of the microvilli, nerve signals are generated in sensory nerve fibers that go to the brain. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.
Sense of Smell
Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell. This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose. Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity (Fig. 15.5). Olfactory cells are modified neurons. Each cell ends in a tuft of about five olfactory cilia, which bear receptor proteins for odor molecules.
Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.
How the Brain Receives Odor Information
Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.
The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.
Page 312Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place? For example, a smell of a certain food may remind you of a favorite vacation. This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3). One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.
The number of olfactory cells declines with age. This can be dangerous if an older person can’t smell smoke or a gas leak. Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.
CHECK YOUR PROGRESS 15.3
Identify the structures of the tongue and nose involved in the senses of taste and smell.
Answer
Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.
Answer
They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.
Answer
Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS
For more information on chemoreceptors, refer to the following discussions:
Section 10.5 describes the function of the respiratory center in the medulla oblongata.
Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 414

reversed prompt

chemicals, nosiceptors- pain-

Answer 414

chemoreceptors

Question 415

Reverse.Prompt

• middle ear structure

1. begins at the tympanic membrane (eardrum) and
2. ends at the oval window and the round window
   
   • a bony wall containing two small openings covered by membranes.
3. ossicles
   
   • Three small bones are found between the tympanic membrane and the oval window.
   • malleus (hammer), the incus (anvil), and the stapes (stirrup)
     
     • because their shapes resemble these objects.
     • The malleus adheres to the tympanic membrane, and the
     • stapes touches the oval window.
4. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx.
   
   • Its purpose is to equalize air pressure across the tympanic membrane.
     
     • When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider.
       
       • As this occurs, we often feel the ears “pop.”
         *

Answer 415

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

Anatomy and Physiology of the Ear

The Middle Ear Structures

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Question 416

We respond to light energy!

our eyes contain____________ sensitive to light rays, thereby provide us with a sense of vision.
Two types of us and when stimulated :

rod cells : black-and-white vision.
cone cells: results in color vision.

Answer 416

What are photoreceptors?

Question 417

Abnormalities of the eye • Myopia (nearsightedness) – eyeball is too long, making it hard to see far away objects • Hyperopia (farsightedness)– eyeball is too short, making it hard to see near objects • Astigmatism – cndition in which the cornea or lens is uneven, leading to a fuzzy image

Question 418

1. genetic mutation
2. defective or is deficient in number.
3. The most common mutation is the inability to see the colors red and green.
   
   1. The gene for red-green color blindness is on the X chromosome;
      
      1. therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5).
      2. This abnormality affects 5–8% of the male population.
      3. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.
4. changes in the physical shape of the eye are
5. There are several forms of color blindness, all of which are attributed to a . In most instances, only one type of cone is

Answer 418

Abnormalities of the Eye

two of the more common vision abnormalities.

Color Blindness

Question 419

Reverse.Prompt

The lens

Answer 419

• is attached to the ciliary body by suspensory ligaments and
• divides the eye into two compartments.
  
  1. The anterior compartment is
     
     • in front of the lens, and the
     • aqueous humor: small amount produced daily, usually leaves daily by way of tiny ducts
     • glaucoma: drainage ducts are blocked
     • 2. posterior compartment
  • is behind it. Normally, it leaves the anterior compartment by way of tiny ducts.

Question 420

Blind Spot

Figure 15.9

Answer 420

• no rods and cones
• located where optic nerve exits retina
• vision impossible in this area
• The two eyes together provide complete vision because the right eye is right of center, and the for the left eye is left of center.

Question 421

Reverse.Prompt

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

Answer 422

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Answer 423

Laser-assisted in situ karatomileusis •

• Need cornea to be thick enough
• • Conjunctiva is cut away from front of eye and folded back to expose cornea •
• A defined amount of corneal tissue is removed to either flatten or increase steepness of cornea curvature •
• Conjunctiva is then put back into place

Question 424

* We are stimulated by mechanical forces and without us keeping balance would be hard
which most often result in ________________of some sort.

* Hearing is the process of airborne sound waves bring converted to __________ waves in the fluids of the inner ear that can be detected by us.
Similarly, we are responding to pressure waves

* when we detect changes in gravity and motion,

* helping us keep our balance.

* We are in the vestibule and semicircular canals of the inner ear.

Answer 424

Mechanoreceptors
pressure
vestibule and semicircular canals of the inner ear

Question 425

reversed prompt

• Interoceptors
  
  • Barocepter
  • homeostasis: regulated by a negative feedback mechanism (see Fig. 4.16).

1. blood pressure rises,
2. baroreceptors signal a regulatory center in the brain.
3. The brain responds by sending out nerve signals to the arterial walls,
4. causing their smooth muscle to relax.
5. The blood pressure then falls.
6. Once blood pressure is returned to normal,
7. the baroreceptors are no longer stimulated.

Answer 425

Barocepter Homeostasis

Question 426

15.4 Sense of VisionLEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Question 427

Reverse.Prompt

Describe the functions of the four types of sensory receptors.

Answer 427

Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Question 428

Sensory receptors

Answer 428

where they are located- exteroreceptors- taste, hearing, vision interreceptors - detect internal stimuli

Answer 429

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 430

Reverse.Prompt

• has 3 layers of neurons
  
  1. The layer closest to the choroid contains the rod cells and cone cells.
  2. A layer of bipolar cells covers the rods and cones.
  3. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve.
• Process: Steps
  
  1. Because ______________-Before rod and cone cells are stimulated, light must penetrate to the back
  2. The rod cells and cone cells synapse with the bipolar cells.
  3. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve.
     
     1. Many more rod cells and cone cells than ganglion cells.
        
        • About 150 million rod cells, 6.5 million cone cells, but only, 1million ganglion cells.
     2. The sensitivity of cones versus rods
        
        • mirrored by how directly they connect to ganglion cells.
          
          • 150 rods activate the same ganglion cell.
            
            1. stimulation of rods results in vision that is blurred and indistinct.
          • some cone cells in the fovea centralis activate only one ganglion cell.
            
            1. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.
     3. As signals pass to bipolar cells and ganglion cells, integration occurs.
        
        • Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals.
     4. Ganglion cells converge to form the optic nerve,
        
        • which transmits information to the visual cortex.
     5. Additional integration occurs in the visual cortex.

Answer 430

Function of the Retina

Fig. 15.9)

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

Figure 15.9

Question 431

Ear

Answer 431

hearing and balance

outer ear-

Question 432

Volume

Answer 432

• is a function of the amplitude (strength) of sound waves.
• Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent.
• The resulting increased stimulation is interpreted by the brain as volume.
• As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

Question 433

Reverse.Prompt

Answer 433

15.4 Sense of Vision

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera, and the transparent cornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped iris. The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Answer 434

Which parts function in balance and which parts function in hearing

Question 435

mechanoreceptors- motion detection, rotational equilibrium, vestibule

Question 436

Sense of Taste

• 4,000 taste buds primarily on the tongue
• walls of the papillae.
• Isolated also present on the hard palate, the pharynx, and the epiglottis.
• chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter.
  
  • These receptors are not clustered in buds, and they do not send taste signals to the brain.
  • Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.

(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images

Answer 436

4,000 taste buds are located primarily on the tongue of adult humans. • We have 5 main types of taste receptors: sweet, sour, salty, bitter, and umami (savory). • Taste buds open at a taste pore; microvilli on cells where molecules bind; information sent to gustatory cortex in parietal lobe • 80-90% of what we perceive as taste is actually due to the sense of smell!

Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.

Answer 437

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Answer 438

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Question 439

reversed prompt

Answer 439

15.1

Question 440

reversed prompt

Sense of touch

Answer 440

The sense of touch depends on pressure receptors sensitive to either strong or slight pressure.
Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation.
Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments.
Signals from proprioceptors make us aware of the position of our limbs.

Question 441

Reverse.Prompt

Abnormalities of the Eye

two of the more common vision abnormalities.

Answer 441

• Color blindness and
  
  • genetic mutation
  • defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.
• changes in the physical shape of the eye are There are several forms of color blindness, all of which are attributed to a . In most instances, only one type of cone is

Question 442

* We continuously send messages to the central nervous system.

* Found in places like the _____________, ________________, _________

* We tell you about the conditions of the external environment.

Answer 442

Exteroceptors
such as those in the

eyes,

ears, and

skin

Question 443

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes

Question 444

reversed prompt

Answer 444

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMES

Upon completion of this section, you should be able to

List the four categories of sensory receptors and describe what stimulus each responds to.

Distinguish between perception and sensation.

Explain the purpose of integration and sensory adaptation.

Answer 445

• Humans have five main types of taste receptors:
• sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”).
  
  • Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami.
• Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes.
• A food can stimulate more than one of these types of taste buds.
• The brain appears to survey the
• overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.

Question 446

reversed prompt

Question 447

reversed prompt

Answer 447

15.1 Overview of Sensory Receptors and Sensations

LEARNING OUTCOMES

Upon completion of this section, you should be able to

List the four categories of sensory receptors and describe what stimulus each responds to.

Distinguish between perception and sensation.

Explain the purpose of integration and sensory adaptation.

A sensory receptor is able to convert a signal from the environment, called a stimulus, into a nerve impulse. This conversion is commonly referred to as sensory transduction. Some sensory receptors are modified neurons, and others are specialized cells closely associated with neurons. Sensory receptors may detect stimuli originating from both the internal and external environments. Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1). Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Table 15.1Exteroceptors

Table Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Sensory ReceptorStimulusCategorySenseSensory Organ

Taste cellsChemicalsChemoreceptorTasteTaste bud

Olfactory cellsChemicalsChemoreceptorSmellOlfactory epithelium

Rod cells and cone cells in retinaLight raysPhotoreceptorVisionEye

Hair cells in spiral organ of the inner earSound wavesMechanoreceptorHearingEar

Hair cells in semicircular canals of the inner earMotionMechanoreceptorRotational equilibriumEar

Hair cells in vestibule of the inner earGravityMechanoreceptorGravitational equilibriumEar

Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

Tutorial: Negative Feedback

Exteroceptors, such as those in the eyes, ears, and skin, continuously send messages to the central nervous system. In this way, they keep us informed regarding the conditions of the external environment.

Types of Sensory Receptors

Sensory receptors in humans can be classified into four categories: chemoreceptors, photoreceptors, mechanoreceptors, and thermoreceptors.

Chemoreceptors respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Photoreceptors respond to light energy. Our eyes contain photoreceptors that are sensitive to light rays and thereby provide us with a sense of vision. Stimulation of the photoreceptors known as rod cells results in black-and-white vision. Stimulation of the photoreceptors known as cone cells results in color vision.

Mechanoreceptors are stimulated by mechanical forces, which most often result in pressure of some sort. When we hear, airborne sound waves are converted to pressure waves in the fluids of the inner ear that can be detected by mechanoreceptors. Mechanoreceptors are responding to pressure waves when we detect changes in gravity and motion, helping us keep our balance. These receptors are in the vestibule and semicircular canals of the inner ear.

The sense of touch depends on pressure receptors sensitive to either strong or slight pressure. Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation. Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments. Signals from proprioceptors make us aware of the position of our limbs.

Thermoreceptors in the hypothalamus and skin are stimulated by changes in temperature. They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).

How Sensation Occurs

Sensory receptors respond to environmental stimuli by generating nerve signals. When the nerve signals arrive at the cerebral cortex of the brain, sensation, the conscious perception of stimuli, occurs.

As discussed in Section 14.4, sensory receptors are the first element in a reflex arc. We are aware of a reflex action only when sensory information reaches the brain. At that time, the brain Page 308integrates this information with other information received from other sensory receptors. Consider what happens if you burn yourself and quickly remove your hand from a hot stove. The brain receives information not only from your skin but also from your eyes, your nose, and all sorts of sensory receptors.

Some sensory receptors are free nerve endings or encapsulated nerve endings, and others are specialized cells closely associated with neurons. Often, the plasma membrane of a sensory receptor contains receptor proteins that react to the stimulus. For example, the receptor proteins in the plasma membrane of chemoreceptors bind to certain chemicals. When this happens, ion channels open, and ions flow across the plasma membrane. If the stimulus is sufficient, nerve signals begin and are carried by a sensory nerve fiber in the PNS to the CNS (Fig. 15.1). The stronger the stimulus, the greater the frequency of nerve signals. Nerve signals that reach the spinal cord first are conveyed to the brain by ascending tracts. If nerve signals finally reach the cerebral cortex, sensation occurs.

Figure 15.1 The role of the CNS and PNS in sensation and sensory perception. After detecting a stimulus, sensory receptors initiate nerve signals in the peripheral nervous system (PNS). These signals give the central nervous system (CNS) information about the external and internal environments. The CNS integrates all incoming information, and then initiates a motor response to the stimulus.

All sensory receptors initiate nerve signals. The sensation that results depends on the part of the brain receiving the nerve signals. Nerve signals that begin in the optic nerve eventually reach the visual areas of the cerebral cortex. Thereafter, we see objects. Nerve signals that begin in the auditory nerve eventually reach the auditory areas of the cerebral cortex. We hear sounds when the auditory cortex is stimulated. If it were possible to switch these nerves, stimulation of the eyes would result in hearing!

Before sensory receptors initiate nerve signals, they also carry out integration, the summing up of signals. One type of integration is called sensory adaptation, which is a decrease in response to a stimulus over time. We have all had the experience of smelling an odor when we first enter a room and then later not being aware of it. When sensory adaptation occurs, sensory receptors send fewer impulses to the brain. Without these impulses, the sensation of the stimuli is decreased. The functioning of our sensory receptors makes a significant contribution to homeostasis. Without sensory input, we would not receive information about our internal and external environments. This information leads to appropriate reflex and voluntary actions to keep the internal environment constant.

CHECK YOUR PROGRESS 15.1

Describe the functions of the four types of sensory receptors.

Answer

Chemoreceptors respond to chemical substances; photoreceptors respond to light energy; mechanoreceptors are stimulated by mechanical forces that result in pressure; thermoreceptors are stimulated by changes in temperature.

Distinguish between sensation and integration.

Answer

Sensation occurs when sensory receptors generate a nerve impulse that arrives at the cerebral cortex; integration is the processing of the information by summation.

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.

Answer

Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

CONNECTING THE CONCEPTS

For more information on the regions of the brain associated with sensation, refer to the following discussions:

Section 14.2 describes the location and function of the reticular activating system (RAS).

Figure 14.11 illustrates the somatosensory regions of the cerebral cortex.

Figure 14.17 illustrates the portions of the peripheral nervous system involved in a reflex arc.

Answer 448

3. Layers of the eye: Retina • The retina contains photoreceptors called rods and cones. • Rods are sensitive to light. • Cones require bright light and respond to wavelengths of light (color). • The macula lutea is an area of the retina densely packed with cones where images are focused. – The fovea centralis is the center of this area, and is the area of highest visual acuity (sharpness of vision)

Question 449

Interoceptors are directly involved in homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16). For example, when blood pressure rises, baroreceptors signal a regulatory center in the brain. The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax. The blood pressure then falls. Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

Question 450

Reverse.Prompt

The functioning of our sensory receptors makes a significant contribution to homeostasis: ​​to keep the internal environment constant.

* Without sensory input, we would not receive information about our internal and external environments.

* This information leads to

appropriate reflex and
voluntary actions

*

Answer 450

Sensory receptors role in homeostasis

Question 451

Reverse.Prompt

Pain Receptors
Like the skin, many internal organs have nociceptors, which respond to chemicals released by damaged tissues. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins, that stimulate pain receptors. Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.
Page 310Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs. This is called referred pain. Some internal organs have a referred pain relationship with areas in the skin of the back, groin, and abdomen. For example, pain from the heart is often felt in the left shoulder and arm. This most likely happens when nerve impulses from the pain receptors of internal organs travel to the spinal cord and synapse with neurons also receiving impulses from the skin. Frequently, this type of referred pain is more common in men than in women. The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

Question 452

The ear functions in hearing and balance.

* 3 divisions

* Outer ear: functions in hearing; filled with air

* Middle ear: functions in hearing; filled with air

* Inner ear: functions in hearing and balance; filled with fluid

Answer 452

Anatomy of the ear

Question 453

Proprioceptors • Mechanoreceptors that are involved in reflex actions that maintain muscle tone, and consequently equilibrium

Question 454

Reverse.Prompt

Table 15.2 summarizes the major structures of the eye and their functions.

Answer 454

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Question 455

reversed prompt

Answer 455

Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium

Question 456

reversed prompt

The ear functions in hearing and balance. • 3 divisions 1. Outer ear: functions in hearing; filled with air 2. Middle ear: functions in hearing; filled with air 3. Inner ear: functions in hearing and balance; filled with fluid

Question 457

1. The pathway for vision begins once light has been focused on the photoreceptors in the retina.
2. Some integration occurs in the retina, where
   
   • nerve signals begin before
   • the optic nerve transmits them to the brain.

Answer 457

Visual Pathway to the Brain​

Answer 458

Sensory receptors • Sensory receptors – dendrites specialized to detect certain types of stimuli – Exteroceptors: detect stimuli from outside the body (e.g., taste, hearing, vision) – Interoceptors: receive stimuli from inside the body (e.g., change in blood pressure) • Directly involved in homeostasis and a part of a negative feedback loop

Answer 459

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

Question 460

Reverse.Prompt

Exteroceptors
such as those in the

eyes,

ears, and

skin

Answer 460

* We continuously send messages to the central nervous system.

* Found in places like the _____________, ________________, _________

* We tell you about the conditions of the external environment.

Question 461

Reverse.Prompt

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Question 462

Reverse.Prompt

• 1. respond to chemical substances in the immediate vicinity. As Table 15.1 indicates, taste and smell, which detect external stimuli, use chemoreceptors. However, so do various other organs sensitive to internal stimuli. Chemoreceptors that monitor blood pH are located in the carotid arteries and aorta. If the pH lowers, the breathing rate increases. As more carbon dioxide is exhaled, the blood pH rises. Nociceptors (also referred to as pain receptors) are a type of chemoreceptor. They are naked dendrites that respond to chemicals released by damaged tissues. Nociceptors are protective, because they alert us to possible danger. For example, without the pain associated with appendicitis, we might never seek the medical help needed to avoid a ruptured appendix.

Answer 462

Chemoreceptors

Question 463

Reverse.Prompt

Sensory receptors pick up changes in the internal and external environment so the body can respond to those changes and maintain homeostasis.

Answer 463

Summarize the importance of sensory receptors in the maintenance of homeostasis in the body.

Answer 464

15.6 Sense of Equilibrium

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Explain how mechanoreceptors are involved in the sense of equilibrium.

Identify the structures of the ear involved in the sense of equilibrium.

Distinguish between rotational and gravitational equilibrium.

Question 465

Abnormalities of the eye • Glaucoma – fluid pressure builds up in the eye • Pinkeye (conjunctivitis)- inflammation of conjunctiva, mucous membrane that covers eyeball and inner part of eyelid

Question 466

• Mechanoreceptors in the
  
  • semicircular canals
  • detect rotational and/or angular movement of the head—rotational equilibrium .
• The three semicircular canals are arranged so there is one in each dimension of space.

1. The base, or ampulla, of each of the three canals is slightly enlarged.
   
   • Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae.
   • Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged.

• As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend.
• This causes a change in the pattern of signals carried by the vestibular nerve to the brain.
• The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium.
• Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Answer 466

Rotational Equilibrium Pathway

(Fig. 15.14a)

Question 467

Reverse.Prompt

15.14

Answer 467

Noise pollution • Loud noises (>85 decibels) or chronic noise can damage inner ear cells. • Environmental noise can cause mental health issues such as inability to concentrate, an increase in irritability, and anxiety. • Noise can cause loss of sleep and productivity, and can lead to anxiety.

Question 468

reversed prompt

Mechanoreceptors
pressure
vestibule and semicircular canals of the inner ear

Answer 468

* We are stimulated by mechanical forces and without us keeping balance would be hard
which most often result in ________________of some sort.

* Hearing is the process of airborne sound waves bring converted to __________ waves in the fluids of the inner ear that can be detected by us.
Similarly, we are responding to pressure waves

* when we detect changes in gravity and motion,

* helping us keep our balance.

* We are in the vestibule and semicircular canals of the inner ear.

Question 469

reversed prompt

Answer 469

Chapter Review

SUMMARIZE

15.1Overview of Sensory Receptors and Sensations

Signal transduction begins with the detection of stimuli by sensory receptors. These receptors may detect stimuli from within the body (interoceptors) or the external environment (exteroceptors). In general, receptors are classified by the types of stimuli they detect:

Chemoreceptors detect chemical stimuli. Nociceptors are a form of chemoreceptor that detects pain.

Photoreceptors detect light stimuli.

Mechanoreceptors detect stimuli generated by mechanical forces.

Thermoreceptors detect stimuli caused by changes in temperature.

All of these classes function as follows:

Sensory receptors perform integration of the incoming signals. They then initiate nerve signals to the spinal cord and/or brain. Sensory adaptation may occur if the stimuli are repeated continuously.

Sensation occurs when nerve signals reach the cerebral cortex.

Perception is an interpretation of sensations.

15.2Somatic Senses

Somatic senses are associated with the skin, muscles, joints, and viscera. The sensory receptors associated with the somatic senses include the following:

Proprioceptors (mechanoreceptors) are involved in reflex actions and help maintain equilibrium and posture.

Cutaneous receptors in the skin sense touch, pressure, and temperature.

Nociceptors detect pain by responding to chemical signals from damaged tissues.

Page 325

15.3Senses of Taste and Smell

Taste and smell are due to chemoreceptors stimulated by molecules in the environment.

Sense of Taste

Receptors for taste are found primarily on the taste buds. Microvilli of taste cells have receptor proteins for molecules that cause the brain to distinguish sweet, sour, salty, bitter, and umami.

Sense of Smell

The cilia of olfactory cells have receptor proteins for molecules that cause the brain to distinguish odors.

15.4Sense of Vision

Vision depends on the eye, the optic nerves, and the visual areas of the cerebral cortex.

Anatomy and Physiology of the Eye

The eye has three layers:

The sclera (outer layer) protects and supports the eye.

The choroid (middle, pigmented layer) absorbs stray light rays.

The retina (inner layer) contains the rod cells (sensory receptors for dim light) and cone cells (sensory receptors for bright light and color).

Function of the Lens: Light enters the eye through the pupil, the size of which is regulated by the iris. The lens, with assistance from the cornea, aqueous humor, and vitreous humor, brings the light rays to focus on the retina, typically on the fovea centralis region of the retina. To see a close object, visual accommodation occurs as the lens becomes round and thick.

Photoreceptors: Two types of photoreceptors are located on the retina: rod cells (black-white vision) and cone cells (color vision). Both contain rhodopsin, which includes retinal (vitamin A). An area called the blind spot lacks rods and cones.

Visual Pathway to the Brain: The visual pathway begins when light strikes photoreceptors (rod cells and cone cells) in the retina. The optic nerves carry nerve impulses from the eyes to the optic chiasma. The nerve impulse leaves the optic chiasma along optic tracts to the thalamus before reaching the primary vision area in the occipital lobe of the brain.

Abnormalities of the Eye

Vision problems may be caused by a buildup of pressure in the eye (glaucoma), genetic factors (color blindness), or the shape of the eye (which can result in being nearsighted, farsighted, or having astigmatism).

15.5Sense of Hearing

Hearing depends on the ear, the cochlear nerve, and the auditory areas of the cerebral cortex.

Anatomy and Physiology of the Ear

The ear has three parts:

In the outer ear, the pinna and the auditory canal direct sound waves to the middle ear.

In the middle ear, the tympanic membrane (including the oval window and round window), and the ossicles (malleus, incus, and stapes) amplify sound waves.

In the inner ear, the semicircular canals and vestibule detect rotational equilibrium; the utricle and saccule detect gravitational equilibrium; and the cochlea houses the spiral organ, which contains mechanoreceptors, hair cells with stereocilia, for hearing.

The auditory tube (or eustachian tube) helps to equalize pressure across the tympanic membrane.

Auditory Pathway to the Brain: The auditory pathway begins when the outer ear receives and the middle ear amplifies sound waves that then strike the oval window membrane.

The mechanoreceptors for hearing are hair cells on the basilar membrane of the spiral organ.

Nerve signals begin in the cochlear nerve and are carried to the primary auditory area in the temporal lobe of the cerebral cortex.

15.6Sense of Equilibrium

The ear also contains mechanoreceptors for equilibrium.

Rotational Equilibrium Pathway

Rotational equilibrium is due to mechanoreceptors (hair cells) in the semicircular canals that detect rotational and/or angular movement of the head.

Gravitational Equilibrium Pathway

Gravitational equilibrium is due to mechanoreceptors (hair cells) in the utricle and saccule that detect head movement in the vertical or horizontal planes. Calcium carbonate granules called otoliths assist in this process.

ASSESS

TESTING YOURSELF

Choose the correct answer for each question.

15.1Overview of Sensory Receptors and Sensations

Which receptors detect stimuli within the body?

interoceptors

exteroceptors

homeoreceptors

reflex receptors

A receptor that detects changes in pH, or specific molecules in the environment, would be classified as a

mechanoreceptor.

photoreceptor.

chemoreceptor.

thermoreceptor.

None of these are correct.

Where does the process of sensation occur in the body?

at the sensory receptor

in the spinal cord

within the synapses between neurons of the PNS

in the cerebral cortex

All of these are correct.Page 326

15.2Somatic Senses

Which type of receptor detects the chemicals released by damaged tissues?

nociceptors

proprioceptors

Meissner corpuscles

Ruffini endings

None of these are correct.

Which type of receptor assists in the maintenance of muscle tone?

nociceptors

proprioceptors

Pacinian corpuscles

Krause end bulbs

All of these are correct.

15.3Senses of Taste and Smell

The senses of taste and smell rely primarily on which type of receptor?

mechanoreceptors

nociceptors

protoreceptors

proprioceptors

chemoreceptors

Olfactory bulbs are located

on the tongue.

in the nasal cavity.

in the brain stem.

in the aorta.

None of these are correct.

15.4Sense of Vision

Label this diagram of a human eye.

Which structure of the eye is incorrectly matched with its function?

lens—focusing

cones—color vision

iris—regulation of amount of light

choroid—location of cones

sclera—protection

Adjustment of the lens to focus on objects close to the viewer is called

convergence.

visual accommodation.

focusing.

constriction.

To focus on objects that are close to the viewer, the

suspensory ligaments must be pulled tight.

lens needs to become more rounded.

ciliary muscle will be relaxed.

image must focus on the area of the optic nerve.

15.5Sense of Hearing

Label this diagram of a human ear.

Which of the following is not involved in the sense of hearing?

auditory canal

tympanic membrane

ossicles

semicircular canals

cochlea

Which one of these correctly describes the location of the spiral organ?

between the tympanic membrane and the oval window in the inner ear

in the utricle and saccule within the vestibule

between the tectorial membrane and the basilar membrane in the cochlear canal

between the nasal cavities and the throat

between the outer and inner ear within the semicircular canalsPage 327

15.6Sense of Equilibrium

Which of the following structures would allow you to know you were upside down, even if you were in total darkness?

utricle and saccule

cochlea

semicircular canals

tectorial membrane

Moving your head forward would be detected by which of the following structures?

the semicircular canals

the utricle and saccule

the cochlea

the auditory canal

None of these are correct.

ENGAGE

THINKING CRITICALLY

Which receptors are activated when you enjoy supper in a pizza restaurant?

Besides the blood pH mentioned, which other homeostatic conditions are monitored by chemoreceptors?

Some sensory receptors, such as those for taste, smell, and pressure, readily undergo the process of sensory adaptation, or decreased response to a stimulus. In contrast, receptors for pain are less prone to adaptation. Why does this make good biological sense? What do you think happens to children who are born without the ability to feel pain normally?

Airport and construction workers are likely to be exposed to continuous, loud noises. What would you predict the long-term effect on their hearing to be? Why?

The acoustic and vestibular nerves travel together to the brain. If a tumor grows on this combined nerve, which sensations will be affected?

Stem cells are currently being used to treat some forms of age-related macular degeneration (AMD). When placed in the retina, these unspecialized cells divide and assume the roles of the damaged retinal cells. How might stem cell therapy be used to treat damage to other senses, such as noise-related damage to hearing? What would be some challenges to this approach?

Health Icon: ©Janis Christie/Digital Vision/Getty Images; Science Icon: ©Antenna/Getty Images; Bioethical Icon: ©JGI/Blend Images LLC

ANSWER KEY

Testing Yourself

Click here for the answers to the Testing Yourself questions.

Answer

Testing Yourself: 1. a; 2. c; 3. d; 4. a; 5. b; 6. e; 7. b; 8. a. retina; b. choroid; c. sclera; d. optic nerve; e. fovea centralis; f. ciliary body; g. lens; h. iris; i. pupil; j. cornea; 9. d; 10. b; 11. b; 12. a. tympanic membrane; b. auditory canal; c. stapes; d. incus; e. malleus; f. oval window; g. semicircular canals; h. vestibule; i. cochlear nerve; j. cochlea; k. auditory tube; l. round window; 13. d; 14. c; 15. a; 16. a

Thinking Critically

Click here for the answers to the Thinking Critically questions.

Answer

Thinking Critically: 1. Just about the entire sensory system: taste, smell, vision (seeing your pizza), as well as receptors for temperature and texture in your mouth. 2. Chemoreceptors also monitor the oxygen and carbon dioxide in the blood as well as some hormones and drugs. 3. Adaptation to sensing stimuli that are not harmful is advantageous. A person can redirect his or her energy to other tasks. If, however, the stimuli causes harm, a person benefits from continuously sensing it and avoiding it. Children born without normal pain perception do not have the normal means to learn to avoid dangerous situations that can lead to severe injury. 4. Hearing receptors are severely damaged by continual loud noise. Without ear protection, the workers may lose their hearing and become deaf. 5. Both hearing and balance will be affected, sometimes severely. 6. Stem cells to regenerate damaged parts of the cochlea might restore hearing. Smell might be restored by stem cells generating new olfactory cells. Equilibrium might be restored by stem cells that could regenerate parts of the inner ear. Challenges to this include directing specific stem cells, which have been developed to have a specific function, to the correct location in the sensory organ.

Question 470

Vision requires

* eyes: 1st) integration of stimuli in eyes: ————> 2nd) nerve signals are sent to

* the brain. cerebral cortex 1/3 of processing visual information.

Answer 470

Vision

Question 471

SCIENCE IN YOUR LIFEWhy does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Question 472

reversed prompt

Question 473

Reverse.Prompt

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Question 474

Abnormalities of the eye • Color blindness – genetic abnormality in which colors, usually red and green, cannot be distinguished; most common in males (if you are color blind you cannot see the “7” in the picture on the left below) • Cataracts – lens of the eye is cloudy

Question 475

reversed prompt

Anatomy of the ear • The ear functions in hearing and balance. • 3 divisions 1. Outer ear: functions in hearing; filled with air 2. Middle ear: functions in hearing; filled with air 3. Inner ear: functions in hearing and balance; filled with fluid

Question 476

Reverse.Prompt

Answer 476

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

Answer 477

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Answer 478

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Question 479

reversed prompt

Answer 479

Tutorial: Negative Feedback

Those of us, for example, in the eyes, ears, and skin, continuously send messages to the central nervous system.

In this way, they keep us informed regarding the conditions of the external environment.

Question 480

sensory receptor

Answer 480

convert a signal from the environment, called a stimulus, into a nerve impulse.
This conversion is commonly referred to as sensory transduction.
modified neurons, and others are specialized cells closely associated with neurons.
Sensory receptors may detect stimuli originating from both the internal and external environments.
Exteroceptors are sensory receptors that detect stimuli from outside the body, such as those that result in taste, smell, vision, hearing, and equilibrium (Table 15.1).
Interoceptors receive stimuli from inside the body. Examples of interoceptors are the baroreceptors (also called pressoreceptors) that respond to changes in blood pressure, osmoreceptors that monitor the body’s water-salt balance, and chemoreceptors that monitor the pH of the blood.

Question 481

reversed prompt

Answer 481

15.1 Overview of Sensory Receptors and Sensations

LEARNING OUTCOMES

Upon completion of this section, you should be able to

List the four categories of sensory receptors and describe what stimulus each responds to.

Distinguish between perception and sensation.

Explain the purpose of integration and sensory adaptation.

Question 482

Reverse.Prompt

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

Question 483

olfaction doesn’t need to go through

Answer 483

thalamus in the brain - goes directly to cerebral cortex

Question 484

Reverse.Prompt

Make a few notes

Answer 484

Check out this table:

Table 15.1Exteroceptors

Table Summary: Table lists the names of different types of sensory receptors in column 1. Other information like their stimulus, category in which they fall into, and so on appear in the other columns.

Question 485

We are the last type of receptors

* found in the hypothalamus and skin are

* stimulated by changes in temperature.

* They respond to both heat and cold and play a major role in the regulation of internal body temperature (see Fig. 4.18).

Answer 485

Thermoreceptors

Question 486

15.5 Sense of Hearing

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the ear involved in hearing.

Summarize how sound waves are converted into nerve signals.

Describe the pathway of sensory information from the ear to the brain.

The ear has two sensory functions: hearing and balance (equilibrium). The sensory receptors for both of these are located in the inner ear. Each consists of hair cells with stereocilia (sing., stereocilium), which are long, stiff microvilli that are sensitive to mechanical stimulation. The stereocilia act as mechanoreceptors.

Anatomy and Physiology of the Ear

Figure 15.12 shows that the ear has three divisions: outer, middle, and inner. The outer ear consists of the pinna (external flap) and the auditory canal. The opening of the auditory canal is lined with fine hairs and sweat glands. Modified sweat glands are located in the upper wall of the canal. They secrete earwax, a substance that helps guard the ear against the entrance of foreign materials, such as air pollutants.

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

The middle ear begins at the tympanic membrane (eardrum) and ends at a bony wall containing two small openings covered by membranes. These openings are called the oval window and the round window. Three small bones are found between the tympanic membrane and the oval window. Collectively, they are called the ossicles. Individually, they are called the malleus (hammer), the incus (anvil), and the stapes (stirrup) because their shapes resemble these objects. The malleus adheres to the Page 320tympanic membrane, and the stapes touches the oval window. An auditory tube, also called the eustachian or pharyngotympanic tube, extends from the middle ear to the nasopharynx. Its purpose is to equalize air pressure across the tympanic membrane. When changing elevation, such as in an airplane, the act of chewing gum, yawning, or swallowing opens the auditory tubes wider. As this occurs, we often feel the ears “pop.”

Whereas the outer ear and the middle ear contain air, the inner ear is filled with fluid. The inner ear has three areas: The semicircular canals and the vestibule are concerned with equilibrium; the cochlea is concerned with hearing. The cochlea resembles the shell of a snail because it spirals.

Auditory Pathway to the Brain

The auditory pathway begins with the auditory canal. Thereafter, hearing requires the other parts of the ear, the cochlear nerve, and the brain.

Through the Auditory Canal and Middle Ear

The process of hearing begins when sound waves enter the auditory canal. Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly. As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles. The pressure is multiplied about 20 times. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

From the Cochlea to the Auditory Cortex

By examining the cochlea in cross-section (Fig. 15.13), you can see that it has three canals. The sensory organ for hearing, called the spiral organ (or the organ of Corti), is located in the cochlear canal. The spiral organ consists of little hair cells and a gelatinous material called the tectorial membrane. The hair cells sit on the basilar membrane, and their stereocilia are embedded in the tectorial membrane.

Figure 15.13 How the spiral organ (organ of Corti) translates sound waves into nerve signals. a. The spiral organ (organ of Corti) is located within the (b) cochlea. c. The spiral organ consists of hair cells resting on the basilar membrane, with the tectorial membrane above. Pressure waves moving through the canals cause the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend. Nerve impulses traveling in the cochlear nerve result in hearing. d. A micrograph of the stereocilia.

(photo): ©P. Motta/SPL/Science Source

Page 322When the stapes strikes the membrane of the oval window, pressure waves move from the vestibular canal to the tympanic canal across the basilar membrane. The basilar membrane moves up and down, and the stereocilia of the hair cells embedded in the tectorial membrane bend. Then, nerve signals begin in the cochlear nerve and travel to the brain. When they reach the auditory cortex in the temporal lobe, they are interpreted as a sound.

Effect of Sound Waves

Each part of the spiral organ is sensitive to different wave frequencies, or pitch. Near the tip, the spiral organ responds to low pitches, such as those of a tuba. Near the base (beginning), it responds to higher pitches, such as those of a bell or a whistle. The nerve fibers from each region along the length of the spiral organ lead to slightly different areas in the auditory cortex. The pitch sensation we experience depends upon which region of the basilar membrane vibrates and which area of the auditory cortex is stimulated.

Volume is a function of the amplitude (strength) of sound waves. Loud noises cause the fluid within the vestibular canal to exert more pressure and the basilar membrane to vibrate to a greater extent. The resulting increased stimulation is interpreted by the brain as volume. As discussed in the Health feature “Noise Pollution,” noise levels above 85 decibels (Table 15.3) may cause permanent hearing loss.

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Table 15.3Noises That Affect Hearing

Table Summary: Table lists the different types of noises in column 1. Other information related to each type of noise appears in columns 2 and 3.

Type of NoiseSound Level (Decibels)Effect

“Boom car,” jet engine, shotgun, rock concertOver 125Beyond threshold of pain; potential for hearing loss high

Nightclub, thunderclapOver 120Hearing loss likely

Earbuds in external ear canal110–120Hearing loss likely

Chain saw, pneumatic drill, jackhammer, symphony orchestra, snowmobile, garbage truck, cement mixer100–200Regular exposure of more than 1 min risks permanent hearing loss

Farm tractor, newspaper press, subway, motorcycle90–100Fifteen minutes of unprotected exposure potentially harmful

Lawn mower, food blender85–90Continuous daily exposure for more than 8 hr can cause hearing damage

Diesel truck, average city traffic noise80–85Annoying; constant exposure may cause hearing damage

CHECK YOUR PROGRESS 15.5

Identify the structures of the ear involved in hearing and provide a function for each.

Answer

The outer ear directs sound into the middle ear, causing vibrations in the tympanic membrane and the ossicles that attach to the inner ear, where fluid stimulates receptors that generate impulses in nerves, sending signals to the brain.

Describe the role of mechanoreceptors in the sense of hearing.

Answer

The hair cells located in the spiral organ of the cochlea are mechanoreceptors, which are sensitive to the movements of fluid in the inner ear.

Summarize how the spiral organ translates sound waves to nerve impulses.

Answer

Pressure waves move through the canals, causing the basilar membrane to vibrate. This causes the stereocilia embedded in the tectorial membrane to bend, generating nerve impulses that travel to the brain.

CONNECTING THE CONCEPTS

For more information on the material in this section, refer to the following discussions:

Section 14.2 describes the function of the cerebral cortex area of the brain in hearing.

Figure 14.15 illustrates the structure of a nerve.

Question 487

reversed prompt

Answer 487

©Amelie Benoist/Science Source
CHAPTER OUTLINE
15.1Overview of Sensory Receptors and Sensations
15.2Somatic Senses
15.3Senses of Taste and Smell
15.4Sense of Vision
15.5Sense of Hearing
15.6Sense of Equilibrium
BEFORE YOU BEGIN
Before beginning this chapter, take a few moments to review the following discussions:
Section 14.1What are the roles of the central and peripheral nervous systems in the body?
Section 14.2What is the role of the primary somatosensory area of the cerebral cortex?
Section 14.2How does the cerebellum help maintain balance?
Improving your Eyesight
John had always worn glasses. As a computer programmer, he often spent 6–10 hours a day looking at a computer screen. Initially, he tried using bifocal and progressive lenses, but they never seemed to adjust his vision correctly. After a visit to his eye doctor, John decided he would like to permanently correct his vision, and remove his need for glasses, by undergoing LASIK surgery.
LASIK, which stands for laser-assisted in situ keratomileusis, is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. During the procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own.
John was an ideal candidate for this type of surgery—he was over 18 years of age and his last two eye exams had indicated that his vision was relatively stable. He knew there would be a small amount of discomfort from the procedure, but his doctor informed him that the procedure was generally safe and that almost 80% of patients improved their vision to the point that they no longer needed glasses.
In this chapter, we will explore the structure and function of not only the eye but also of the other sensory organs that contribute information to our central nervous system for interpretation.
As you read through the chapter, think about the following questions:
What is the role of the cornea in vision?
How does the eye normally adjust to looking at objects at different distances?

Question 488

reversed prompt

Key Concepts to Focus On

1. What are sensory receptors?
2. How do we detect the sense of taste and smell?
3. What is the anatomy of the eye?
4. How do we focus images?
5. What are some eye abnormalities?
6. What is the anatomy of the ear?
7. Which parts function in balance and which parts function in hearing?

Question 489

Reverse.Prompt

4,000 taste buds are located primarily on the tongue of adult humans. • We have 5 main types of taste receptors: sweet, sour, salty, bitter, and umami (savory). • Taste buds open at a taste pore; microvilli on cells where molecules bind; information sent to gustatory cortex in parietal lobe • 80-90% of what we perceive as taste is actually due to the sense of smell!

Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.

Answer 489

Sense of Taste

• 4,000 taste buds primarily on the tongue
• walls of the papillae.
• Isolated also present on the hard palate, the pharynx, and the epiglottis.
• chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter.
  
  • These receptors are not clustered in buds, and they do not send taste signals to the brain.
  • Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.

(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images

Answer 490

SCIENCE IN YOUR LIFE

What are “ear tubes”?

The auditory tubes of children tend to be oriented more horizontally than those of adults. Because of this, fluid may accumulate in the tubes, allowing for an infection to occur. These infections are called otitis media, and they are often painful. Extended cases of otitis media may produce long-term hearing loss.

A procedure called a tympanostomy places small tubes in the tympanic membrane, allowing these fluids to drain more easily, thus reducing the chance of infection. In most cases, the tubes fall out of the membrane over time, but sometimes they need to be removed by a physician.

Question 491

reversed prompt

Question 492

reversed prompt

Answer 492

How does sensation occur? • Sensory receptors respond to environmental stimuli. • Nerve impulses travel to the cerebral cortex and sensation (conscious perception of stimuli) occurs. • Integration, the summing of signals occurs, and nerve signals can be initiated • Sensory adaptation, decrease in stimulus response, can occur with repetitive stimuli (i.e., odor, sound).

Question 493

reversed prompt

Sensory Receptors, Skin, Pain, Temperature Regulation

Cutaneous Receptors

Figure 15.3

Answer 493

1. located in dermis
2. touch, pressure, pain, temperature
3. skin sensory receptors (see Figure 15.3)
4. several types of these are sensitive to fine touch and specialized
   
   • ​​unlike the ear where the skin only shows free nerve endings (pain receptors), yet the skin of the ear is sensitive to all sensations
   • give a person specific information, such as the location of the touch, as well as its shape, size, and texture
5. Types
   
   • make the skin sensitive to touch, pressure, pain, and temperature (warmth and cold). The dermis is a
   • Meissner corpuscles
     
     1. fingertips, palms, lips, tongue, nipples, penis, and clitoris.
   • Merkel discs
     
     1. are found where the epidermis meets the dermis. A free nerve ending called a root hair plexus winds around the base of a hair follicle. This receptor responds if the hair is touched.
        
        1. Krause end bulbs are
   • Two types of cutaneous receptors sensitive to pressure are
     
     1. Pacinian corpuscles: onion-shaped sensory receptors that lie deep inside the dermis.
     2. Ruffini endings: are encapsulated by sheaths of connective tissue and contain lacy networks of nerve fibers.
   • Temperature receptors
     
     1. are simply free nerve endings in the epidermis.
     2. Some free nerve endings are responsive to cold; more numerous
     3. others respond to warmth.
     4. no known structural differences.
        *

Question 494

The sense of touch depends on pressure receptors sensitive to either strong or slight pressure.
Baroreceptors located in certain arteries detect changes in blood pressure, and stretch receptors in the lungs detect the degree of lung inflation.
Proprioceptors respond to the stretching of muscle fibers, tendons, joints, and ligaments.
Signals from proprioceptors make us aware of the position of our limbs.

Answer 494

Sense of touch

Question 495

Reverse.Prompt

Vision: Focusing; Purpose of the Lens

Answer 495

focuses images on the retina:

* cornea: Focusing starts
lens and the humors: continues as the rays pass through

* The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus.

* If the eye is too long or too short,

* the person may need corrective lenses to bring the image into focus.

* The image on the retina is inverted (upside down) and reversed from left to right.

Question 496

Reverse.Prompt

Anatomy of the eye

Answer 496

• 2 compartments 1. Anterior compartment: between the cornea and lens; filled with a clear fluid called aqueous humor – this liquid is continuously produced each day and drains through small ducts 2. Posterior compartment: most of the eye, behind the lens; contains a gelatinous material called vitreous humor that holds the retina in place and supports the lens – this liquid you are born with and remains; no more is produced

Question 497

BIOLOGY TODAY Health

Noise Pollution

Though we can sometimes tune out its presence, unwanted noise is all around us. Noise pollution is noise from the environment that is annoying, distracting, and potentially harmful. It comes from airplanes, cars, lawn mowers, machinery, and our own loud music and that of our neighbors. It is present at our workplaces, in public spaces like amusement parks, and at home. Its prevalence allows loud noise to have a potentially high impact on our welfare.

Noise and Health

How does noise affect human health? Perhaps the greatest worry about noise pollution is that exposure to loud (over 85 decibels) or chronic noises can damage cells of the inner ear and cause hearing loss (Fig. 15B). When we are young, we often do not consider the damage that noise may be doing to our spiral organ. The stimulation of loud music is often sought by young people at rock concerts without regard to the possibility that their hearing may be diminished as a result. Over the years, loud noises can bring deafness and accompanying depression when we are older adults.

Figure 15B Loud noise damages the hair cells in the spiral organ. a. Normal hair cells in the spiral organ of a guinea pig. b. Damaged cells. This damage occurred after 24-hour exposure to a noise level equivalent to that at a rock concert (see Table 15.3). Hearing is permanently impaired because lost cells will not be replaced, and damaged cells may also die.

(both): ©Dr. Yeohash Raphael, Kresge Institute/University of Michigan, Ann Arbor

Noise can affect well-being by other means, too. Data from studies of environmental noise can be difficult to interpret because of the presence of other confounding factors, including physical or chemical pollution. The tolerance level for noise also varies from person to person. Nonetheless, laboratory and field studies show that noise may be detrimental in nonauditory ways. Its effects on mental health include annoyance, inability to concentrate, and increased irritability. Long-term noise exposure from air or car traffic may impair cognitive ability, language learning, and memory in children. Noise often causes loss of sleep and reduced productivity and can induce stress. Additionally, several studies have demonstrated a link between noise pollution and cardiovascular health, specifically hypertension.

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Answer 498

Anatomy of the ear • The ear functions in hearing and balance. • 3 divisions 1. Outer ear: functions in hearing; filled with air 2. Middle ear: functions in hearing; filled with air 3. Inner ear: functions in hearing and balance; filled with fluid

Answer 499

What are some eye abnormalities? •

Answer 500

Figure 15.12 The three divisions of the human ear. The external ear consists of the pinna (the structure commonly referred to as the “ear”) and the auditory canal. The tympanic membrane separates the external ear from the middle ear. In the middle ear, the malleus (hammer), the incus (anvil), and the stapes (stirrup) amplify sound waves. In the inner ear, the mechanoreceptors for equilibrium are in the semicircular canals and the vestibule. The mechanoreceptors for hearing are in the cochlea.

Question 501

• flexible, transparent, and concave structure. •
• Visual accommodation occurs when the lens changes shape to focus light on the retina and form an image. •
• As we age, the lens loses elasticity, and we use glasses to correct for this. 15.4 Sense of Vision The lens a. Focusing on distant object b

Answer 501

The lens is a

Question 502

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?
Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.
Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.
CHECK YOUR PROGRESS 15.2
Describe how the body uses proprioceptors to indicate the position of the arms and legs.
Answer
By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.
Answer
Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.
Answer
Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS
For more information on the material in this section, refer to the following discussions:
Figure 4.9 provides a more detailed look at the structure of human skin.
Section 13.2 provides an overview of muscle fiber contraction.
Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 503

Reverse.Prompt

Rotational Equilibrium Pathway

Answer 503

• Mechanoreceptors in the
  
  • 3 semicircular canals: one in each dimension of space.
    
    • base, or ampulla, of each slightly enlarged
      
      • • stereocilia on hair cells are embedded within a gelatinous material called a cupula
          
          • Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged.
            
            • As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend.
              
              • This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.
  • detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a).
  • The three semicircular canals are arranged so there is The .

Question 504

reversed prompt

Pain Receptors, nociceptors,

Answer 504

• 1. Like the skin, many internal organs have them
  2. which respond to chemicals released by damaged tissues.
  3. When inflammation occurs because of mechanical, thermal, or electrical stimuli or toxic substances, cells release chemicals, called prostaglandins,
  • that stimulate pain receptors.
  • Aspirin and ibuprofen reduce pain by inhibiting the enzymes that synthesize these chemicals.
    4. Sometimes, stimulation of internal pain receptors is felt as pain from the skin as well as the internal organs.
  • This is called referred pain.
    
    • Some internal organs have a referred pain relationship with areas in the
      
      1. skin of the back,
      2. groin, and
      3. abdomen.
    • For example, pain from the heart is often felt in the left shoulder and arm.
      
      1. This most likely happens when nerve impulses from the pain receptors of internal organs
         
         • travel to the spinal cord and
         • synapse with neurons also receiving impulses from the skin.
    • Frequently, this type of referred pain is more common in men than in women.
    • The nonspecific symptoms that women often experience during a heart attack may delay a diagnosis.

*

Answer 505

Depends on 10-20 million olfactory cells (modified neurons) in the roof of the nasal cavity • Odor molecules activate specific combination of receptor proteins for recognition of specific smells and information is sent directly to the olfactory cortex in the temporal lobe from the olfactory bulb

Question 506

Reverse.Prompt

Gravitational Equilibrium Pathway

Answer 506

movement of the head in the vertical or horizontal planes,

*

Question 507

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes

Question 508

Reverse.Prompt

CHECK YOUR PROGRESS 15.6

State the location and function of the structures involved in maintaining balance.

Answer

All structures are in the inner ear and involve mechanoreceptors. For rotational equilibrium—semicircular canals, ampullae, cupula, stereocilia, hair cells, vestibular nerve, supporting cells, and endolymph; for gravitational equilibrium—utricle, saccule, otoliths, otolithic membrane, hair cells, supporting cells, and vestibular nerve.

Describe how rotational equilibrium is achieved.

Answer

Fluid within the semicircular canal moves and displaces a cupula, causing the stereocilia of the hair cells to bend. This causes a change in the pattern of signals sent to the brain by the vestibular nerve.

Contrast rotational and gravitational equilibrium and explain how the two work together to maintain balance.

Answer

Rotational equilibrium occurs when the head is moved side-to-side and gravitational equilibrium occurs when the head is moved up and down. They work together to keep the head, and body, in position according to gravity.

CONNECTING THE CONCEPTS

For more information on the sense of equilibrium, refer to the following discussions:

Section 14.1 examines the structure of a neuron and the generation of a nerve impulse.

Section 14.2 explains the role of the cerebellum in the processing of sensory information regarding balance.

CONCLUSION

Following the surgery, John received eyedrops and some pain medication to help relieve the discomfort associated with the procedure. He was also informed not to expect immediate changes in his vision. While some patients see improvements in their vision as early as the day after the surgery, it takes 2 to 3 months for their vision to stabilize.

John’s doctor scheduled several appointments to assess how his vision was progressing. At his first appointment, he complained of some redness and dryness in his eyes, but was informed by his doctor that this was a normal outcome of the surgery.

His doctor was pleased with John’s progress, and informed him that most patients will have vision close to 20/20, but even with LASIK it was possible his vision could still change over time.

Question 509

Through the Auditory Canal and Middle Ear

Steps of the hearing process

Answer 509

1. sound waves enter the auditory canal.
2. when a large number of waves strike the tympanic membrane, it (vibrates) ever so slightly
   
   • auditory ossicles attach to one another: malleus to incus, incus to stapes. The malleus is attached to the inner wall of the tympanic membrane. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes.
3. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles; about 20 times.
4. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate. In this way, the pressure is passed to the fluid within the cochlea.

Question 510

Reverse.Prompt

Function of Photoreceptors

Rods

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

(Fig. 15.8b)

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

Answer 510

1. rhodopsin in this type of photoreceptor
   
   • The visual pigment in them
   • deep purple pigment Rhodopsin is a
   • complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A.
2. Process:
   
   • When a _____________absorbs light splits into opsin and retinal.
   • This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane.
   • The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases.
   • Thereafter, signals go to other neurons in the retina.
3. Characteristics/Classification/Features
   
   • very sensitive to light and, therefore, are suited to
   • night vision.
   • Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision.
   • plentiful throughout the retina, except the fovea
   • Therefore these also provide us with
     
     • peripheral vision
     • perception of motion.

Question 511

Cutaneous receptors

Answer 511

What receptors are in the dermis and make the skin sensitive to touch, pressure, pain, and temperature?

Question 512

reversed prompt

Answer 512

Types of sensory receptors • Chemoreceptors – respond to nearby chemicals – Nociceptors (pain receptors) – chemoreceptors that respond to chemicals released by damaged tissue • Photoreceptors – respond to light energy • Mechanoreceptors – respond to mechanical forces such as pressure • Thermoreceptors – stimulated by temperature changes

Answer 513

SCIENCE IN YOUR LIFEWhat are phantom sensation and phantom pain?
Suppose a person loses a foot and a leg due to an injury. In addition to dealing with loss of a limb, an amputee often must cope with the phenomenon of phantom sensation or phantom pain—or both. Phantom sensation is a painless awareness of the amputated limb. For example, a patient whose foot and lower leg have been removed may have an itchy or tingly sensation in the “foot,” even though the foot is no longer there. Similarly, phantom pain can be sensed as originating from the absent body part. Researchers believe that any stimulus (such as a touch) to the stump will fool the brain into a perceived sensation, because the brain has received signals from the leg and foot for such a long time.
Phantom sensation may last for years but usually disappears without treatment. Phantom pain must be treated with a combination of medication, massage, and physical therapy.
CHECK YOUR PROGRESS 15.2
Describe how the body uses proprioceptors to indicate the position of the arms and legs.
Answer
By detecting the degree of muscle relaxation, the stretch of the tendons, and the movement of ligaments.

Summarize the role of each type of cutaneous receptor.
Answer
Meissner corpuscles, Krause end bulbs, Merkel disks, and root hair plexus are sensitive to fine touch. Pacinian corpuscles and Ruffini endings are sensitive to pressure. Temperature receptors are free nerve endings.

Explain why the sensation of pain is important for the maintenance of homeostasis.
Answer
Skin receptors that lead to pain sensation alert organisms to changes in the external environment that can upset homeostasis and cause harm. Nociceptors in internal organs are sensitive to chemicals released by damaged tissues. The perception of pain allows the body to recognize potential dangers in the external and internal environments and respond to them.

CONNECTING THE CONCEPTS
For more information on the material in this section, refer to the following discussions:
Figure 4.9 provides a more detailed look at the structure of human skin.
Section 13.2 provides an overview of muscle fiber contraction.
Section 14.2 presents the gate control theory of how the brain responds to input from pain receptors.

Question 514

reversed prompt

Answer 514

15.4 Sense of Vision

LEARNING OUTCOMES

Upon completion of this section, you should be able to

Identify the structures of the human eye.

Explain how the eye focuses on near and far objects.

Describe the role of photoreceptors in vision.

Summarize the abnormalities of the eye that produce vision problems.

Vision requires the work of the eyes and the brain. As we will see, integration of stimuli occurs in the eyes before nerve Page 313signals are sent to the brain. Still, researchers estimate that at least a third of the cerebral cortex takes part in processing visual information.

Anatomy and Physiology of the Eye

The eye is an elongated sphere about 2.5 cm in diameter. It has three layers, or coats: the sclera, the choroid, and the retina (Fig. 15.6). The outer layer is made up of the white, fibrous sclera, and the transparent cornea, which is made of collagen fibers. The cornea is known as the window of the eye.

Figure 15.6 The structures of the human eye. The sclera (the outer layer of the eye) becomes the cornea, and the choroid (the middle layer) is continuous with the ciliary body and the iris. The retina (the inner layer) contains the photoreceptors for vision. The fovea centralis is the region where vision is most acute.

The choroid is the thin, middle coat. It has an extensive blood supply, and its dark pigment absorbs stray light rays that photoreceptors have not absorbed. This helps visual acuity. Toward the front, the choroid becomes the doughnut-shaped iris. The iris regulates the size of the pupil, a hole in the center of the iris through which light enters the eye. The color of the iris (and therefore the color of the eyes) correlates with its pigmentation. Heavily pigmented eyes are brown, and lightly pigmented eyes are green or blue. Behind the iris, the choroid thickens and forms the circular ciliary body. The ciliary body contains the ciliary muscle, which controls the shape of the lens for near and far vision.

SCIENCE IN YOUR LIFE

What is pinkeye?

At some point in their lives, most people have suffered from conjunctivitis, or pinkeye. Conjunctivitis is the inflammation of a mucous membrane called the conjunctiva, which covers the eye (except the cornea) and the inner part of the eyelid. The purpose of the conjunctiva is to lubricate the eye and keep it from drying out. In the case of viral conjunctivitis, the most common type, this membrane becomes inflamed as part of an immune response against viral pathogens. Viral conjunctivitis is highly contagious; individuals with the condition must be careful not to spread the disease. However, not all conjunctivitis is contagious; allergies and other medical conditions can cause pinkeye-like symptoms. Treatment usually involves the use of eyedrops that help lubricate the eye and reduce inflammation.

The lens is attached to the ciliary body by suspensory ligaments and divides the eye into two compartments. The anterior compartment is in front of the lens, and the posterior compartment is behind it. The anterior compartment is filled with a clear, watery fluid called the aqueous humor. A small amount of aqueous humor is continually produced each day. Normally, it leaves the anterior compartment by way of tiny ducts. When a person has glaucoma, these drainage ducts are blocked and aqueous humor builds up. If glaucoma is not treated, the resulting pressure compresses the arteries that serve the nerve fibers of the retina, where photoreceptors are located. The nerve fibers begin to die because of lack of nutrients, and the person gradually loses his or her vision. Eventually, total blindness can result.Page 314

The third layer of the eye, the retina, is located in the posterior compartment. This compartment is filled with a clear, gelatinous material called the vitreous humor. The vitreous humor holds the retina in place and supports the lens. The retina contains photoreceptors called rod cells and cone cells. The rods are very sensitive to light, but they do not detect color. Therefore, at night or in a darkened room, we see only shades of gray. The cones, which require bright light, are sensitive to different wavelengths of light. This sensitivity gives us the ability to distinguish colors. The retina has a very special region called the fovea centralis where cone cells are densely packed. Light is normally focused on the fovea when we look directly at an object. This is helpful because the sharpest images are produced by the fovea centralis. Sensory fibers from the retina form the optic nerve, which takes nerve signals to the visual cortex.

Table 15.2 summarizes the major structures of the eye and their functions.

Table 15.2Structures of the Eye

Table Summary: Table is divided into information for different structures of the eye grouped under sclera, choroid, retina, and other in column 1. The functions of each structure appear in the next column.

StructureFunction

ScleraProtects and supports the eye

CorneaRefracts light rays

PupilAdmits light

ChoroidAbsorbs stray light

Ciliary bodyHolds lens in place, accommodation

IrisRegulates light entrance

RetinaContains photoreceptors for sight

Rod cellsMake black-and-white vision possible

Cone cellsMake color and acute vision possible

Fovea centralisContains mostly cones for acute vision

Other

LensRefracts and focuses light rays

HumorsTransmit light rays and support the eye

Optic nerveTransmits impulses to the visual cortex

Function of the Lens

The cornea, assisted by the lens and the humors, focuses images on the retina. Focusing starts with the cornea and continues as the rays pass through the lens and the humors. The image produced is much smaller than the object, because light rays are bent (refracted) when they are brought into focus. If the eye is too long or too short, the person may need corrective lenses to bring the image into focus. The image on the retina is inverted (upside down) and reversed from left to right.

Visual accommodation occurs for close vision. During visual accommodation, the lens changes its shape to bring the image into focus on the retina. The shape of the lens is controlled by the ciliary muscle, within the ciliary body. When we view a distant object, the ciliary muscle is relaxed, causing the suspensory ligaments attached to the ciliary body to be taut. The ligaments put tension on the lens and cause it to remain relatively flat (Fig. 15.7a). When we view a near object, the ciliary muscle contracts, releasing the tension on the suspensory ligaments. The lens becomes round and thick due to its natural elasticity (Fig. 15.7b). Thus, contraction or relaxation of the ciliary muscle allows the image to be focused on the retina. Close work requires contraction of the ciliary muscle, so it often causes muscle fatigue, known as eyestrain. Eyestrain is more common after the age of 40, because the lens loses some of its elasticity and is unable to accommodate. It is also common among those who work with computers, because the intense focusing causes the person to blink less, allowing the eyes to dry out. Eyedrops and/or corrective lenses, either eyeglasses or contact lenses, may be necessary to reduce eyestrain.

Figure 15.7 Focusing light on the retina. Light rays from each point on an object are bent by the cornea and the lens in such a way that an inverted and reversed image of the object forms on the retina. a. When focusing on a distant object, the lens is flat because the ciliary muscle is relaxed and the suspensory ligament is taut. b. When focusing on a near object, the lens accommodates—it becomes rounded because the ciliary muscle contracts, causing the suspensory ligament to relax.

Visual Pathway to the Brain

The pathway for vision begins once light has been focused on the photoreceptors in the retina. Some integration occurs in the retina, where nerve signals begin before the optic nerve transmits them to the brain.

Function of Photoreceptors

Figure 15.8a illustrates the structure of the photoreceptors called rod cells and cone cells. Page 315Both rods and cones have an outer segment joined to an inner segment by a short stalk. Pigment molecules are embedded in the membrane of the many disks present in the outer segment. Synaptic vesicles are located at the synaptic endings of the inner segment.

Figure 15.8 The two types of photoreceptors in the eye. a. The outer segment of rods and cones contains stacks of membranous disks, which contain visual pigments. b. In rods, the membrane of each disk contains rhodopsin, a complex molecule containing the protein opsin and the pigment retinal. When rhodopsin absorbs light energy, it splits, releasing retinal, which sets in motion a cascade of reactions that causes ion channels in the plasma membrane to close. Thereafter, nerve signals go to the brain.

(a): ©Science Source

The visual pigment in rods is a deep purple pigment called rhodopsin (Fig. 15.8b). Rhodopsin is a complex molecule made up of the protein opsin and a light-absorbing molecule called retinal, a derivative of vitamin A. When a rod absorbs light, rhodopsin splits into opsin and retinal. This leads to a cascade of reactions and the closure of ion channels in the rod cell’s plasma membrane. The release of inhibitory transmitter molecules from the rod’s synaptic vesicles ceases. Thereafter, signals go to other neurons in the retina. Rods are very sensitive to light and, therefore, are suited to night vision. Carrots, and other orange and yellow vegetables, are rich in vitamin A, so it is true that eating carrots can improve your night vision. Rod cells are plentiful throughout the retina, except the fovea. Therefore, rods also provide us with peripheral vision and perception of motion.

The cones, on the other hand, are located primarily in the fovea and are activated by bright light. They allow us to detect the fine detail and the color of an object. Color vision depends on three types of cones, which contain pigments called the B (blue), G (green), and R (red) pigments. Each pigment is made up of retinal and opsin, but there is a slight difference in the opsin structure of each. This accounts for their individual absorption patterns. Various combinations of cones are believed to be stimulated by in-between shades of color.

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Function of the Retina

The retina has three layers of neurons (Fig. 15.9). The layer closest to the choroid contains the rod cells and cone cells. A layer of bipolar cells covers the rods and cones. The innermost layer contains ganglion cells, whose sensory fibers become the optic nerve. Only the rod cells and cone cells are sensitive to light; therefore, light must penetrate to the back of the retina before the rods and cones are stimulated.

Figure 15.9 The structure of the retina. a. The retina is the inner layer of the eye. b. Rod and cone cells, located at the back of the retina nearest the choroid, synapse with bipolar cells, which synapse with ganglion cells. Integration of signals occurs at these synapses. Cone cells, in general, distinguish more detail than do rod cells. c. Micrograph shows that the sclera and choroid are relatively thin compared to the retina, which has several layers of cells.

(c): ©McGraw-Hill Education/Dennis Strete, photographer

The rod cells and cone cells synapse with the bipolar cells. Next, signals from bipolar cells stimulate ganglion cells whose axons become the optic nerve. Notice in Figure 15.9 that there are many more rod cells and cone cells than ganglion cells. Although the precise number is not known, the retina has around 150 million Page 316rod cells and 6.5 million cone cells, but only 1 million ganglion cells. The sensitivity of cones versus rods is mirrored by how directly they connect to ganglion cells. As many as 150 rods may activate the same ganglion cell. No wonder the stimulation of rods results in vision that is blurred and indistinct. In contrast, some cone cells in the fovea centralis activate only one ganglion cell. This explains why cones, especially in the fovea centralis, provide us with a sharper, more detailed image of an object.

As signals pass to bipolar cells and ganglion cells, integration occurs. Therefore, considerable processing occurs in the retina before ganglion cells generate nerve signals. Ganglion cells converge to form the optic nerve, which transmits information to the visual cortex. Additional integration occurs in the visual cortex.

Blind Spot

Figure 15.9 also shows that there are no rods and cones where the optic nerve exits the retina. Therefore, no vision is possible in this area. You can prove this to yourself by putting a dot to the right of center on a piece of paper. Use your right hand to move the paper slowly toward your right eye, and make sure you look straight ahead. The dot will disappear at one point—this is your right eye’s blind spot. The two eyes together provide complete vision because the blind spot for the right eye is not the same as the blind spot for the left eye. The blind spot for the right eye is right of center, and the blind spot for the left eye is left of center.

From the Retina to the Visual Cortex

To reach the visual cortex, nerve impulses are carried by the optic nerves from the eyes to the optic chiasma (Fig. 15.10). The optic chiasma has an X shape, formed by a crossing-over of optic nerve fibers. After exiting the Page 317optic chiasma, the optic nerves continue as optic tracts. Fibers from the right half of each retina converge and continue on together in the right optic tract. Similarly, the nerve fibers from the left half of each retina join to form the left optic tract, traveling together to the brain.

Figure 15.10 The function of the optic chiasma. Because of the optic chiasma, data from the right half of each retina go to the right visual cortex, and data from the left half of the retina go to the left visual cortex. These data are then combined to allow us to see the entire visual field.

The optic tracts sweep around the hypothalamus, and most fibers synapse with neurons in nuclei (masses of neuron cell bodies) within the thalamus. Axons from the thalamic nuclei form optic radiations that take nerve impulses to the visual cortex within the occipital lobe. The image is split in the visual cortex. This division of incoming information happens because the right visual cortex receives information from the right optic tract, and the left visual cortex receives information from the left optic tract. For good depth perception, the right and left visual cortices communicate with each other. Also, because the image is inverted and reversed, it must be righted in the brain for us to correctly perceive the visual field.

Abnormalities of the Eye

Color blindness and changes in the physical shape of the eye are two of the more common vision abnormalities. There are several forms of color blindness, all of which are attributed to a genetic mutation. In most instances, only one type of cone is defective or is deficient in number. The most common mutation is the inability to see the colors red and green. The gene for red-green color blindness is on the X chromosome; therefore, males (who possess only one X chromosome) are more susceptible (see Section 21.5). This abnormality affects 5–8% of the male population. If the eye lacks cones that respond to red wavelengths, green colors are accentuated, and vice versa.

Distance Vision

If you can see from 20 ft what a person with normal vision can see from 20 ft, you are said to have 20/20 vision. Persons who can see close objects but can’t see the letters on an optometrist’s chart from 20 ft are said to be nearsighted. Nearsighted people can see close objects better than they can see objects at a distance. The shape of the eye in these individuals is elongated, and when they attempt to look at a distant object, the image is brought to focus in front of the retina (Fig. 15.11a). They can see close objects because their lens can compensate for the elongated shape of the eye. To see distant objects, these people can wear concave lenses, which spread the light rays, so that the image focuses on the retina.

Figure 15.11 How corrective lenses correct vision problems. a. A concave lens in nearsighted persons focuses light rays on the retina. b. A convex lens in farsighted persons focuses light rays on the retina. c. An uneven lens in persons with astigmatism focuses light rays on the retina.

Persons who can easily see the optometrist’s chart but cannot see close objects well are farsighted. These individuals can see distant objects better than they can see close objects. The shape of their eye is shortened, and when they try to see close objects, the image is focused behind the retina (Fig. 15.11b). When the object is distant, the lens can compensate for the shortened shape of the eye. When the object is close, these persons can wear convex lenses to increase the bending of light rays, so that the image can be focused on the retina.

When the cornea or lens is uneven, the image is fuzzy. The light rays cannot be evenly focused on the retina. This condition, called astigmatism, can be corrected by an unevenly ground lens to compensate for the uneven cornea (Fig. 15.11c).

As we introduced in the chapter opener, many people today opt to have LASIK surgery instead of wearing lenses. LASIK surgery is discussed in the Health feature “Correcting Vision Problems.”Page 318

BIOLOGY TODAY Health

Correcting Vision Problems

Poor vision can be due to a number of problems, some more serious than others. Several of these, such as cataracts and glaucoma, are conditions that often require medical attention.

Cataracts and Glaucoma

Cataracts develop when the lens of the eye becomes cloudy. Normally, the lens is clear, allowing light to pass through easily. A cloudy lens allows less light to reach the retina and slowly causes vision loss. Fortunately, a doctor can surgically remove the cloudy lens and replace it with a clear plastic lens, which often restores the light level passing through the lens and improves the patient’s vision.

Glaucoma is caused by a buildup of fluid pressure inside the eye and may lead to a decrease in vision. The condition may eventually cause blindness. Eyedrops and oral medications are often prescribed to help reduce the interior pressure. If eyedrops and medications are not capable of controlling the pressure, surgery may be the only option. During glaucoma surgery, the doctor uses a laser to create tiny holes in the eye where the cornea and iris meet. This increases fluid drainage from the eye and decreases the pressure inside the eye.

The Benefits of LASIK Surgery

For many people, a sign of aging is the slow and steady decrease of their ability to see up close, a condition called presbyopia. Difficulty in reading small print, or reading in low-light conditions, is usually the first sign of presbyopia. The condition tends to begin in the late thirties and is common by age 55. Reading in low-light situations becomes more difficult, and letters begin to look fuzzy when reading close up. Many people who suffer from presbyopia experience headaches while reading. Although, historically, people accommodated for the condition by using a magnifying lens, today most people wear bifocal lenses. Bifocals are designed to correct vision at a distance of 12 to 18 inches. These lenses work well for most people while reading, but pose a problem for those who use a computer. Computer monitors are usually 19 to 24 inches away. This forces bifocal lens wearers to constantly move their head up and down in an attempt to switch between the close and distant viewing sections of the bifocals. Another solution is to wear contact lenses with one eye corrected to see close objects and the other eye corrected to see distant objects. The same type of correction can be done with LASIK surgery (Fig. 15A). LASIK, which stands for laser-assisted in situ keratomileusis, is generally a safe and effective treatment option for a wide array of vision problems.

Figure 15A LASIK surgery.

©Arctic-Images/Iconica/Getty Images

LASIK is a quick and relatively painless procedure that involves the use of a laser to permanently change the shape of the cornea. For the majority of patients, LASIK improves their vision and reduces their dependency on corrective lenses. The ideal LASIK candidate is over 18 years of age and has had a stable contact or glasses prescription for at least 2 years. Patients need to have a cornea thick enough to allow the surgeon to safely create a clean corneal flap of appropriate depth. Typically, patients affected by common vision problems (nearsightedness, astigmatism, or farsightedness) respond well to LASIK. Anyone who suffers from any disease that decreases his or her ability to heal properly after surgery is not a candidate for LASIK. Candidates should thoroughly discuss the procedure with their eye-care professional before electing to have LASIK. People need to realize that the goal of LASIK is to reduce their dependency on glasses or contact lenses, not to eliminate them completely.

Individuals who suffer from cataracts, advanced glaucoma, or corneal or other eye diseases are not considered for LASIK. Patients who expect LASIK to completely correct their visual problems and make them totally independent of their corrective lenses are not good candidates, either.

The LASIK Procedure

During the LASIK procedure, a small flap of tissue (the conjunctiva) is cut away from the front of the eye. The flap is folded back, exposing the cornea, allowing the surgeon to remove a defined amount of tissue from the cornea. Each pulse of the laser removes a small amount of corneal tissue, allowing the surgeon to flatten or increase the steepness of the curve of the cornea. After the procedure, the flap of tissue is put back into place and allowed to heal on its own. LASIK patients receive eyedrops or medications to help relieve the discomfort associated with the procedure. Improvements to vision begin as early as the day after the surgery, but typically take 2 to 3 months. Most patients will have vision close to 20/20, but the chances for improved vision are based in part on how good the person’s eyes were before the surgery.

As with any surgery, complications are possible. Adverse effects include a sensation of having something in the eye or having blurred vision. The individual might also see halos around objects or be very sensitive to glare. In addition, dryness can cause eye irritation. Typically, these effects are temporary, and the rate of complications following surgery is very low. Always consult with your doctor before considering any type of surgery.

Questions to Consider

Using Figure 15.11 as a guide, explain how LASIK surgery corrects the flow of light into the eye.

What problems might be experienced by people who wear contacts that cause the two eyes to focus at different distances?

CHECK YOUR PROGRESS 15.4

Identify the structures of the eye and provide a function of each.

Answer

Sclera—protects and supports eyeball; cornea—refracts light rays; pupil—admits light; choroid—absorbs stray light; ciliary body—contains ciliary muscle which functions in accommodation; iris—regulates light entrance; retina—contains sensory receptors; rod cells and cone cells—detect wavelengths of light; fovea centralis—acute vision; lens—refracts and focuses light; humors—transmit light and support eyeball; optic nerve—transmits impulses to brain.

Describe the two types of photoreceptors and state the function of each.

Answer

Rod cells function in black-and-white vision and are very sensitive to light. Cone cells are activated by bright light and function in color vision.

Summarize the movement of sensory information from the photoreceptors to the visual cortex.

Answer

Photoreceptors (rods and cones) synapse with bipolar cells, which synapse with ganglion cells whose axons become the optic nerve. These cross at the optic chiasma and then connect to the thalamic nucleus, which is connected to the visual cortex.

CONNECTING THE CONCEPTS

For more information on the material presented in this section, refer to the following discussions:

Table 9.8 describes the function and dietary sources of vitamin A.

Section 14.2 describes the function of the visual association area in the cerebral cortex of the brain.

Section 21.5 explores the pattern of inheritance associated with color blindness.

Answer 515

Study all screen shots and slides over and over again

Question 516

Anatomy of the ear • The ear functions in hearing and balance. • 3 divisions 1. Outer ear: functions in hearing; filled with air 2. Middle ear: functions in hearing; filled with air 3. Inner ear: functions in hearing and balance; filled with fluid

Answer 517

What is the anatomy of the ear? •

Question 518

The ear functions in hearing and balance.

Answer 518

• 3 divisions 1. Outer ear: functions in hearing; filled with air 2. Middle ear: functions in hearing; filled with air 3. Inner ear: functions in hearing and balance; filled with fluid

Answer 519

SCIENCE IN YOUR LIFE

Why does rubbing your closed eyes (as you might if you’re tired or your eyes itch) produce a visual sensation?

The eye is a flexible container filled with fluid and a soft, gelatinous material. Compressing the eyes by rubbing on them increases pressure in the eyes. In turn, the photoreceptors of the eye are stimulated by the increased eye pressure. When the nerve signals are conveyed to the brain, the brain senses “vision.” We “see stars” because nerve signals from the eyes can only result in sight.

Question 520

Outer ear

Answer 520

1. Divisions of the ear: Outer ear • Includes – Pinna: the external ear flap that catches sound waves – Auditory canal: directs sound waves to the tympanic membrane • Lined with fine hairs and modified sweat glands that secrete ear wax called ceruminous glands

Question 521

The vestibular nerve originates in the semicircular canals, saccule, and utricle. It takes nerve signals to the brain stem and cerebellum (Fig. 15.14). Through its communication with the brain, the vestibular nerve helps us achieve equilibrium, but other structures in the body are also involved. For example, in Section 15.5, we mentioned that proprioceptors are necessary for maintaining our equilibrium. Vision, if available, usually provides extremely helpful input the brain can act upon. To explain, let’s take a look at the two sets of mechanoreceptors for equilibrium.

Answer 521

Figure 15.14 The mechanoreceptors of the inner ear and the sense of balance. a. Rotational equilibrium is coordinated by receptors in the ampullae of the semicircular canals. b. Gravitational equilibrium is coordinated by receptors in the utricule and saccule located near the semicircular canals.

Answer 522

Regulating Noise Pollution

Noise pollution has been a concern for several decades. In 1972, the Noise Control Act was passed as a means for coordinating federal noise control and research and to develop noise emission standards. The aim was to protect Americans from “noise that jeopardizes their health or welfare.” The Environmental Protection Agency (EPA) had federal authority to regulate noise pollution, and its Office of Noise Abatement and Control (ONAC) worked on establishing noise guidelines. However, the activities of the ONAC were transferred to state and local governments in 1981. Today, there is no national noise policy, although the EPA does maintain standards on noise pollution on its website: www.epa.gov.

Workplace noise exposure is controlled by the Occupational Safety and Health Administration (OSHA). OSHA has set guidelines for workplace noise. OSHA regulations require that protective gear be provided if sound levels exceed certain values. This may include noise-reducing earmuffs and other protective methods for people who work around big equipment. However, OSHA guidelines don’t cover things like telephone ringing and computer noise that may be present in a nonindustrial environment such as an open-plan office. Aviation noise and traffic noise reduction plans are overseen by the Department of Transportation, the Federal Aviation Administration (FAA), and the Federal Highway Administration (FHWA), respectively. Local governments often have legislation that controls noise levels in public places, such as downtown areas and public parks. However, without national standards, the laws vary by location.

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Answer 523

15.3 Senses of Taste and SmellLEARNING OUTCOMES
Upon completion of this section, you should be able to
Compare and contrast the senses of taste and smell.
Identify the structures of the tongue and the olfactory areas of the nose.
Summarize how the brain receives taste and odor information.
Taste and smell are called chemical senses, because their receptors are sensitive to molecules in the food we eat and the air we breathe. Page 311Chemoreceptors are plasma membrane receptors that bind to particular molecules. Taste cells and olfactory cells are examples of chemoreceptors.
Sense of Taste
In adult humans, approximately 4,000 taste buds are located primarily on the tongue (Fig. 15.4). Many taste buds lie along the walls of the papillae. These small elevations on the tongue are visible to the naked eye. Isolated taste buds are also present on the hard palate, the pharynx, and the epiglottis. Researchers have identified chemoreceptors in the human lung that are sensitive only to chemicals that normally taste bitter. These receptors are not clustered in buds, and they do not send taste signals to the brain. Stimulation of these receptors causes the airways to dilate, leading the researchers to speculate about implications for new medications to treat diseases such as asthma.
Figure 15.4 The tongue and the sense of taste. a. Papillae on the tongue contain taste buds sensitive to sweet, sour, salty, bitter, and umami. b. Photomicrograph and enlargement of the papillae. c. Taste buds occur along the walls of the papillae. Taste cells in microvilli possess receptor proteins for certain molecules. When molecules bind to the receptor proteins, nerve signals are generated and go to the brain, where the sensation of taste occurs.
(b, both): ©Clouds Hill Imaging Ltd./Corbis Documentary/Getty Images
Humans have five main types of taste receptors: sweet, sour, salty, bitter, and umami (from the Japanese, meaning “savory, delicious”). Foods rich in certain amino acids, such as the common seasoning monosodium glutamate (MSG), as well as certain flavors of cheese, beef broth, and some seafood, produce the taste of umami. Taste buds for each of these tastes are located throughout the tongue, although certain regions may be slightly more sensitive to particular tastes. A food can stimulate more than one of these types of taste buds. The brain appears to survey the overall pattern of incoming sensory impulses and to take a “weighted average” of their taste messages as the perceived taste.
How the Brain Receives Taste Information
Taste buds open at a taste pore. They have supporting cells and a number of elongated taste cells that end in microvilli. When molecules bind to receptor proteins of the microvilli, nerve signals are generated in sensory nerve fibers that go to the brain. Signals reach the gustatory (taste) cortex, located primarily in the parietal lobe, where they are interpreted as particular tastes.
Sense of Smell
Approximately 80–90% of what we perceive as “taste” actually is due to the sense of smell. This accounts for the dulled taste of food when we have a head cold or a stuffed-up nose. Our sense of smell depends on 10 to 20 million olfactory cells located in olfactory epithelia high in the roof of the nasal cavity (Fig. 15.5). Olfactory cells are modified neurons. Each cell ends in a tuft of about five olfactory cilia, which bear receptor proteins for odor molecules.
Figure 15.5 The sense of smell. a. The olfactory epithelium in humans is located high in the nasal cavity. b. Olfactory cells end in cilia that have receptor proteins for specific odor molecules. The cilia of each olfactory cell can bind to only one type of odor molecule (signified here by color). If a rose causes olfactory cells to be stimulated by “blue” and “green” odor molecules, then neurons designated by blue and green in the olfactory bulb are activated. The primary olfactory area of the cerebral cortex interprets the pattern of stimulation as the scent of a rose.
How the Brain Receives Odor Information
Each olfactory cell has only 1 out of an estimated 1,000 different types of receptor proteins. Nerve fibers from similar olfactory cells lead to the same neuron in the olfactory bulb (an extension of the brain). An odor contains many odor molecules, which activate a characteristic combination of receptor proteins. For example, a rose may stimulate olfactory cells, designated by the blue and green dots in Figure 15.5, whereas a dandelion may stimulate a different combination. An odor’s signature in the olfactory bulb is determined by which neurons are stimulated. When the neurons communicate this information via the olfactory tract to the olfactory areas of the cerebral cortex, we know we have smelled either a rose or a carnation.
The olfactory cortex is located in the temporal lobe. Some areas of the olfactory cortex receive smell sensations, and other areas contain olfactory memories.
Page 312Have you ever noticed that a certain aroma vividly brings to mind a certain person or place and can re-create emotions you feel about that person or place? For example, a smell of a certain food may remind you of a favorite vacation. This is because the olfactory bulbs have direct connections with the limbic system and its centers for emotion and memory (see Section 14.3). One investigator showed that when subjects smelled an orange while viewing a painting, they later recalled memories of the painting more vividly and had many deep feelings about the painting.
The number of olfactory cells declines with age. This can be dangerous if an older person can’t smell smoke or a gas leak. Older people also tend to apply excessive amounts of perfume or cologne before they can detect its smell.
CHECK YOUR PROGRESS 15.3
Identify the structures of the tongue and nose involved in the senses of taste and smell.
Answer
Taste cells within the taste buds on the tongue are chemoreceptors that detect food molecules. Olfactory cells within the olfactory epithelium of the nasal cavity are modified neurons that detect odor molecules.

Compare and contrast the function of the chemoreceptors on the tongue and in the nose.
Answer
They both respond to chemical stimuli. In the tongue, there are five main types of taste receptors, and the stimulus is direct. In the nose, there are olfactory cells with about 1,000 types of receptors, and the stimulus can be distant.

Summarize the pathway of sensory information regarding taste and smell from the receptors to the brain.
Answer
Nerve signals generated by taste receptors go to the gustatory cortex in the parietal lobe of the brain where the sensation of taste occurs. In the nasal cavity, odor molecules stimulate olfactory cells to activate neurons in the olfactory bulb of the brain, which sends the information to the cerebral cortex where smells are perceived.

CONNECTING THE CONCEPTS
For more information on chemoreceptors, refer to the following discussions:
Section 10.5 describes the function of the respiratory center in the medulla oblongata.
Section 14.3 explains the role of the limbic system in maintaining memories, such as smell and taste.

Question 524

1. The process of hearing begins when sound waves enter the auditory canal.
   
   • Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules.
     
     1. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly.
        
        • As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes.
        • The malleus is attached to the inner wall of the tympanic membrane.
     2. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes.
     3. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles.
        
        • The pressure is multiplied about 20 times.
     4. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate.
     5. In this way, the pressure is passed to the fluid within the cochlea.

Answer 524

Through the Auditory Canal and Middle Ear

Answer 525

Questions to Consider

Given that noise pollution induces stress, what other body systems may be affected?

At a local level, what do you think could be done to curb noise pollution in your neighborhood?

Question 526

Rotational Equilibrium Pathway

Mechanoreceptors in the semicircular canals detect rotational and/or angular movement of the head—rotational equilibrium (Fig. 15.14a). The three semicircular canals are arranged so there is one in each dimension of space. The base, or ampulla, of each of the three canals is slightly enlarged. Little hair cells, whose stereocilia are embedded within a gelatinous material called a cupula, are found within the ampullae. Each ampulla responds to head rotation in a different plane of space because of the way the semicircular canals are arranged. As fluid within a semicircular canal flows over and displaces a cupula, the stereocilia of the hair cells bend. This causes a change in the pattern of signals carried by the vestibular nerve to the brain. The brain uses information from the hair cells within each ampulla of the semicircular canals to maintain equilibrium. Appropriate motor output to various skeletal muscles can correct our present position in space as needed.

Why does spinning around cause you to become dizzy? When we spin, the cupula slowly begins to move in the same direction we are spinning, and bending of the stereocilia causes hair cells to send messages to the brain. As time goes by, the cupula catches up to the rate we are spinning, and the hair cells no longer send messages to the brain. When we stop spinning, the slow-moving cupula continues to move in the direction of the spin and the stereocilia bend again, indicating we are moving. Yet the eyes know we have stopped. The mixed messages sent to the brain cause us to feel dizzy.Page 323

Question 527

reversed prompt

Answer 527

15.1 Overview of Sensory Receptors and SensationsLEARNING OUTCOMESUpon completion of this section, you should be able toList the four categories of sensory receptors and describe what stimulus each responds to.Distinguish between perception and sensation.Explain the purpose of integration and sensory adaptation.

Question 528

Reverse.Prompt

Auditory Pathway to the Brain

Answer 528

1. auditory canal.
2. Thereafter, hearing requires the other parts of the ear,
3. the cochlear nerve, and the
4. brain.

Question 529

• 2. Layers of the eye: Choroid •
• Choroid – middle layer that absorbs light rays not absorbed by the retina –
• Iris: donut-shaped, colored structure that regulates the size of the pupil –
• Ciliary body: structure behind the iris that contains a muscle that controls the shape of the lens
• • Lens – attached to the ciliary body; refracts and focuses light rays

Question 530

reversed prompt

Answer 530

Key Concepts to Focus On • What are sensory receptors? • How do we detect the sense of taste and smell? • What is the anatomy of the eye? • How do we focus images? • What are some eye abnormalities? • What is the anatomy of the ear? • Which parts function in balance and which parts function in hearing

Question 531

reversed prompt

2. Divisions of the ear: Middle ear • Includes – Tympanic membrane (eardrum): membrane that vibrates to carry sound waves to the bones – Ossicles (malleus, incus, stapes): 3 small bones that amplify sound waves – Auditory tube/pharyngotympanic tube (previously known as Eustachian tube): a tube that connects from the throat to the middle ear and is used to equalize pressure so the eardrum does not burst • This tube has a 45 degree tilt in adults, but only 10 degree tilt in children, so with a more horizontal angle, kids get more ear infections

Question 532

reversed prompt

Question 533

Interoceptors are directly involved in this process whose steps are listed below:

blood pressure rises, baroreceptors signal a regulatory center in the brain.
The brain responds by sending out nerve signals to the arterial walls, causing their smooth muscle to relax.
The blood pressure then falls.

* Once blood pressure is returned to normal, the baroreceptors are no longer stimulated.

* Tutorial: Negative Feedback

Answer 533

homeostasis and are regulated by a negative feedback mechanism (see Fig. 4.16).

Question 534

Reverse.Prompt

Through the Auditory Canal and Middle Ear

Answer 534

1. The process of hearing begins when sound waves enter the auditory canal.
   
   • Just as ripples travel across the surface of a pond, sound waves travel by the successive vibrations of molecules.
     
     1. Ordinarily, sound waves do not carry much energy. However, when a large number of waves strike the tympanic membrane, it moves back and forth (vibrates) ever so slightly.
        
        • As you know, the auditory ossicles attach to one another: malleus to incus, incus to stapes.
        • The malleus is attached to the inner wall of the tympanic membrane.
     2. Thus, vibrations of the tympanic membrane cause vibration of the malleus and, in turn, the incus and stapes.
     3. The magnitude of the original pressure wave increases significantly as the vibrations move along the auditory ossicles.
        
        • The pressure is multiplied about 20 times.
     4. Finally, the stapes strikes the membrane of the oval window, causing it to vibrate.
     5. In this way, the pressure is passed to the fluid within the cochlea.

Question 535

Reverse.Prompt

Figure 15.12

• 1. three divisions: outer, middle, and inner.
• The outer ear consists of the pinna (external flap) and the auditory canal. ​​
  
  • The opening of the auditory canal is lined with fine hairs and sweat glands.
  • Modified sweat glands are located in the upper wall of the canal.
    
    • They secrete earwax, protective

Answer 535

Anatomy and Physiology of the Ear