Sensory Physiology - Textbook

Question 1

sensory receptor?

Answer 1

We use the term sensory receptor to refer to a cell that is specialized to detect incoming sensory stimuli.

Question 2

What are the ways of classifying sensory receptors?

Answer 2

1. Method One:
   Type of Stimulus: Sensory receptors are often categorized by the type of stimulus they detect:
   Mechanoreceptors: Respond to mechanical forces such as pressure, vibration, and touch.
   Thermoreceptors: Detect changes in temperature.
   Nociceptors: Sensitive to pain-causing stimuli.
   Photoreceptors: Detect light (found in the eyes).
   Chemoreceptors: Respond to chemical stimuli, including taste and smell.
2. Location: Receptors can also be classified based on their location in relation to the stimulus and the body:
   Exteroceptors: Detect external stimuli, such as skin receptors for touch, temperature, and pain.
   Interoceptors: Detect internal stimuli, providing information about internal conditions (e.g., pH, oxygen levels).
   Proprioceptors: Found in muscles, tendons, and joints, these provide a sense of body position and movement.

(they can also be classified by Structure or rate of adaptation sometimes.)

Question 3

adequate stimulus?

Answer 3

The preferred or most sensitive type of stimulus for a receptor.

Question 4

polymodal receptors?

–>Most common type in humans?

Answer 4

Receptors that can detect more than one class of stimulus.

–> Most common type is Nociceptors, which detect pain.

Question 5

For the following stimuli, determine whether the receptor involved is a mechanoreceptor, a chemoreceptor, or a photoreceptor: (a) blood oxygen, (b) acceleration, (c) light, (d) sound waves, (e) blood glucose.

Answer 5

For the given stimuli, the types of sensory receptors involved are as follows:

(a) Blood oxygen - Chemoreceptor: These receptors detect changes in the chemical composition, such as oxygen levels in the blood.

(b) Acceleration - Mechanoreceptor: Specifically, these are found within the vestibular system in the inner ear, helping to sense motion and balance through the detection of mechanical forces caused by movement and gravity.

(c) Light - Photoreceptor: These are located in the retina of the eye and are specifically designed to detect light.

(d) Sound waves - Mechanoreceptor: These are found in the inner ear where they detect mechanical vibrations caused by sound waves.

(e) Blood glucose - Chemoreceptor: These receptors detect chemical changes, including the concentration of glucose in the blood.

Question 6

For the following stimuli, determine whether the receptor involved is an interocepter, proprioceptor, or exteroceptor: (a) blood oxygen, (b) acceleration, (c) light, (d) sound waves, (e) blood glucose.

Answer 6

(a) Blood oxygen - Interoceptor: These receptors detect changes within the body’s internal environment, such as the oxygen levels in the blood.

(b) Acceleration - Proprioceptor: These receptors are involved in sensing the position and movement of the body, particularly through the vestibular system which helps in maintaining balance and orientation.

(c) Light - Exteroceptor: These receptors respond to stimuli originating outside the body, such as light affecting the eyes.

(d) Sound waves - Exteroceptor: These receptors also detect external stimuli, in this case, sound waves impacting the ears.

(e) Blood glucose - Interoceptor: Like those that monitor blood oxygen, these receptors detect internal metabolic conditions, specifically glucose levels in the blood.

Question 7

population coding?

Answer 7

Population coding is a neural mechanism where groups of neurons collectively represent information, enhancing the brain’s ability to process complex stimuli. Each neuron within a population may respond differently to a stimulus, contributing to a richer overall representation. This coding strategy increases the accuracy and stability of neural responses by averaging out individual neuron variability and provides redundancy, which ensures reliability even if some neurons fail. Population coding is crucial for efficient and robust information processing, allowing the brain to handle diverse sensory, motor, and cognitive tasks.

Question 8

How does lateral inhibition improve acuity?

Answer 8

Lateral inhibition is a process by which neurons use their network connections to enhance contrast in the information being processed, thereby improving sensory acuity. This occurs when a neuron firing actively inhibits its neighbors, making its own signal appear stronger in comparison. For example, in the visual system, cells in the retina help sharpen images by inhibiting the response of surrounding cells to light. This inhibition enhances the boundaries between regions of different light intensities, significantly improving the clarity and focus of the visual image. Essentially, lateral inhibition helps the sensory systems emphasize differences in the sensory input, which enhances the perception of fine details.

Question 9

Threshold of Detection?

Answer 9

The weakest stimulus that produces a response in a receptor 50 percent of the time.

Question 10

range fractionation?

Answer 10

Having multiple different receptors each with a different range of stimulus detection ability, in order to have some receptors covering all parts of the possible spectrum.

Question 11

Tonic vs Phasic Receptors?

Answer 11

Tonic receptors are slow-adapting receptors that respond continuously to a stimulus as long as it persists. They provide constant feedback about the duration and intensity of a stimulus, making them essential for detecting and monitoring stimuli that require sustained attention, such as pressure, pain, or joint position.

Phasic receptors are fast-adapting receptors that respond quickly to changes in a stimulus but then quickly decrease their firing rate if the stimulus remains constant. Phasic receptors are particularly useful for detecting changes or new events in the environment, such as the start or end of a touch or sound. They help to signal changes in sensory information rather than the steady state of a stimulus, allowing organisms to react to dynamic changes around them.

Question 12

How does lateral inhibition enhance contrast?

Answer 12

Lateral inhibition is when a neuron is activated by a stimulus and sends inhibitory signals to the neurons around it. This enhances contrast because the neuron that stays activated will pinpoint a much more precise spot for the stimulus activation.

Question 13

Explain the advantages of encoding sensory signals logarithmically.

Answer 13

Logarithmic encoding compresses the wide range of possible input intensities into a manageable scale.

By encoding intensity logarithmically, sensory systems can remain sensitive to smaller changes in stimulus intensity even when those changes occur at high absolute levels.

Logarithmic encoding helps the brain use its limited number of neurons and synaptic connections more efficiently.

Question 14

vomeronasal organ?

Answer 14

The vomeronasal organ (VNO), also known as Jacobson’s organ, is a specialized part of the olfactory system in many vertebrates, primarily used for detecting pheromones, chemicals that carry information between individuals of the same species. Located at the base of the nasal cavity, the VNO is separate from the main olfactory system and is particularly well-developed in animals like reptiles and rodents, though its presence and functionality in humans are subjects of ongoing research and debate. The VNO senses chemical signals involved in social and reproductive behaviors, such as aggression, territoriality, and mating, contributing crucially to an animal’s communication repertoire.

Question 15

odorant-binding proteins?

Answer 15

Odorant-binding proteins (OBPs) are crucial for the function of the olfactory system, primarily by facilitating the transport and detection of odorant molecules within the nasal cavity. These soluble proteins bind to hydrophobic odor molecules, transporting them through the aqueous environment of the nasal mucus to the olfactory receptors on sensory neurons. OBPs enhance the sensitivity and specificity of olfactory detection by protecting the odorants from degradation, selectively binding different odorants, and possibly modulating olfactory receptor responses. This makes them key players in the precise and efficient perception of smells.

They’re thought to be involved in allowing lipophilic odorant molecules to dissolve in the aqueous mucus layer.

Question 16

The olfactory system codes information by using what is termed a combinatorial code. What are the advantages of using a combinatorial code to detect incoming chemical stimuli?

Answer 16

The olfactory system utilizes a combinatorial code to detect and interpret the vast array of chemical stimuli in the environment, a process that involves the activation of multiple types of olfactory receptors by a single odorant molecule. This coding strategy offers several advantages: it allows for a high degree of sensitivity and specificity in odor detection, enabling the differentiation between thousands of unique odors even when they share similar chemical properties. Additionally, the combinatorial nature of this code means that slight changes in the chemical structure of odorants can be distinctly recognized, providing a nuanced perception of smells. This complexity and precision in odor recognition enhance an organism’s ability to make fine distinctions in their environment, which is critical for survival and reproduction.

Question 17

What would happen to the ability to smell if a drug that inhibited adenylate cyclase were applied to the olfactory epithelium of a vertebrate? Would this drug affect the sensing of pheromones if applied to the vomeronasal epithelium?

Answer 17

Administering a drug that inhibits adenylate cyclase in the olfactory epithelium of a vertebrate would likely impair the ability to smell. Adenylate cyclase is crucial for the transduction of olfactory signals; it catalyzes the conversion of ATP to cyclic AMP (cAMP), which then opens ion channels allowing for the influx of ions that create a depolarizing current, leading to an action potential. By inhibiting this enzyme, the signal transduction pathway is disrupted, reducing or possibly eliminating the ability to detect odors.

Regarding the effect of such a drug on the vomeronasal epithelium, which is primarily involved in detecting pheromones, the impact might be different. The vomeronasal organ (VNO) uses a distinct signal transduction mechanism that does not typically rely on adenylate cyclase and cAMP for signal processing. Instead, the VNO primarily utilizes a phospholipase C (PLC) pathway, leading to the production of inositol trisphosphate (IP3) and diacylglycerol (DAG), and a subsequent increase in intracellular calcium levels. Therefore, a drug that inhibits adenylate cyclase would likely not affect the sensing of pheromones if applied to the vomeronasal epithelium, as this pathway does not involve adenylate cyclase for signal transduction.

Question 18

True or False?

The gustatory (taste) system is able to discriminate between more molecules than the olfactory system (smell)

Answer 18

FALSE!

(he gustatory system (taste) is NOT able to discriminate among thousands of different molecules, and the OLFACTORY system IS able to!

Question 19

G protein gustducin?

Answer 19

Gustducin is a type of G protein specifically involved in taste signal transduction, primarily associated with detecting bitter, sweet, and umami flavors. Found in taste receptor cells on the tongue, gustducin activates upon the binding of tastants to their respective G protein-coupled receptors. Once activated, gustducin initiates a cascade of intracellular events, including the activation of phosphodiesterases and changes in ion channel activity, ultimately leading to neuronal depolarization and the transmission of taste signals to the brain. This process enhances the ability to distinguish and respond to various chemical compounds in food.

Question 20

True or False?
Unlike olfactory receptor cells, which are bipolar sensory neurons, taste receptor cells are epithelial cells that release neurotransmitter onto a primary afferent neuron.

Answer 20

True!

Question 21

Compare and contrast olfaction and gustation in vertebrates.

Answer 21

Olfaction and gustation are both chemical sensing systems in vertebrates but serve different primary functions and operate through distinct mechanisms. Olfaction, or smell, detects airborne chemical molecules through receptors in the olfactory epithelium located in the nasal cavity. This system can identify a vast array of odors due to a large number of receptor types, each sensitive to different molecular features, and employs a combinatorial coding scheme that allows for nuanced perception of complex scents.

Gustation, or taste, detects chemicals dissolved in saliva through taste buds primarily located on the tongue. It is generally limited to recognizing five basic tastes: sweet, sour, salty, bitter, and umami. Each taste is sensed by specific receptors (ion channels or G protein-coupled receptors) which, unlike in olfaction, correspond directly to a limited set of perceived qualities. Taste signals are less complex compared to smell and are critical for evaluating food and drink for ingestion and digestion.

Question 22

How would the response of a taste receptor cell differ between a food that is slightly salty and a food that is very salty? How would this affect action potential generation in the afferent neuron?

Answer 22

The response of a taste receptor cell to different concentrations of salt varies primarily in the intensity and frequency of the action potentials generated. For a food that is slightly salty, the salt concentration activates specific receptors (likely ion channels sensitive to sodium ions) on the taste receptor cells but at a lower level. This results in a relatively lower frequency of action potentials in the afferent neuron connected to these taste cells.

In contrast, a food that is very salty provides a higher concentration of sodium ions, leading to a more robust activation of these same receptors. This results in a higher influx of sodium ions into the taste receptor cells, generating a stronger depolarization and subsequently a higher frequency of action potentials.

The rate of action potential generation is crucial because it codes for the intensity of the taste stimulus. A higher frequency of action potentials typically signifies a more intense taste sensation, which in the case of salt, translates to a saltier taste. Therefore, the brain interprets the increased firing rate as a stronger salty flavor. This differential response in action potential frequency allows the nervous system to discern not just the type of taste but also its concentration or intensity.

Question 23

What are the two main types of mechanoreceptor proteins in animals?

Answer 23

The two main types of mechanoreceptor proteins in animals:

1. ENaC (epithelial sodium channels)
2. TRP (transient receptor potential) channels

Question 24

1. Baroreceptors?
2. Tactile Receptors?
3. Propreoceptors?

Answer 24

1. Baroreceptors are specialized sensory neurons located primarily in the walls of the carotid arteries and the aorta. They function as mechanoreceptors that detect changes in blood pressure by sensing the stretch of the blood vessel walls. When blood pressure rises, the increased stretching of these arteries stimulates the baroreceptors to send more frequent action potentials to the brain, particularly to the cardiovascular control centers in the medulla oblongata. Conversely, a decrease in blood pressure reduces this stretch and the rate of action potential firing decreases. The brain responds to these signals by adjusting heart rate, blood vessel dilation, and overall cardiovascular tone to maintain stable blood pressure levels, playing a critical role in the homeostatic regulation of the circulatory system.
2. Tactile receptors are specialized sensory structures in the skin and other tissues that detect mechanical stimuli such as touch, pressure, vibration, and stretch. These receptors vary in their structure and function, which determines their sensitivity to different kinds of tactile information. For example, Meissner’s corpuscles are sensitive to light touch and small vibrations, while Pacinian corpuscles detect deeper pressure and higher frequency vibrations. Merkel cells respond to steady pressure and texture, and Ruffini endings sense skin stretch and sustain pressure. Each type of receptor contributes to the overall sense of touch, allowing organisms to perceive and interact with their environment effectively.
3. Proprioceptors are sensory receptors found primarily in muscles, tendons, joints, and the inner ear. They play a critical role in providing the central nervous system with information about body position and movement. This sensory feedback enables the maintenance of balance and posture, coordination of movements, and the sense of body position in space. Examples of proprioceptors include muscle spindles, which detect changes in muscle length; Golgi tendon organs, which sense changes in muscle tension; and joint receptors, which provide information about joint position and movement. This feedback is essential for executing smooth and coordinated voluntary actions.

Question 25

Pacinian corpuscles?

Answer 25

Pacinian corpuscles are specialized mechanoreceptors that detect transient pressure and high-frequency vibration. They are located deep in the skin, in joint capsules, and around some organs. Each Pacinian corpuscle consists of a nerve ending surrounded by concentric layers of collagen, resembling the layers of an onion. These layers are highly responsive to rapid changes in pressure, enabling the detection of vibrations and touch stimuli that quickly come and go. When pressure is applied and then changes or moves, it causes deformation of the corpuscle’s layers, mechanically gating the ion channels of the neuron, leading to depolarization and the initiation of an action potential. This makes Pacinian corpuscles one of the fastest-adapting receptors in the sensory system, capable of detecting sudden disturbances transmitted through soft tissues.

Question 26

1. ___________ on the surface of skeletal muscles monitor the length of the muscle. Each muscle spindle consists of modified muscle fibers called intrafusal fibers enclosed in a connective tissue capsule.
2. ______________ are located at the junction between a skeletal muscle and a tendon. These receptors are stimulated by changes in the tension in the tendon.
3. _____________ are located in the capsules that enclose the joints. Several types of receptors are in this category, including receptors similar to free nerve endings, Pacinian corpuscles, and Golgi tendon organs. These receptors detect pressure, tension, and movement in the joint.

Answer 26

1. Muscle spindles on the surface of skeletal muscles monitor the length of the muscle. Each muscle spindle consists of modified muscle fibers called intrafusal fibers enclosed in a connective tissue capsule.
2. Golgi tendon organs are located at the junction between a skeletal muscle and a tendon. These receptors are stimulated by changes in the tension in the tendon.
3. Joint capsule receptors are located in the capsules that enclose the joints. Several types of receptors are in this category, including receptors similar to free nerve endings, Pacinian corpuscles, and Golgi tendon organs. These receptors detect pressure, tension, and movement in the joint.

Question 27

What are possible advantages of having both tonic and phasic touch receptors in the skin of vertebrates?

Answer 27

Tonic receptors provide continuous feedback about the presence of stimuli. They adapt slowly to a stimulus, which means they can signal the duration and steady presence of pressure or touch. This is essential for tasks that require sustained grip or continuous monitoring of object placement, such as holding a tool or maintaining balance.

Phasic receptors, on the other hand, respond rapidly but briefly to changes in stimulation, quickly adapting to a constant stimulus. They are particularly useful for detecting changes in our environment, such as the initial contact or release of an object. This helps in tasks requiring fine motor skills, like typing or manipulating small objects, where the onset and cessation of touch need to be precisely registered.

Together, these receptors allow vertebrates to integrate detailed information about both static and dynamic changes in their surroundings. This combination enhances sensory perception, providing a comprehensive understanding of tactile stimuli that aids in complex behaviors, improves spatial awareness, and heightens overall survival capabilities.

Question 28

Why do insects have complex touch organs, rather than isolated sensory neurons associated with the body surface as in mammals?

Answer 28

Insects have complex touch organs because their exoskeletal structure necessitates specialized adaptations for sensing their environment. Unlike mammals, which have their sensory neurons spread across the soft and flexible skin, insects possess a hard, rigid exoskeleton that does not itself have sensory capabilities. As a result, insects require specialized structures like setae (hair-like structures) and campaniform sensilla (dome-shaped structures) which are embedded in the exoskeleton but connected to sensory neurons. These organs amplify mechanical stimuli from the environment and transmit these signals to the nervous system, allowing the insect to detect touch, pressure, vibrations, and even air currents, crucial for navigation, predator avoidance, and communication. This setup enables insects to maintain acute sensory perception despite their protective, non-flexible outer layer.

Question 29

statocysts?

Answer 29

Many invertebrates have organs called statocysts that they use to detect the orientation of their bodies with respect to gravity.

Statocysts are hollow, fluid-filled cavities that are lined with mechanosensory neurons, and contain dense particles of calcium carbonate called statoliths.

When the orientation of the animal changes, the statolith moves across the sheet of mechanoreceptors.

Each statocyst is composed of a globelike structure called the macula, and three cristae, each oriented in a different plane. The cristae and macula contain statoliths that move in response to mechanical stimuli.

Question 30

tympanal organs?

Answer 30

The most sensitive insect ears are called tympanal organs. A tympanal organ consists of a very thin region of the cuticle, called the tympanum, located over an air space similar to the air space in a drum.

Sound waves cause the thin tympanum to vibrate, causing the air within the air space to vibrate. A chordotonal organ in this air space detects these vibrations, and sends signals in the form of action potentials to the nervous system.

(Tympanal organs are found on many locations on the insect body, including the legs, abdomen, thorax, and wing base!)

Question 31

kinocilium vs stereocilia?

Answer 31

Most vertebrate hair cells have a single long cilium, the kinocilium, and many shorter projections, called stereocilia.

Question 32

tip link?

Answer 32

The stereocilia are connected to each other and the kinocilium by a series of small fibers that cause the bundle of hair cells to act as a single unit. One particular type of these fibers, called a tip link, connects the top of each shorter stereocilium to the side of the adjacent taller one.

Question 33

neuromasts?

Answer 33

Neuromasts are sensory organs found primarily in aquatic vertebrates, including fish and some amphibians, as part of their lateral line system. These organs are key for detecting water currents and vibrations, which helps these animals navigate, orient themselves, hunt prey, and avoid predators in their environment. Neuromasts can be located both externally on the skin and internally in canals within the body.

Each neuromast consists of a cluster of hair cells, similar to those found in the inner ear of vertebrates, surrounded by support cells and gelatinous structures called cupulae. The hair cells contain hair-like projections (stereocilia and kinocilia) that extend into the cupula. When water motion or pressure changes displace the cupula, it causes the stereocilia to bend, thereby converting mechanical stimulus into electrical signals that are transmitted to the brain via sensory nerves. This allows the fish to detect and respond to changes in their aquatic environment effectively.

Question 34

Outer Ear vs Middle Ear vs Inner ear? (brief description)

Answer 34

The external structures are called the outer ear and in mammals consist of the pinna, which forms the distinctive shapes of mammalian ears, and the auditory canal. The auditory canal leads to the middle ear, which contains a series of small bones that transfer sound waves to the inner ear. The inner ear is embedded within the skull and consists of a series of fluid-filled membranous sacs and canals.

Question 35

vestibular apparatus?

Answer 35

The vestibular apparatus of the inner ear detects movements or changes in body position.

Question 36

utricle and saccule?

Answer 36

The utricle and saccule are two of the major components of the vestibular system located in the inner ear, primarily responsible for detecting linear accelerations and head position relative to gravity. These structures help with balance and spatial orientation in vertebrates.

Utricle: This is slightly larger than the saccule and is oriented horizontally when the head is in an upright position. The utricle is sensitive primarily to horizontal movements and the effect of gravity when the head tilts. It plays a crucial role in detecting acceleration and deceleration movements, such as when starting or stopping walking.

Saccule: Located vertically when the head is upright, the saccule responds to vertical movements and contributes to the perception of verticality and vertical acceleration, such as when going up or down in an elevator.

Question 37

In most vertebrates, the saccule also contains a small extension called the ________. In birds and mammals, the lagena is greatly extended and is called the cochlear duct (in birds), or the _______ (in mammals)

Answer 37

In most vertebrates, the saccule also contains a small extension called the lagena. In birds and mammals, the lagena is greatly extended and is called the cochlear duct (in birds), or the cochlea (in mammals)

Question 38

The utricle and saccule contain a series of mineralized _____.

Answer 38

The utricle and saccule contain a series of mineralized otoliths.

Question 39

tympanic membrane

Answer 39

The tympanic membrane, also known as the eardrum, is a thin, cone-shaped membrane that separates the external ear from the middle ear and vibrates in response to sound waves.

Question 40

oval window

Answer 40

The oval window is a membrane-covered opening that leads from the middle ear to the inner ear. It receives vibrations from the last of the three small bones in the middle ear—the stapes—and transmits these vibrations to the fluid of the cochlea in the inner ear.

Question 41

malleus, incus and stapes?

Answer 41

The malleus, incus, and stapes, commonly known as the hammer, anvil, and stirrup, respectively, are the three tiny bones of the middle ear. They form a connected chain that transmits and amplifies sound from the tympanic membrane to the oval window. The malleus attaches to the tympanic membrane, receives vibrations from it, transfers these vibrations to the incus, which in turn passes them to the stapes, and finally, the stapes pushes against the oval window to transmit sounds into the inner ear.

Question 42

What’s different about Cetacean ears, compared to normal mammal ears?

Answer 42

Cetacean outer ears do not perform the sound collecting and amplifying function that is typical of the ears of land mammals. Cetacean ears do not have pinnae, the ear canal is small and plugged with debris and wax, and it does not connect to the tympanic membrane.

Question 43

perilymph?

Answer 43

Perilymph is the fluid that fills the space within the bony labyrinth, which houses the cochlea, semicircular canals, utricle, and saccule of the inner ear. This fluid is similar in composition to cerebrospinal fluid and is located in the scala tympani and scala vestibuli of the cochlea. Perilymph facilitates the transmission of sound vibrations from the oval window to the cochlear duct and helps to cushion the inner ear’s delicate structures.

Question 44

endolymph

Answer 44

Endolymph is the fluid within the membranous labyrinth, which is enclosed by the bony labyrinth. It fills the cochlear duct (scala media), the central part of the cochlea, and the membranous portions of the vestibular system. Endolymph is unique in that it has a high potassium ion concentration, contrasting with the high sodium ion concentration of perilymph. This ionic difference is crucial for the electrochemical processes that enable hearing and balance.

Question 45

organ of Corti

Answer 45

The Organ of Corti is the sensory organ of hearing, located within the cochlear duct (scala media) of the cochlea on the basilar membrane. It contains hair cells, which are the actual sensory receptors. These hair cells are mechanically stimulated by the vibration of the basilar membrane caused by sound waves, and they convert these mechanical vibrations into electrical signals that are sent to the brain via the auditory nerve.

Question 46

basilar membrane?

Answer 46

The Basilar Membrane is a narrow, flexible membrane that runs the length of the cochlea, separating the scala tympani from the scala media. It supports the Organ of Corti and plays a key role in the auditory process. Its mechanical properties vary along its length (being stiffer at the base and more flexible at the apex), which allows it to resonate at different frequencies along its length. This mechanical property helps in the tonotopic mapping of sound frequencies, where higher frequencies cause peak vibrations closer to the base and lower frequencies peak towards the apex. This differential movement is critical for the perception of pitch.

Question 47

inner hair cells
and
outer hair cells

Answer 47

These two hair cells line the vertebrate inner ear.

Inner hair cells detect sounds, and outer hair cells help to amplify sounds.

Question 48

round window of the cochlea

Answer 48

The round window of the cochlea is a flexible, membrane-covered opening in the bone of the cochlea in the inner ear. It serves as a secondary boundary for the perilymphatic fluid that fills the cochlea. Located just below the oval window, the round window plays a critical role in the functioning of the inner ear by allowing the fluid inside the cochlea to move. When sound waves travel through the cochlea and cause the oval window to move inwards, the round window bulges outward. This complementary movement helps to relieve pressure within the cochlea, enabling the fluid to transmit the sound vibration effectively throughout the inner ear structures for proper auditory processing. The round window thus acts as a pressure release valve, ensuring that the fluid vibrations caused by incoming sounds can propagate along the basilar membrane without any back pressure that would otherwise dampen the sensory response.

Question 49

place coding?

Answer 49

Place coding is a mechanism in the auditory system where different frequencies of sound waves are detected by specific regions of the cochlea in the inner ear. This coding occurs on the basilar membrane within the cochlea, which is structured to be more responsive to high frequencies at its base and to low frequencies at its apex. As sound waves enter the cochlea, they generate traveling waves along the basilar membrane, peaking at locations specific to each frequency due to variations in the membrane’s stiffness and width. Hair cells located at these peak points convert the mechanical wave into nerve impulses, which are then transmitted to the brain. The brain interprets these signals based on where they originated along the cochlea, allowing it to distinguish different sound frequencies through this tonotopic organization.

Question 50

What would happen to sound transduction if the endolymph of the vertebrate inner ear had high Na+ and low K+?

Answer 50

If the endolymph of the vertebrate inner ear had high Na+ and low K+, it would significantly disrupt sound transduction. Normally, the endolymph has an unusually high concentration of K+ and a very low concentration of Na+, which is critical for the normal functioning of hair cells within the cochlea.

In the typical scenario, when sound waves cause the basilar membrane to move, the hair cells’ stereocilia deflect, leading to the opening of mechanically-gated ion channels at their tips. These channels are selectively permeable to K+, allowing K+ ions from the high-K+ endolymph to flow into the relatively low-K+ cytoplasm of the hair cells. This influx of K+ causes depolarization of the hair cell, leading to neurotransmitter release at the base of the hair cell, which then stimulates the auditory nerve fibers.

If the ionic composition of the endolymph were reversed to high Na+ and low K+, the usual ion gradient needed for hair cell depolarization would be altered. Na+ ions might enter the cells instead of K+, but because hair cells are specifically adapted to use K+ for depolarization, the normal depolarization process would be inefficient or might not occur at all. This could lead to a failure in neurotransmitter release and, consequently, no transmission of auditory signals to the brain. Essentially, hearing would likely be impaired or completely lost if the ionic gradients were reversed in this manner.

Question 51

How does the structure of the basilar membrane of the mammalian ear allow fine discrimination of different sound frequencies?

Answer 51

The basilar membrane of the mammalian ear plays a crucial role in the ability to discriminate different sound frequencies due to its unique structure and mechanical properties. It stretches from the base to the apex of the cochlea, varying in width and stiffness along its length. This gradient in physical characteristics is key to its function in frequency discrimination.

At the base of the cochlea, the basilar membrane is narrower and stiffer, which makes it responsive to high-frequency sounds. As you move towards the apex of the cochlea, the membrane becomes wider and more flexible, making it more responsive to lower frequencies. This structural variation allows the basilar membrane to act as a frequency analyzer.

When sound waves enter the cochlea, they create waves within the cochlear fluid that travel along the basilar membrane. The point at which these waves peak and subsequently cause the greatest displacement of the membrane varies depending on the frequency of the sound. High-frequency sounds cause peak displacements closer to the base of the cochlea, while lower-frequency sounds peak closer to the apex.

This spatial variation in vibration along the basilar membrane means that different frequencies of sound stimulate different sets of hair cells located along its length. Each set of hair cells is connected to nerve fibers that transmit these specific frequency signals to the brain, where they are interpreted as sounds of varying pitches. This finely tuned structure allows for the precise discrimination of a wide range of sound frequencies, which is essential for tasks such as understanding speech and appreciating music.

Question 52

Ciliary vs Rhabdomeric photoreceptors?

Answer 52

Ciliary photoreceptors have a single cilium protruding from the cell, often with a highly folded ciliary membrane that forms lamellae or disks that contain photopigments, the molecules specialized for absorbing the energy coming from incoming photons.

In contrast, in rhabdomeric photoreceptors (also called microvillus photoreceptors) the apical surface that contains the photopigments is elaborated into multiple outfoldings called microvillar projections.

Ciliary Photoreceptors: These are primarily found in vertebrates. The light-detecting part of a ciliary photoreceptor, known as the outer segment, is formed from the membrane of a modified cilium. The outer segment contains stacks of membranous discs which house photopigments like rhodopsin. When light hits these pigments, it triggers a signal transduction cascade that ultimately leads to changes in the cell’s membrane potential. Ciliary receptors typically participate in image-forming vision and are highly efficient in detecting light due to their intricate organization and concentration of photopigments.

Rhabdomeric Photoreceptors: Commonly found in invertebrates such as insects and mollusks, these photoreceptors have a different structure. Their light-sensing part consists of microvilli – tiny, finger-like projections that extend from the cell body. These microvilli contain the photopigments responsible for light detection. Rhabdomeric photoreceptors are part of a larger structure called a rhabdom, where several photoreceptors aggregate their microvilli to increase light absorption. Unlike ciliary receptors, rhabdomeric ones often function more directly through modulation of ion channels by the light-activated photopigments, leading to a change in the cell’s membrane potential.

Question 53

rods and cones?

Answer 53

Rods are highly sensitive to light and function best in low-light conditions, making them crucial for night vision. They do not mediate color vision and have a relatively low spatial resolution, which means they are better at detecting motion and general shapes rather than detailed images. Rods are more numerous than cones in the human retina, with about 120 million rod cells concentrated mainly around the periphery.

Cones, on the other hand, require more light to function and are vital for color vision and seeing fine detail. Humans typically have three types of cones, each sensitive to different wavelengths of light (red, green, and blue), which allows for the perception of a wide range of colors. Cones also contribute to high visual acuity, enabling the detection of fine details. There are about 6 million cones in the human retina, mostly concentrated in the central fovea, the area of the retina responsible for sharp central vision.

Question 54

Retina vs Fovea?

Answer 54

The retina and the fovea are integral parts of the eye, each serving distinct roles in vision.

The retina is a thin layer of tissue that lines the back inside wall of the eye and is responsible for receiving and processing light. This sensory membrane contains millions of photoreceptor cells (rods and cones) that detect light and convert it into electrical signals. These signals are then transmitted to the brain via the optic nerve. The retina is crucial for all aspects of vision, including color perception, low-light vision, and detail resolution.

The fovea is a small, specialized region within the retina that is essential for sharp central vision, which is important for activities like reading, driving, and any task where visual detail is of primary importance. The fovea is densely packed with cones, the photoreceptors needed for detecting color and fine detail. This area lacks rods and is structured in such a way that other layers of the retina are displaced, allowing light to directly strike the cones without any dispersion, providing the clearest and most detailed visual input. The foveal region is surrounded by the macula, which also plays a key role in high-resolution vision.

Question 55

chromophore

Answer 55

A chromophore is a molecule, or part of a molecule, that is responsible for its color. This component of a molecule absorbs certain wavelengths of light and reflects or transmits others, which is what makes it visible to the human eye as colored. Chromophores play a crucial role in various biological, chemical, and technological applications.

In biological contexts, chromophores are key components of visual pigments in the photoreceptor cells of the eye. For example, the chromophore in the rods and cones of the retina is called retinal, a derivative of vitamin A. This particular chromophore undergoes a change in its structure when it absorbs light, triggering a series of biochemical reactions that eventually convert the light signal into a neural signal, a process central to vision.

Question 56

opsin?

Answer 56

Opsins are G protein–coupled receptors that are covalently linked to the chromophore.

All of these photopigments, however, consist of a vitamin A–derived chromophore bound to a G protein in the opsin gene family (rhodopsin, iodopsin, porphyropsin, melanopsin… etc)

Question 57

Compare and contrast phototransduction in rhabdomeric and ciliary photoreceptors.

Answer 57

Phototransduction in rhabdomeric and ciliary photoreceptors involves converting light into neural signals, but the mechanisms and structural components of these receptors differ significantly, reflecting their adaptation to different evolutionary paths and functional requirements.

Rhabdomeric photoreceptors, primarily found in invertebrates like mollusks and arthropods, use microvillar structures that increase their surface area, where the phototransductive machinery is located. These receptors typically employ a phosphoinositide signaling cascade. The activation of this pathway leads to the opening of ion channels that depolarize the cell, a mechanism that contrasts with how signals are processed in ciliary types.

Ciliary photoreceptors, on the other hand, are typical in vertebrates and are characterized by their ciliary structure, which originates from a modified cilium. These photoreceptors utilize a cyclic nucleotide (cGMP) cascade, where light exposure leads to a reduction in cGMP levels, causing ion channels to close and the cell to hyperpolarize. This difference in response to light (hyperpolarization versus depolarization) is a fundamental contrast between the two types.

Furthermore, ciliary receptors are often more directly involved in image-forming vision, with their structural setup allowing for a high degree of spatial resolution. Rhabdomeric receptors are sometimes linked to non-image-forming vision, like circadian photoentrainment, although they too can contribute to image-forming vision in some species.

Question 58

Compare and contrast the structure and function of rods and cones. Do all vertebrates have these photoreceptors?

Answer 58

Rods are slender and elongated cells, optimized for high sensitivity to light. They contain a stack of membrane discs that house the photopigment rhodopsin, which is extremely sensitive to light.
Rods are highly sensitive to light and are thus responsible for vision under low light conditions (scotopic vision). They do not mediate color vision and have low spatial acuity, but are very effective at detecting motion.

Cones are shorter and tapered, and they contain fewer membranous discs than rods. The discs in cones are embedded in the outer membrane rather than being detached, which may facilitate rapid recovery and recycling of photopigments. Cones contain photopigments known as cone opsins, which are less sensitive to light but provide color sensitivity and higher spatial resolution.
Cones require more light to function and thus contribute to daytime (photopic) and color vision. There are typically three types of cones in humans, each sensitive to different wavelengths of light (red, green, and blue), which allows for detailed color discrimination and high visual acuity.

Question 59

What are the 4 main types of eyes?

Answer 59

1. Flat sheet
2. Cup shaped
3. Vesicular
4. convex

Question 60

Flat sheet eyes?

Answer 60

Flat-sheet eyes contain a layer of photoreceptor cells that form a primitive retina lined with a pigmented epithelium. These eyes provide some sense of light direction, and may allow the detection of contrasts between light and dark. Many animal groups have eyes of this type, although they are most often seen in larval forms or as accessory eyes in adults. However, the limpet Patella has a simple patch of pigmented cells that serve as its primary eyes.

Question 61

Cup-shaped eyes?

Answer 61

Cup-shaped eyes are similar to flat-sheet eyes, except that the retinal sheet is folded to form a narrow aperture. These eyes provide much better discrimination of light direction and intensity, and allow improved detection of contrasts between light and dark. The most advanced cup-shaped eyes, such as those of the Nautilus, a cephalopod, have extremely small, pinhole-sized openings. The pinhole blocks most of the light from entering the eye so that an incoming point light source illuminates a single point on the retina, forming an image. This design is similar to a primitive type of camera called a pinhole camera. Pinhole camera eyes can form images, although the resolution is poor and the image is dim. In order to form a crisp image, the aperture (pinhole) must be small, but a small aperture lets in only a small amount of light, resulting in a dim image. Thus, there is a compromise between image clarity and image intensity.

Question 62

Vesicular eyes?

Answer 62

Vesicular eyes and modern cameras solve this conflict by inserting a lens into the pinhole aperture. A lens takes multiple sources of light and refracts them, focusing the light from a single source onto a single point on the retina. The challenge in developing a good vesicular eye is that the lens must fit precise specifications in order to provide a clear image. However, even a bad lens is better than no lens at all, and provides an improvement over a pinhole camera–type eye. Vesicular eyes are found in some mollusks, but only the cephalopod mollusks have the capacity to alter the shape or position of the lens to focus the image. Like cephalopods, vertebrates have complex vesicular eyes with a lens that can be used to generate a sharp, focused image.

Question 63

Convex eyes?

Answer 63

Convex eyes are present in many annelids, mollusks, and arthropods. In these eyes, the individual photoreceptors radiate outward from the base, forming a convex, rather than a concave, light-gathering surface.

The most complex convex eyes are the compound eyes of the arthropods.

Question 64

ommatidia?

Answer 64

Ommatidia are the individual units of compound eyes found in arthropods, such as insects and crustaceans. Each ommatidium is a miniature eye itself, complete with a cornea, a lens-like crystalline cone, light-sensitive photoreceptor cells forming a structure called the rhabdom, and pigment cells that isolate each ommatidium to minimize optical interference. This arrangement allows each ommatidium to capture light from a specific part of the visual field, contributing to a pixelated, mosaic view of the environment. Compound eyes excel in detecting motion and have a wide field of view, though they typically offer lower resolution than the camera-type eyes seen in vertebrates. This type of vision is particularly advantageous for navigation, predator avoidance, and prey capture among arthropods.

(like the eyes of flies!)

Question 65

sclera

Answer 65

The sclera is the opaque, fibrous, protective outer layer of the human eye that is commonly referred to as the “white of the eye.” It forms the supporting wall of the eyeball and is continuous with the clear cornea at the front of the eye. The sclera provides structural integrity and protection for the delicate components of the eye, maintaining its shape and safeguarding it from external injuries or shocks. It is composed mainly of collagen and elastic fiber, which give it both toughness and flexibility. The sclera also serves as an attachment point for the muscles that control eye movement, enabling the eye to look in different directions.

Question 66

aqueous humor vs vitreous humor?

Answer 66

Aqueous Humor: This is a clear, watery fluid that fills the space between the cornea and the lens, known as the anterior chamber of the eye. The aqueous humor is produced by the ciliary body, a structure located behind the iris. This fluid helps to maintain intraocular pressure, provides nutrients to the avascular structures of the eye (like the lens and cornea), and removes metabolic wastes. The aqueous humor flows through the pupil and drains out of the eye via the trabecular meshwork at the angle where the iris and cornea meet.

Vitreous Humor: This is a transparent, gel-like substance that fills the much larger space behind the lens, making up the bulk of the eye’s volume, known as the vitreous chamber. The vitreous humor is composed mostly of water mixed with collagen fibers and hyaluronic acid, giving it a jelly-like consistency. This structure helps to stabilize the eye and provide physical support to the retina. The vitreous humor also plays a role in maintaining the shape of the eye and, unlike aqueous humor, does not continually regenerate but stays relatively stable throughout life.

Question 67

What are the advantages of a vesicular eye compared with a pinhole-type eye?

Answer 67

The vesicular eye, equipped with a lens, excels in light gathering and image formation, allowing for high visual acuity and the ability to focus on objects at different distances. This type of eye adjusts its aperture to accommodate varying light conditions, enhancing vision in both bright and dim environments. In contrast, the pinhole-type eye, simpler in structure without a lens, provides a greater depth of field and lacks chromatic aberration, meaning all distances are moderately focused without color distortion. However, it cannot match the vesicular eye in terms of image detail and light sensitivity, making the latter more suitable for animals requiring precise visual abilities.

Question 68

In order to produce focused images of objects at various distances, the eye must ensure that the focal point falls on the retina, a process termed _______________. Because the location and shape of the cornea are fixed, the cornea does not participate in accommodation. Instead, the lens must either change position relative to the retina, or change shape.

Answer 68

In order to produce focused images of objects at various distances, the eye must ensure that the focal point falls on the retina, a process termed accommodation. Because the location and shape of the cornea are fixed, the cornea does not participate in accommodation. Instead, the lens must either change position relative to the retina, or change shape.

Question 69

True or false?

Humans lack the ability to sense other sensory modalities such as electrical and magnetic fields.

Answer 69

true!

Question 69

Would you expect laser eye surgery (which affects the shape of the cornea) to be effective in an aquatic vertebrate? Why or why not?

Answer 69

Laser eye surgery, which reshapes the cornea to correct refractive errors, would likely be ineffective in aquatic vertebrates. This is because water has a similar refractive index to the cornea, which minimizes the cornea’s role in focusing light in aquatic environments. Instead, the lens plays a more critical role in the vision of aquatic vertebrates by providing the necessary refraction to focus light onto the retina. Consequently, altering the cornea’s shape would not significantly impact vision for these animals as it does for terrestrial vertebrates, where the cornea substantially contributes to focusing light.

Question 70

passive vs active electroreception?

Answer 70

Passive and active electroreception are two distinct methods used by aquatic animals to perceive electrical signals in their environment. Passive electroreception involves detecting the natural electrical fields generated by other organisms, such as muscle contractions or bioelectric fields, which help predators locate prey or enable navigation and communication. This mode is common in sharks, rays, and some bony fish. In contrast, active electroreception is characterized by the emission of electrical pulses by an animal, which then detects the changes in these pulses caused by the presence and properties of objects in the environment, effectively allowing them to “see” through electrical images. This method is used by electric fishes like the electric eel and knifefish, which can control the frequency and amplitude of their electric discharges to obtain detailed information about their surroundings, even in murky waters where other senses might be less effective.

Question 71

what happens when two wave type electric fish encounter eachother?

Answer 71

When two wave-type electric fish encounter each other, their electrical signals interact and produce beats. The fish detect the beats, and the beat frequency helps them to interpret the signals coming from other electric fish. The high sensitivity of electric fish to the beats generated by interacting electrical signals explains why these electric fish must be producing an EOD in order to easily detect signals from other electric fish.

Question 72

what is magnetoreception?

Answer 72

Magnetoreception, or the ability to detect magnetic fields (also called magnetoception), is widely distributed throughout the animal kingdom. Migratory birds, homing salmon, and many other organisms use Earth’s magnetic field to help them navigate.

Question 73

Magnetite?

Answer 73

Magnetite is a natural mineral that responds to magnetic fields, and thus could be the basis for magnetoreception in animals.

Particles resembling it have been found in the sensory neurons of these animals.

Question 74

If the pit organs of a pit viper use the same receptor (TRPA1) as do nociceptors, why doesn’t the viper detect pain rather than heat when it is tracking its prey?

Answer 74

The pit organs of a pit viper, despite using the same type of receptor (TRPA1) as nociceptors (pain receptors in mammals), are specialized to detect infrared radiation (heat) rather than pain. This specificity arises from the adaptation of these receptors in the pit organ to respond primarily to thermal stimuli rather than mechanical or chemical signals that typically induce pain. When a pit viper tracks its prey, the TRPA1 receptors in its pit organs detect the heat emitted by the prey’s body, allowing the snake to effectively locate and target it, especially in low-light conditions. The differentiation in function, despite the similarity in receptor type, highlights a remarkable example of evolutionary adaptation where the same type of receptor is used for different purposes across species.

Question 75

The human retina contains functional cryptochromes that can rescue cryptochrome mutants in Drosophila and restore magnetoreception. But humans do not appear to have the ability to detect magnetic fields. In fact, in the absence of other directional cues, humans tend to walk in circles. How can we have functional cryptochromes but no magnetoreception?

Answer 75

Although human retinas contain functional cryptochromes, which are capable of restoring magnetoreception in cryptochrome-deficient Drosophila, humans do not exhibit magnetoreceptive behavior. This discrepancy likely arises because the presence of cryptochrome alone is insufficient to confer magnetoreception. In species with known magnetoreception, such as birds, these cryptochromes are part of a more complex sensory system that includes neuronal wiring and brain structures specifically dedicated to processing magnetic field information. Humans lack this specialized neural architecture. Thus, while our cryptochromes can perform similar molecular functions as those in species with magnetoreception, the absence of an integrated system to interpret and use the information derived from magnetic fields results in the lack of magnetoreceptive abilities. This underscores the importance of both molecular components and specific neural circuitries in the development of sensory capabilities.

Question 76

What is the difference between a sense organ and a sensory receptor?

Answer 76

A sense organ is a specialized structure consisting of multiple tissues that work together to perceive specific types of external stimuli, such as light, sound, or touch, and translate them into signals that can be processed by the nervous system. Examples include the eye, ear, and skin. In contrast, a sensory receptor is a specific cell or cell component, often located within a sense organ, that is specialized to respond to a particular type of stimulus. These receptors can be individual cells or distinct cellular structures that detect mechanical forces, chemicals, temperature, and other physical phenomena. While sense organs contain and support sensory receptors, sensory receptors are the specific sites where stimulus detection occurs.

Question 77

Explain labeled-line coding and give an example of the kinds of sensory information that can be encoded by this method.

Answer 77

Labeled-line coding is a sensory encoding system where specific neurons or pathways are dedicated to conveying information about one type of stimulus. In this model, each sensory receptor is “labeled” for a specific modality (e.g., taste, sound, light), and the brain interprets the input from each receptor along a direct, unambiguous line, assuming that any activation signals the presence of its specific stimulus.

An example of labeled-line coding can be seen in the gustatory system (taste). Taste receptors on the tongue are specialized to detect one of the basic tastes: sweet, sour, salty, bitter, and umami. Each type of taste receptor responds to particular chemical components in food. For instance, sweet receptors respond to sugars like glucose and fructose. When these receptors are activated, they send signals through specific neural pathways to the brain, which interprets these signals as the taste “sweet.” This direct pathway from a specific type of receptor through a dedicated set of neurons to a brain region that interprets the signal as a specific taste demonstrates the labeled-line coding model.

Question 78

What are the primary stimulus modalities detected by animal sensory receptors?

Answer 78

Mechanoreception: Detection of mechanical changes or disturbances such as touch, sound, pressure, and vibration. Examples include tactile receptors in the skin and hair cells in the auditory and vestibular systems.

Chemoreception: Sensing chemical stimuli, which includes taste (gustation) and smell (olfaction). Chemoreceptors also detect internal body chemicals, like CO2 levels in the blood.

Photoreception: Sensing light, involved in vision. Photoreceptors in the eyes, such as rods and cones in vertebrates, detect light intensity and color.

Thermoreception: Detecting changes in temperature, enabling the sensation of heat and cold. Certain receptors respond to external temperature changes, while others monitor the internal temperature of the body.

Electroreception: Sensing electrical fields, which is especially prominent in aquatic animals like sharks and rays, helping them to navigate and locate prey by detecting the electrical impulses emitted by other organisms.

Magnetoreception: Sensing magnetic fields, which is thought to aid in navigation, particularly in migratory species like birds, turtles, and some fishes.

Nociception: Detecting harmful stimuli that could cause injury, leading to the sensation of pain. Nociceptors are present in skin, joints, and organs across various animal groups.

Question 79

What is the relationship between the intensity of a stimulus and the response of the primary afferent neuron? How do neurons encode changes in stimulus intensity?

Answer 79

The relationship between the intensity of a stimulus and the response of primary afferent neurons is characterized by an increase in the frequency of action potentials as stimulus intensity increases. This is known as rate coding. Primary afferent neurons encode changes in stimulus intensity by varying the rate at which they fire action potentials. A more intense stimulus triggers the neuron to fire more rapidly, which is interpreted by the central nervous system as a stronger stimulus. This modulation allows the nervous system to detect and respond to changes in the environment by translating physical stimulus intensity into a neural code that reflects different levels of stimulus strength.

Question 80

Outline the similarities and differences between the receptors involved in the detection of odorants and the receptors involved in the detection of pheromones in mammals.

Answer 80

Mammalian odorant and pheromone receptors both function as G Protein-Coupled Receptors (GPCRs) and are located in the nasal tissues, playing critical roles in the detection of airborne chemicals. However, they differ significantly in their specific functions and locations. Odorant receptors are found in the olfactory epithelium and detect a wide range of volatile chemicals, contributing broadly to the sense of smell. In contrast, pheromone receptors are typically located in the vomeronasal organ (VNO) and are specialized to detect pheromones, which are specific chemicals released by other individuals of the same species to trigger particular behavioral or hormonal responses. This specialization allows mammals to respond to environmental odors and social signals distinctly, ensuring both survival and reproductive success.

Question 81

Outline the similarities and differences between the receptors involved in odorant detection in mammals and insects.

Answer 81

In both mammals and insects, the receptors involved in odorant detection are protein structures embedded in the membranes of olfactory sensory neurons, which bind specific molecules from the environment and initiate a signal transduction pathway leading to a neural response. However, there are key differences in their molecular structure and diversity. Mammalian olfactory receptors are primarily G-protein-coupled receptors, comprising a large gene family with about 1,000 different receptors in humans, allowing a broad and nuanced perception of smells. Insects, on the other hand, utilize not only G-protein-coupled receptors but also ionotropic receptors (e.g., the insect olfactory receptors and ionotropic glutamate receptors), which are typically more direct in converting chemical binding into electrical signals. Insects tend to have fewer types of olfactory receptors than mammals but achieve specificity and diversity in odor detection through various combinations of these receptors and their unique patterns of expression.

Question 82

Compare and contrast the signal transduction mechanisms used by gustatory receptors to detect the primary types of tastants.

Answer 82

Gustatory receptors in the taste system detect five primary types of tastants: sweet, umami, bitter, salty, and sour, each using distinct signal transduction mechanisms. Sweet, umami, and bitter tastes are detected through G-protein-coupled receptors (GPCRs). For sweet and umami tastes, activation of these GPCRs leads to a signaling cascade involving the release of second messengers that ultimately result in neurotransmitter release. Bitter taste utilizes a similar GPCR system but often involves a larger family of receptors, allowing detection of a wide variety of potentially toxic substances. In contrast, salty and sour tastes generally involve ion channels rather than GPCRs. Salt taste is primarily detected through sodium channels that directly allow sodium ions to enter the cell, generating an action potential. Sour taste detection is mediated by ion channels that detect the presence of protons (H+), which can block specific potassium channels or activate proton-specific channels, leading to depolarization of the taste cell. Thus, while sweet, umami, and bitter rely on metabotropic mechanisms, salty and sour are governed by ionotropic mechanisms, reflecting their direct chemical nature.

Question 83

Compare the mammalian gustatory system with that in insects.

Answer 83

The gustatory systems of mammals and insects are distinct both structurally and functionally, tailored to their specific ecological needs. Mammalian taste receptors are localized in taste buds on the tongue, responding to five basic tastes through mechanisms like G-protein-coupled receptors for sweet, umami, and bitter, and ion channels for salty and sour. In contrast, insects have gustatory receptors on various body parts, such as mouthparts, legs, and antennae, which detect a range of chemical cues using gustatory receptor proteins that often directly influence ion channels. While mammalian taste primarily aids in food selection, insect gustation is integral not only to feeding but also to behaviors such as oviposition and pheromone detection, demonstrating a broader ecological application.

Question 84

What are the major families of mechanoreceptor proteins involved in touch, proprioception, and hearing?

Answer 84

The major families of mechanoreceptor proteins involved in touch, proprioception, and hearing in various organisms include the Piezo, TREK/TRAAK, and DEG/ENaC protein families. Piezo proteins are critical for detecting mechanical forces on the skin and internal organs, playing a pivotal role in touch and proprioception. The TREK/TRAAK family, part of the larger group of potassium channels, is involved in responding to mechanical stretch and temperature changes, influencing both proprioceptive and nociceptive pathways. DEG/ENaC proteins, sodium channels, are essential for mechanosensation in both vertebrates and invertebrates, crucial for tasks ranging from touch sensation to hearing. These diverse families allow organisms to detect and respond to a wide range of mechanical stimuli, from gentle touch to the vibrations associated with sound.

Question 85

List four major types of vertebrate touch receptors and identify them as either slowly adapting or rapidly adapting receptors.

Answer 85

Merkel cells: Slowly adapting; respond to light touch, detail about object shapes and textures.

Meissner’s corpuscles: Rapidly adapting; sensitive to changes in texture and low-frequency vibrations.

Ruffini endings: Slowly adapting; detect skin stretch, important for finger position and object manipulation.

Pacinian corpuscles: Rapidly adapting; respond to high-frequency vibrations and deep pressure.

Question 86

Outer hair cells respond to sounds, but they do not make synaptic connections with afferent neurons that carry sound information to the brain. What is their role in hearing?

Answer 86

Outer hair cells in the cochlea play a crucial role in hearing not by transmitting sound information to the brain, but by enhancing auditory sensitivity and frequency resolution. These cells act as mechanical amplifiers; when they detect sound, they change their length and stiffness in response to the vibrations. This action increases the motion of the basilar membrane, selectively amplifying specific frequencies of sound. By doing so, outer hair cells fine-tune the vibrations within the cochlea, making it possible for inner hair cells to transmit more precise signals through afferent neurons to the brain. This amplification is essential for normal hearing sensitivity and for the ability to discern different sound frequencies, contributing significantly to our detailed and nuanced perception of sound.

Question 86

Using the vertebrate ear as an example, outline some of the ways in which sensory systems amplify environmental stimuli.

Answer 86

In the vertebrate ear, several mechanisms amplify environmental sound stimuli to enhance hearing. The outer ear captures sound waves and funnels them toward the tympanic membrane (eardrum), which vibrates in response. These vibrations are then mechanically amplified by the ossicles (malleus, incus, and stapes) in the middle ear, effectively increasing the force of the sound waves as they are transferred to the oval window of the cochlea. Inside the cochlea, the movement of fluid within the cochlear duct displaces the basilar membrane, translating these mechanical forces into neural signals. Additionally, outer hair cells in the organ of Corti dynamically respond to the fluid motion by changing length, which serves to selectively amplify specific frequencies by enhancing the movement of the basilar membrane, thereby providing a finer auditory sensitivity and selectivity. These mechanisms collectively enable the ear to convert minute variations in air pressure into detailed auditory information.

Question 87

Outline two possible scenarios for the evolution of animal photoreceptors.

Answer 87

Ciliary Photoreceptors Evolution: This theory suggests that photoreceptors evolved from a common ancestral ciliary cell type that originally functioned in light detection. Over time, these cells specialized and adapted to more complex vision tasks in animals such as vertebrates. The ciliary photoreceptors, like those found in human rods and cones, utilize a single cilium that extends from the cell body and is embedded with photopigments, which are crucial for capturing light.

Rhabdomeric Photoreceptors Evolution: Alternatively, rhabdomeric photoreceptors, primarily found in invertebrates like mollusks and arthropods, might have evolved from a distinct lineage of light-sensitive cells. These photoreceptors are characterized by their microvillar structures, which are densely packed with photopigments. They likely arose from an ancestral cell that used microvilli for light absorption, diversifying later into the more complex rhabdomeric organs seen in many invertebrates today.

Question 88

Explain how the properties of vertebrate rods make them suitable for photoreception in dim light.

Answer 88

First, rods contain a high concentration of the photopigment rhodopsin, which is extremely sensitive to low light levels. This allows rods to respond to even a few photons of light, making them much more sensitive than cones, which require brighter light. Additionally, rods have a structure that maximizes their light-absorbing capability; they feature a long, cylindrical outer segment packed with stacks of membrane discs where rhodopsin is embedded. This design increases the surface area available for light capture. Moreover, rods benefit from a phenomenon called spatial summation, where signals from multiple rods converge onto a single neuron, amplifying the light signal and enhancing sensitivity in low-light conditions. This makes rods particularly well-suited for night vision and seeing in poorly lit environments.

Question 89

Explain the role of the following types of cells in the mammalian retina, using one or two sentences for each answer: rods, cones, horizontal cells, bipolar cells, amacrine cells, retinal ganglion cells.

Answer 89

Rods: Specialized for low-light vision, rods are highly sensitive to light and allow for night vision but do not support color discrimination.
Cones: Responsible for color vision and high-acuity vision in well-lit conditions, cones are less sensitive to low light but crucial for detecting fine detail and different colors.
Horizontal cells: These cells integrate and regulate input from multiple photoreceptor cells, aiding in contrast enhancement and spatial resolution by modulating the output of rods and cones.
Bipolar cells: Acting as a relay between photoreceptors (rods and cones) and ganglion cells, bipolar cells transmit signals from the photoreceptors to the ganglion cells, with different types either enhancing or inhibiting the signal based on the visual information received.
Amacrine cells: These cells interact at the level of the inner synaptic layer, influencing the bipolar cell-ganglion cell synapses, and are involved in complex processing tasks like image refinement and motion sensitivity.
Retinal ganglion cells: The final output neurons of the retina, ganglion cells receive inputs from bipolar and amacrine cells and send visual information to the brain via the optic nerve, encoding and transmitting image details critical for further visual processing.

Question 90

Explain how changing the shape of the mammalian lens allows objects at different distances to be brought into focus.

Answer 90

In mammals, the ability to focus on objects at varying distances, known as accommodation, is achieved by changing the shape of the lens. The lens is a flexible, transparent structure situated behind the iris and is controlled by the ciliary muscles surrounding it. When viewing objects that are close, the ciliary muscles contract, which reduces the tension on the zonule fibers connected to the lens, allowing the lens to become thicker and more curved. This increased curvature enhances the lens’s refractive power, enabling it to focus light from near objects onto the retina. Conversely, when viewing distant objects, the ciliary muscles relax, the zonule fibers tighten, and the lens becomes flatter, decreasing its curvature and refractive power to focus light from far away onto the retina. This dynamic adjustment of the lens shape is crucial for maintaining clear vision across different visual distances.

Question 91

Compare the receptors found in the pit organs of some snakes to the thermoreceptors found in other vertebrates.

Answer 91

The pit organs in snakes, such as pit vipers, and the thermoreceptors found in other vertebrates both serve the function of detecting temperature changes but differ significantly in their sensitivity and ecological roles. Pit organs are highly specialized infrared sensory receptors that enable snakes to detect very slight changes in ambient temperature, allowing them to “see” the heat radiated by warm-blooded prey, even in total darkness. This organ contains a membrane that can detect temperature changes as minute as 0.003°C, which is much more sensitive than the general thermoreceptors found in other vertebrates. In contrast, typical vertebrate thermoreceptors are distributed throughout the skin and internal organs and help regulate body temperature by responding to external and internal temperature changes, but with less precision compared to the specialized pit organs of snakes. Thus, while both serve thermoreceptive functions, the pit organs are adapted for precise hunting and navigation, whereas general thermoreceptors are crucial for overall thermoregulation.

Question 92

Compare active electrolocation with echolocation.

Answer 92

Active electrolocation and echolocation are sensory modalities used by different animal species to navigate and locate objects in their environment, but they function through distinct mechanisms and are adapted to different ecological niches. Active electrolocation, utilized by certain fish species such as the elephant nose fish, involves the generation of electrical fields and the detection of distortions in these fields caused by objects or other living organisms. This method is particularly effective in murky or dark waters where visual cues are limited. In contrast, echolocation, employed by bats and some marine mammals like dolphins, involves emitting sound waves and detecting the echoes that bounce back from objects. This method is highly effective in air or clear water environments, enabling precise localization and identification of objects over greater distances. While both systems process environmental information to create a perceptual map of the surroundings, they utilize different physical phenomena—electric fields versus sound waves—to achieve this.

Question 93

Mechanoreceptors do not depolarize in response to light, no matter how intense the stimulus, but the eye responds to a mechanical stimulus (such as pressing on the eyeball) if the stimulus is sufficiently large. Why might this be?

Answer 93

Mechanoreceptors and photoreceptors in the eye are specialized to respond to different types of stimuli due to their distinct structures and activation mechanisms. Mechanoreceptors, such as those in the skin and other tissues, are sensitive to mechanical forces like pressure or stretch, but they lack the molecular machinery necessary to respond to light, which requires a photopigment that changes structure when struck by photons. Conversely, the retina’s photoreceptors, rods, and cones contain these photopigments specifically tuned to detect light. However, a sufficiently large mechanical stimulus, such as pressing on the eyeball, can indirectly affect the retina and photoreceptors, creating a sensation of light (known as phosphenes), not because the mechanoreceptors are responding to light, but because the mechanical pressure is indirectly stimulating the photoreceptors. This phenomenon illustrates how sensory systems are finely tuned to their specific types of stimuli yet can sometimes cross-activate under abnormal conditions.

Question 94

Do taste receptors use labeled-line coding? Why or why not?

Answer 94

Taste receptors primarily use labeled-line coding to convey specific taste information from the tongue to the brain. In this system, each receptor is tuned to a particular type of tastant (sweet, sour, salty, bitter, or umami) and sends its signal along a dedicated neural pathway. This allows the brain to recognize distinct tastes based on which neurons are activated. For example, cells that respond to sweet stimuli connect to specific brain regions associated with the perception of sweetness, ensuring that taste signals are not mixed up, allowing for clear and distinct taste perceptions. This system illustrates how labeled-line coding provides specificity and accuracy in sensory processing, essential for the discernment of different tastes.

Question 95

Receptors for fine touch are typically located in the shallow layers of the skin, while receptors for stronger touch stimuli are typically located in deeper layers. Why might this be so?

Answer 95

Receptors for fine touch, such as Merkel cells and Meissner’s corpuscles, are typically found in the shallow layers of the skin because these layers are more sensitive to light touch and slight pressure, allowing for greater precision in sensing textures and fine details. These receptors are adapted to detect subtle deformations of the skin’s surface, making them well-suited for tasks like reading Braille or feeling the texture of an object. Conversely, receptors for stronger touch stimuli, such as Pacinian corpuscles, are located deeper in the dermis or in subcutaneous tissue. These deeper receptors are adapted to respond to more significant pressures and vibrations, which travel further into the body. Their deeper location helps in buffering against damage and provides a robust response to high-pressure stimuli, important for proprioception and awareness of our body’s interactions with the environment.

Question 96

Hair cells have prominent cilia on their apical surface. Why do these cilia increase the sensitivity of a hair cell to mechanical stimuli?

Answer 96

Hair cells in sensory systems such as the inner ear possess prominent cilia called stereocilia on their apical surface, which significantly enhance their sensitivity to mechanical stimuli. These cilia amplify minute movements through leverage, transforming small vibrations into larger motions that effectively open ion channels linked by tip links. This mechanism allows the stereocilia to convert mechanical energy into electrical signals in a highly sensitive and directional manner. The extensive array and organization of these cilia increase the surface area, capturing more stimuli and providing precise detection of the direction and magnitude of mechanical forces. Thus, these cilia are fundamental in translating physical motion into neural information, crucial for hearing and balance.

Question 97

Why do the inner ears of most vertebrates have three semicircular canals and not just one?

Answer 97

The inner ears of most vertebrates contain three semicircular canals, each oriented roughly perpendicular to the other two. This arrangement allows for the detection of rotational movements across all three dimensions of space. The canals are filled with fluid, and each canal responds to rotations in a different plane: horizontal, vertical, and oblique. As the head moves, the fluid inside the respective canal lags behind due to inertia, and this movement relative to the canal walls bends hair cells embedded in a gel-like structure called the cupula located at one end of each canal. The bending of these hair cells generates nerve impulses that inform the brain about the direction and speed of the rotation. Having three canals provides a full spatial representation necessary for maintaining balance and coordinating movement accurately in a three-dimensional world.

Question 98

Peripheral vision is the ability to detect objects outside the center of the visual field. Vertebrates vary in the extent of their peripheral vision. What differences would you expect in the retina of an animal with excellent peripheral vision, compared to one with poor peripheral vision?

Answer 98

Animals with excellent peripheral vision, such as many prey species, tend to have a retina that is more uniformly populated with photoreceptor cells across a wider area, allowing them to detect movements and threats from a broader range of angles. This includes a larger distribution of rods, which are more sensitive to light and motion than cones, enhancing their ability to see in lower light conditions which often characterize peripheral regions of the visual field. In contrast, animals with poor peripheral vision, like predators that focus on detail and color in the center of their visual field, often have a denser concentration of cones in a central area known as the fovea, enabling sharp central vision but reducing the scope and sensitivity of their peripheral vision. This specialization allows them to better capture and focus on specific targets directly ahead of them.

Question 99

Humans have only three types of cone photoreceptors, but can distinguish thousands of colors. How is this possible?

Answer 99

Humans can distinguish thousands of colors despite having only three types of cone photoreceptors (red, green, and blue) through a process known as color mixing and the phenomenon of color opponency. Each type of cone responds to different wavelengths of light, and the brain combines the input from these cones in various proportions to perceive a wide spectrum of colors. This trichromatic theory of color vision explains how various combinations and intensities of the red, green, and blue signals can create the perception of virtually any color. Additionally, the color opponency mechanism, which involves the processing of colors in opposing pairs (red-green, blue-yellow), enhances color discrimination and helps in distinguishing subtle differences in color despite the limited types of cone receptors.

Question 100

What predictions could you make about what would happen to vision in an individual with a degenerative disease that destroyed the horizontal cells of the retina?

Answer 100

If an individual had a degenerative disease that destroyed the horizontal cells of the retina, several significant changes in visual perception could be predicted. Horizontal cells are crucial for integrating and regulating the input from multiple photoreceptors, particularly influencing contrast enhancement and color perception by inhibiting nearby photoreceptors and bipolar cells. Without horizontal cells, the individual would likely experience a decrease in visual contrast, making it harder to distinguish edges and contours. Additionally, there might be a reduction in the ability to perceive fine detail and a decrease in the dynamic range of vision, leading to difficulties in adapting to different lighting conditions. Overall, the visual system’s ability to process complex visual scenes effectively would be significantly impaired.