Module 7: Sensory System

Question 1

What are some of the sensory systems that the body uses to detect external changes rapidly?

Answer 1

Somatosensory system (touch), visual system, auditory and vestibular system, olfactory system (smell), and gustatory system (taste).

Question 2

What is transduction of environmental information?

Answer 2

Transduction of environmental information is how information from the external environment is turned into language the brain understands (action potentials).

Question 3

What must happen in order for brain to know what is happening outside the body?

Answer 3

Environmental stimuli (energy) like light, heat, touch, or sound must first be detected by sensory receptors, which then convert the information into action potentials.

Question 4

What are some examples of environmental stimuli?

Answer 4

Mechanical stimuli: touch, pressure, vibration, proprioception, sound

Chemical stimuli: taste, pain, odours

Electromagnetic stimulus: light

Other stimuli: gravity, motion, acceleration, heat

Question 5

Which receptors do stimuli need to be detected and converted into action potentials?

Answer 5

Environmental stimuli come in different forms and will require different receptors to detect the stimulus and then convert it to action potentials.

Question 6

How is a mechanical stimulus detected?

Answer 6

A mechanical stimulus, like touching or vibrating the skin, will stretch the sensory receptors in the skin and open ion channels, causing a depolarization of the sensory neuron producing an action potential.

Question 7

How is a chemical stimulus detected?

Answer 7

A chemical stimulus, like a sour taste on the tongue or an odour in the nose, binds with a receptor, causing a depolarization and then an action potential.

Question 8

How is light energy detected?

Answer 8

Light energy is absorbed by photoreceptors of the eye (rods and cones in the retina) and eventually produces action potentials.

Question 9

How is gravity and motion detected?

Answer 9

Gravity and motion can also be detected by hair cells in the vestibular system, which convert this form of external stimulus to action potentials.

Question 10

Can all receptors only detect one type of stimulus?

Answer 10

No, some receptors can detect more than one type of stimulus.

Question 11

What is an adequate stimulus?

Answer 11

An adequate stimulus is the particular form of environmental stimulus to which the sensory receptor is most sensitive.

Question 12

What is the adequate stimulus for the rod and cones?

Answer 12

The adequate stimulus for the rod and cone cells found in the retina of the eye is light. Sensory receptors do respond to other forms of energy but not in an optimal way. For example, rod and cone cells of the eye also respond to pressure on the eyeball.

Question 13

What happens when you gently probe the surface of your hand with the tip of a pencil?

Answer 13

When you gently probe the surface of your hand with the tip of a pencil, you will periodically hit a cold receptor. At this location, the pencil tip provides adequate stimulus for the activation of the cold receptor. The result is a feeling of “cool” in the location where you touched the surface of the hand.

Question 14

What happens once a sensory receptor is stimulated by an environmental stimulus?

Answer 14

Once the sensory receptor is stimulated by an environmental stimulus, it will cause a change ion permeability, leading to a local depolarization.

Question 15

What is a receptor (or generator) potential?

Answer 15

The local depolarization after the sensory receptor is stimulated is called a generator or receptor potential.

Question 16

How is an action potential generated after the sensory receptor is stimulated?

Answer 16

Since the receptor does not have voltage-gated ion channels necessary to fire an action potential, the receptor potential must spread to an area on the sensory neuron that does contain these channels. This is usually at the first node of Ranvier on the axon. The action potential will then be generated and propagated along the axon and into the spinal cord.

Question 17

How is an action potential generated in receptors with no axons?

Answer 17

In receptors with no axons (like hair cells in the inner ear), the depolarization has to spread to the synapse to result in the release of a neurotransmitter.

Question 18

What are some shared characteristics between receptor potentials and EPSPs and IPSPs?

Answer 18

1) They are generally depolarizing, but can be hyperpolarizing as well.
2) They are caused by an increase in permeability to sodium ions (Na+), or potassium ions (K+) in the case of a hyperpolarizing stimulus.
3) They are local and do not propagate down the neuron like an action potential but spread like an EPSP, decreasing with time and distance from the stimulus.
4) They are proportional to the strength of the stimulus – the stronger the stimulus, the larger the receptor potential and the more likely to fire an action potential.

Question 19

When holding a heavier object, how are the large number of action potentials generated?

Answer 19

The weight of the object was “coded” into the action potentials (heavier object = more action potentials per second). The heavier weight will trigger the receptor to produce a large receptor potential, and this large potential will trigger many action potentials on the sensory neuron’s axon. This burst of high-frequency action potentials will eventually reach the brain where you will become consciously aware of the heavier weight in your hand.

Question 20

What is the function of the somatosensory system?

Answer 20

The somatosensory system detects and processes the sensations of touch, vibration, temperature, and pain – the majority of which originate in the skin. Detecting each sensation requires several different sensory receptors within the skin, each developed to detect its adequate stimulus.

Question 21

What are the receptors in the skin referred to as? List them.

Answer 21

The receptors in the skin are referred to as cutaneous receptors. They include the following:

1) Hair follicle receptors that are sensitive to fine touch and vibration
2) Free nerve endings that respond to pain and temperature (hot and cold)
3) Meissner’s corpuscles that detect low-frequency vibrations (between 30 and 40 cycles per sec) and touch
4) Ruffini’s corpuscles that detect touch
5) Pacinian corpuscles that detect high-frequency vibrations (250 to 300 cycles per sec) and touch

Question 22

What is the receptive field?

Answer 22

The receptive field is the area on the surface of the skin where an adequate stimulus will activate a particular receptor to fire an action in the neuron. Any stimulus applied outside the receptor field will not generate an action potential.

Question 23

What are the two spinal tracts the action potentials reach the brain using?

Answer 23

1) The spinothalamic (anterolateral) tract

2) Dorsal column, medial lemniscal system

Question 24

What information does the spinothalamic (anterolateral) tract transmit?

Answer 24

The spinothalamic (anterolateral) tract transmits information dealing with very basic sensations like pain, temperature, and crude touch.

Question 25

What happens to the information transmitted in the spinothalamic (anterolateral) tract?

Answer 25

The information from the sensory neuron (first order neuron) enters the spinal cord, where it synapses with a second order neuron. This neuron crosses to the opposite or contralateral side of the spinal cord and ascends to a region of the brain called the thalamus. The thalamus acts as a relay station for almost all sensory information (except smell). A second synapse with a third order neuron occurs here and then travels to the somatosensory cortex. NOTE: sensory information from the right side of the body goes to the left side of the brain and vice versa.

Question 26

What information does the dorsal column, medial lemniscal system transmit?

Answer 26

The dorsal column, medial lemniscal system transmits information associated with the more advanced sensations of fine detailed touch, proprioception (muscle sense), and vibration.

Question 27

What happens to the information transmitted in the dorsal column, medial lemniscal system?

Answer 27

The information from the sensory neuron (first order neuron) enters the spinal cord and immediately travels up the spinal cord before crossing to the contralateral side (unlike the spinothalamic system). In the upper spinal cord, the sensory neuron synapses with a second order neuron, which then crosses to the opposite side of the spinal cord. From here, it continues to the thalamus, where it synapses again onto a third order neuron that then travels to the somatosensory cortex. NOTE: sensory information from the right side of the body goes to the left side of the brain and vice versa.

Question 28

What happens once the sensory information reaches the brain?

Answer 28

Once the sensory information has reached the brain, it travels to the primary somatosensory cortex, which is located in the parietal lobe on the postcentral gyrus behind the central sulcus.

Question 29

How is the primary somatosensory cortex arranged?

Answer 29

The primary somatosensory cortex is arranged in a very specific manner. The information arriving at this cortex is “geographically preserved”. This topographical representation of the body on the surface of the cortex is called the somatosensory homunculus.

Question 30

How is the human body represented on the homunculus?

Answer 30

The human body is represented somewhat out of scale. Some of the representative areas are out of proportion. This is because some areas on the cortex, like the areas dealing with the hand, tongue, and lips, receive more sensory information and require more of the brain to process that information. The hands, tongue, and lips are the most sensitive part of the body; they contain many more sensory receptors that any other part.

Question 31

How are the body parts arranged on the somatosensory homunculus?

Answer 31

From lateral to medial: pharynx, tongue, lips, face, nose, eye, thumb, fingers, hand, forearm, arm, head, back, leg, foot, genitals.

Question 32

What is the function of the visual system?

Answer 32

The visual system detects light, converts it into action potentials, and sends these to the primary visual areas for processing. Once processed, we become aware of our visual world and are able to distinguish and recognize features in our external environment.

Question 33

What does the visual system consist of?

Answer 33

The visual system consists of the eye (which contains photoreceptors that convert light to action potentials), the visual pathway (which transmits the action potentials), and the primary visual area in the occipital lobe of the brain (which processes the incoming signals).

Question 34

Explain the functions of the parts of the eye and how light is processed.

Answer 34

After passing through the cornea, the amount of light is regulated by the iris, which can constrict with bright light or dilate in low light. The lens flips the light (upside down and backwards) and focuses it onto the retina at the back of the eye. The retina contains photoreceptors called rods and cones. The rods and cones actually point toward the back of the head. The center of your vision is focused onto a part of the retina called the fovea. This area has the highest concentration of cone cells.

Question 35

Discuss the ideal light conditions, photopigments, and location of rods.

Answer 35

Rods are extremely sensitive to light and, therefore, function best under low light conditions. They contain one type of photopigment (a chemical sensitive to light) and, consequently, do not detect colour. Rods are located mostly in the region of the retina outside and around the fovea.

Question 36

Discuss the ideal light conditions, photopigments, and location of cones.

Answer 36

Cones function best under bright light and are ideal for detecting detail. There are three different types of cone cells, each with a different photopigment and each sensitive to one primary colour. The cones are principally located in the region of the fovea where they are found in large concentrations.

Question 37

Do rods and cones generate action potentials?

Answer 37

No, rods and cones do not have axons and, therefore, do not generate action potentials. However, they do generate receptor potentials that cause the release of an inhibitory neurotransmitter from their synaptic ending.

Question 38

What are the other cells of the retina? What is their importance in generating action potentials?

Answer 38

The retina contains a pigment layer at the very back of the eye that absorbs excess light. Other cells in the retina include bipolar cells, ganglion cells, horizontal cells, and amacrine cells. Rod and cone cells do not generate action potentials. These other cells are responsible for the integration of information from the rods and cones and the production of action potentials.

Question 39

Explain how the visual system works “backwards”.

Answer 39

The light striking the retina has been flipped upside down and backwards due to the lens. When depolarized, the rod and cone cells release an inhibitory neurotransmitter, shutting off bipolar cells. But, most importantly, when light strikes the retina, it does not excite and depolarize the rod and cone cells.

Question 40

Does the light hyperpolarize or depolarize these cells in the retina?

Answer 40

The light actually hyperpolarizes these cells and shuts them off. Since these cells release an inhibitory neurotransmitter when depolarized in the dark, they inhibit the bipolar cells. When light strikes the photoreceptors, they hyperpolarize, shut off, and stop releasing inhibitory neurotransmitter. The bipolar cells, which can depolarize spontaneously (by themselves), now become activated. Eventually, the depolarization of the bipolar cells may lead to an action potential in the ganglion cells.

Question 41

How is light transformed into action potentials?

Answer 41

In the dark, Na+ are flowing into the photoreceptors, producing a depolarization. This leads to release of the inhibitory neurotransmitter. However, when light strikes the rod and cone cells, it closes these Na+ channels. With less Na+ coming in and K+ leaking out, the cell hyperpolarizes. With the rods and cones hyperpolarized, no inhibitory neurotransmitter is released and the bipolar cell depolarizes.

Question 42

What are the four primary eye movements?

Answer 42

1. Saccades
2. Smooth pursuit
3. Vestibular ocular reflex (VOR)
4. Vergences

Question 43

What are saccades?

Answer 43

Saccades are rapid, jerky movements of the eye. Saccades are used to rapidly move the eye to the object of interest (e.g. gazing around a room while holding your head still or reading these words on your computer).

Question 44

What is smooth pursuit?

Answer 44

Smooth pursuit is a smooth movement of the eyes that is made to keep a moving object of interest focused on the fovea (e.g. following the flight of a bird through the sky while keeping your head still).

Question 45

What is the vestibular ocular reflex (VOR)?

Answer 45

The vestibular ocular reflex (VOR) is an eye movement made when you focus your attention on an object and then move your head back and forth or shake it up and down (e.g. staring at someone with whom you are disagreeing or agreeing).

Question 46

What are vergences?

Answer 46

Vergences are eye movements that are made when an object of interest is approaching or moving away from you. When the object is moving away, the eyes diverge; when the object moves closer, the eyes converge (e.g. staring at a pencil while moving it away from and toward your face).

Question 47

What is the function of the auditory system?

Answer 47

The auditory system converts sound waves from the external environment into action potentials that travel to the auditory system of the brain.

Question 48

At what frequencies does most acute hearing occur?

Answer 48

Although a typical, healthy human ear can detect sound frequencies ranging from 20 waves per second (or Hz) to 20,000 Hz, our best, most acute hearing occurs in the range of 1,000 to 3,000 Hz.

Question 49

What are the three parts the large, basic structural features of the auditory system can be divided into?

Answer 49

1) The external or outer ear contains the ear or auricle and the external auditory canal.
2) The middle ear consists of the eardrum (or tympanic membrane), the ear ossicles (which are made up of 3 bones – the malleus, incus, and stapes), and the Eustachian tube.
3) The inner ear consists of the vestibular apparatus, which is involved with the sense of balance, and the cochlea for the processing of sound.

Question 50

What can the hollow area inside the cochlea be divided into?

Answer 50

The hollow area inside the cochlea be divided into three compartments: an upper scala vestibuli (also called vestibular duct), a middle cochlear duct, and a lower scala tympani.

Question 51

What separates the cochlear duct and the tympanic duct? What does it contain?

Answer 51

Separating the cochlear duct and the tympanic duct is the basilar membrane, which contains the organ of Corti.

Question 52

What is the organ of Corti?

Answer 52

The organ of Corti is where the sound waves are converted to action potentials by special hair cells.

Question 53

What are the hair cells in the organ of Corti embedded in?

Answer 53

These hair cells are embedded in the tectorial membrane.

Question 54

When a tree falls in a forest and nothing is there to hear it, does it make a sound?

Answer 54

No. The falling tree will hit the ground and cause air to rush out from underneath. This rushing wave of air will travel in all directions like waves on water. Sound is not created until this wave of air pressure hits parts of the ear (or microphone) and turns it into electrical information (action potentials in the CNS) that is then interpreted as sound.

Question 55

What is the difference between sound frequency and sound intensity, or loudness.

Answer 55

The frequency of a sound wave is the number of waves (or cycles) per unit time, while the intensity (or loudness) is expressed by the height (or amplitude) of the sound wave.

Question 56

What happens when the airwaves created by the falling tree travel through the air and reach the outer ear?

Answer 56

These waves are funneled into the external auditory canal and strike the tympanic membrane, causing it to flex back and forth. The levering action of the ear ossicles amplifies the pressure waves that strike the tympanic membrane.

Question 57

What do the ear ossicles do?

Answer 57

The ear ossicles cause the oval window to vibrate; this is a small membrane-covered opening directly underneath the stapes. Since the ear ossicles amplify the vibrations of the tympanic membrane and since the oval window is much smaller than this membrane, the waves are amplified 15 to 20 times their original amount.

Question 58

What does the fluid inside the cochlea do?

Answer 58

Fluid inside the cochlea, called perilymph, transmits the waves to the hair cells embedded in the basilar membrane, which detect the vibrations and turn them into action potentials in the auditory nerve.

Question 59

List the steps of the auditory system.

Answer 59

1) Sound waves strike the tympanic membrane and cause it to vibrate.
2) Vibration of the tympanic membrane causes the three bones of the middle ear to vibrate.
3) Footplate of the stapes vibrates in the oval window.
4) Vibration of the footplate causes the perilymph in the scala vestibuli to vibrate, which in turn causes displacement of the basilar membrane.
5) Short wave lengths from high pitched sounds cause displacement of the basilar membrane near the oval window. This movement is detected by hair cells of the spiral organ.
6) Long wave lengths from low pitched sounds cause displacement of the basilar membrane far from the oval window. This movement is detected by hair cells of the spiral organ.
8) When the vibrations reach the perilymph in the scala tympani, they travel to the round window where they are dampened.

Question 60

How do we hear different frequencies of sound?

Answer 60

The way we hear different frequencies of sound is due to the vibration of the basilar membrane (or basement membrane) located in cochlea. The pressure waves in the fluid created by vibrations of the oval window produce a travelling wave on the basement membrane, which reaches a peak at different regions of the membrane. This happens because the membrane is not consistent along its length.

Question 61

Describe the basilar membrane at different parts of cochlea.

Answer 61

The basilar membrane is wide and thin at the top of the cochlea and narrow and thick at the base near the oval window. Not only is its shape not uniform, but its tension also varies along its length, being “tight” at the base and “loose” at the top.

Question 62

Which hair cells are stimulated by certain frequencies? Describe the length and stiffness of hair cells along the membrane.

Answer 62

Low frequencies will stimulate hair cells as the apex (top) of the cochlea, and high frequencies will stimulate hair cells on the membrane near the oval window. The length and stiffness of the hair cells also differ slightly along the length of the membrane; this is another way we can detect different frequencies.

Question 63

What happens when the basilar membrane vibrates? (hair cells, depolarization, neurotransmitters)

Answer 63

When the basilar membrane vibrates, the hair cells are bent, causing ion channels to open and the depolarization of the cells. The depolarization causes the release of a neurotransmitter from the hair cells, exciting neurons of the auditory nerve which then fire action potentials.

Question 64

What is the effect of louder sounds on the basilar membrane, hair cells, and action potentials.

Answer 64

The louder the sound, the stronger the vibration of the basilar membrane, the more bent the hair cells, the more neurotransmitter released, and the higher the frequency of action potentials produced.

Question 65

Where do the signals flow to from the ear?

Answer 65

These signals flow to the auditory cortex located in the temporal lobe of the brain.

Question 66

Where is the vestibular system located? What is its function?

Answer 66

The vestibular system is located in your inner ear next to the cochlea and is responsible for maintaining balance, equilibrium, and postural reflexes.

Question 67

How does the vestibular system perform its functions?

Answer 67

The vestibular system performs these functions by detecting linear and rotational motion and the position of your head relative to the rest of the body.

Question 68

What is the vestibular apparatus also responsible for?

Answer 68

The vestibular apparatus is also responsible for one of the types of eye movements called the vestibular ocular reflex (VOR).

Question 69

What are the two primary structures of the vestibular apparatus? How many of these structures are there?

Answer 69

1) The semicircular canals, which detect rotational or angular accelerations of the head.
2) The otolith organs, which detect linear accelerations.

There are three semicircular canals in each apparatus; each one will detect movement in each plane of motion. There are two otolith organs: one for detecting linear acceleration in the vertical plane (up and down) and another for accelerations on the horizontal plane.

Question 70

What fluid are the semicircular canals filled with?

Answer 70

The semicircular canals are filled with a fluid called endolymph.

Question 71

What is the swelling at the end of each canal called? What does this swelling contain? What does that contain?

Answer 71

At the end of each canal is a swelling called the ampula. Inside the ampula is a sensory region called the crista ampullaris. The crista ampullaris contains the sensory hair cells, which are fixed at their base, while the cilia are embedded in a gelatinous material called cupula.

Question 72

What happens when our head is rotated to the left?

Answer 72

When our head is rotated to the left, the endolymph inside the canals lags behind and seemingly moves to the right. The endolymph hits the cupula and bends the hair cells embedded in it. When the hair cells are bent in a particular direction, they will depolarize and fire action potentials, sending signals to the brain. When bent in the opposite direction, they will hyperpolarize, sending no signals to the brain.

Question 73

What do the otolith organs detect?

Answer 73

The otolith organs detect linear accelerations and decelerations and the position of your head when it is tilted.

Question 74

What are the two otolith organs in each vestibular apparatus? What do they detect?

Answer 74

1) The utricle detects horizontal accelerations and decelerations (like when you are in a car).
2) The saccule detects vertical accelerations and decelerations (like when you are in an elevator).

Both organs act together to detect head tilts.

Question 75

What does each otolith organ contain?

Answer 75

Each otolith organ contains many hair cells that are anchored at their base and have their cilia embedded in a gelatinous membrane. The gelatinous membrane has otolith crystals embedded in it to give it weight and inertia during movements.

Question 76

What happens in the otolith organ when the body is at rest vs when it accelerates?

Answer 76

When body is at rest, there is a regular series of action potentials being produced in the vestibular nerve. When the body accelerates in either the vertical or horizontal plane, the otolith crystals initially lag behind and seem to move in the opposite direction to the acceleration. This bends cilia of the hair cells in the opposite direction, causing them to increase frequency of action potentials in the vestibular nerve (the faster the acceleration, the higher the action potential frequency).

Question 77

What happens in the otolith organs when the body is moving at a constant velocity?

Answer 77

When the body is moving at a constant velocity, the hair cells return to their “resting state” as do the frequency of action potentials.

Question 78

What happens in the otolith organs when the body decelerates?

Answer 78

When the body begins to decelerate, the hair cells bend in the other direction, which causes the frequency of action potentials to decrease further from the resting state (the more rapid the deceleration, the lower the action potential frequency).

Question 79

How does one tiny hair cell perform such amazing functions in two widely different systems?

Answer 79

The secret lies in the cilia at the top of the hair cells. When the hair cells are at rest, they release a small resting level of neurotransmitter from their base onto the sensory nerve, which fires action potentials. When the smaller stereocilia bend toward the larger kinocilium (e.g. during an acceleration), the hair cell releases more neurotransmitter, causing more action potentials in the sensory nerve. When the stereocilia bend away from the kinocilium (e.g. during a deceleration), the hair cell releases less neurotransmitter, resulting in fewer action potentials.