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Student Protocol Sensory Physiology In this experiment, you will become familiar with your senses, sensory-related phenomena, some sensory illusions, and will make observations based on your own senses. Everyone in the group should complete each exercise. You do not need the PowerLab or LabChart for this experiment. Written by staff of ADInstruments. Background Conventionally, five senses are described: sight, hearing, taste, smell, and touch (visual, auditory, gustatory, olfactory, and tactile senses, respectively). This is clearly an oversimplification, and additional sensory modalities include pain, temperature, balance, and body position. Depending on your method of classification, there are between nine and 21 human senses. Additionally, there are some other candidates, such as the sensory awareness of hunger and thirst, which may or may not fall within this classification. Each sense works in fundamentally the same way. A stimulus is transduced by a specialized receptor cell, which directly (when the receptor is part of a neuron) or indirectly (by releasing neurotransmitters) activates a sensory neuron. Some of these receptors have the ability to adapt to stimuli, which refers to the process by which a sensory system becomes insensitive to a continuing source of stimulation. Many tactile receptors, such as skin receptors, adapt quickly. This is necessary to keep our clothes from driving us crazy. Most nociceptors (pain receptors) do not adapt, requiring medications such as acetaminophen and morphine to be used to interrupt the pain signal to the brain. This lack of adaptation is essential in receptors designed to protect us from our environment. Vision generally describes the ability to detect electromagnetic energy. The visible range for humans is from about 380 nm to 750 nm. This is often referred to as the visible spectrum. The brain interprets the image collected by the photo receptive cells in the eye as sight. There are two types of cells in the mammalian eye. Cones are primarily responsible for color differentiation and rods are responsible for contrast (light and dark) resolution. The cones are found predominantly in the fovea, the region of highest visual acuity. Rods are not found in this area but are distributed fairly evenly throughout the remainder of the retina. The optic disc where the nerves and retinal blood vessels enter and exit is devoid of receptors and is referred to as the blind spot. Page 1 of 22 ©2010

Sensory Physiology Student Protocol

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Page 1: Sensory Physiology Student Protocol

Student Protocol

Sensory PhysiologyIn this experiment, you will become familiar with your senses, sensory-related phenomena, some sensory illusions, and will make observations based on your own senses. Everyone in the group should complete each exercise. You do not need the PowerLab or LabChart for this experiment.

Written by staff of ADInstruments.

BackgroundConventionally, five senses are described: sight, hearing, taste, smell, and touch (visual, auditory, gustatory, olfactory, and tactile senses, respectively). This is clearly an oversimplification, and additional sensory modalities include pain, temperature, balance, and body position. Depending on your method of classification, there are between nine and 21 human senses. Additionally, there are some other candidates, such as the sensory awareness of hunger and thirst, which may or may not fall within this classification.

Each sense works in fundamentally the same way. A stimulus is transduced by a specialized receptor cell, which directly (when the receptor is part of a neuron) or indirectly (by releasing neurotransmitters) activates a sensory neuron. Some of these receptors have the ability to adapt to stimuli, which refers to the process by which a sensory system becomes insensitive to a continuing source of stimulation. Many tactile receptors, such as skin receptors, adapt quickly. This is necessary to keep our clothes from driving us crazy. Most nociceptors (pain receptors) do not adapt, requiring medications such as acetaminophen and morphine to be used to interrupt the pain signal to the brain. This lack of adaptation is essential in receptors designed to protect us from our environment. Vision generally describes the ability to detect electromagnetic energy. The visible range for humans is from about 380 nm to 750 nm. This is often referred to as the visible spectrum. The brain interprets the image collected by the photo receptive cells in the eye as sight. There are two types of cells in the mammalian eye. Cones are primarily responsible for color differentiation and rods are responsible for contrast (light and dark) resolution. The cones are found predominantly in the fovea, the region of highest visual acuity. Rods are not found in this area but are distributed fairly evenly throughout the remainder of the retina. The optic disc where the nerves and retinal blood vessels enter and exit is devoid of receptors and is referred to as the blind spot.

Hearing, or audition, is the sense of sound perception and results from tiny hair fibers in the inner ear detecting motion of the membrane. This membrane (ear drum) vibrates in response to changes in air pressure. Humans with perfect hearing can detect vibrations in the range of 20 to 20,000 Hz.

Taste, or gustation, is one of the two main chemical senses. It is well-known that there are at least four types of taste receptors (taste buds) on the tongue: sweet, salt, sour, and bitter (Figure 1). The existence of a fifth receptor that detects the amino acid glutamate was recently confirmed. This umami receptor detects a flavor commonly found in meat and in artificial flavorings such as monosodium glutamate. The actual sense of taste is a combination of taste receptors, olfactory receptors, touch, temperature, and sight.

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Sensory Physiology Student Protocol

Figure 1. Taste buds.

Smell, or olfaction, is the other chemical sense. There are hundreds of olfactory receptors, each binding to a particular molecular feature. All of these receptors are found in a specialized region in the roof of the nasal cavity. Each odor molecule fits into binding site on a receptor neuron triggering an action potential (Figure 2).

Figure 2. Olfactory cells.

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Sensory Physiology Student Protocol

Touch, or tactition, is the sense of pressure perception. There are several types of specialized tactile receptors that may be found in the skin, muscles, and viscera (Figure 3). These range from simple nerve endings found in hair follicles to the relatively complex Pacinian corpuscles embedded in tissues. Each type is thought to respond to different intensities and frequencies of pressures.

Figure 3. Skin receptors.

Nociception is the perception of pain. Pain is an adaptive interpretation of the stimulus, not the stimulus itself. Generally speaking, there are two types of pain. “Fast pain" is carried from injured tissue by myelinated A-delta fibers. This is the sharp pain you feel when you slam the car door on your thumb. "Slow pain" is the dull aching you feel afterwards and is delivered to the central nervous system by unmyelinated C fibers. Nociceptors consist of free nerve endings embedded in the skin, muscles, joints, and viscera that respond to chemical, thermal, or mechanical stimuli.

Thermoception is the sense of heat and cold. Cold receptors are sensitive to temperatures lower than about 37 °C. Warm receptors are sensitive from 37 °C to about 45 °C. Above this temperature, nociceptors are activated. Temperature receptors are found in the subcutaneous layers of the skin. The receptors adapt between 20 °C and 40 °C. For example, the cool air-conditioned room stops feeling cold after a brief period of time. At high and low temperatures these receptors do not adapt, thereby helping to prevent temperature related injury to tissues. The homeostatic thermoceptors, which provide feedback on internal body temperature, are quite different. They are located close to the hypothalamus in the brain and are responsible for setting the internal thermostat.

Equilibrioception is the perception of balance and is related to the vestibular system in the inner ear (Figure 4). The vestibular system has two components, the semicircular canals, which are filled with endolymph fluid and detect rotatory movements of the head, and the otolith organs (utricle and saccule) that detect linear acceleration and the effects of gravity,

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Sensory Physiology Student Protocol

respectively. Each of the ampulla of the semicircular canals contains a receptor apparatus, the crista ampullaris. This consists of a gelatinous, wedge-shaped structure that blocks off the ampulla and prevents flow of the endolymph. The cilia of the receptor cells and the otoconia, which are calcium carbonate crystals, are embedded in this gel. When the head moves, the gel is distorted and the cilia bend allowing rotatory movements of the head to be detected. The weight of the otoconia allows detection of gravitational forces.

Figure 4. The vestibular apparatus.

Proprioception is the perception of body position and is often described as the unconscious awareness of where the various regions of the body are located at any one time. This can be demonstrated by closing your eyes and moving a hand or foot around. Stretch receptors in the joints and muscles feed this three-dimensional information back to the brain. Assuming proper proprioceptive function, at no time will you lose awareness of where the limb is, even though it is not being detected by any of the other senses. Proprioception is called the joint position sense.

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Sensory Physiology Student Protocol

Required Equipment LabChart software PowerLab Data Acquisition Unit Pins Black ink pen with white barrel or piece of white paper and tape Two pieces of white paper Small black object Small colored object (one color) Small flashlight Paper clips Ruler Three small buckets Hot, cold, and lukewarm water Apple, cut into small pieces Raw potato, cut into small pieces Raw onion, cut into small pieces Three beakers Table sugar Table salt Citric acid (orange or lemon) Cotton balls or cotton buds Swivel chair

Procedure

Exercise 1: Convergence of GazeIn this exercise, you will learn the criteria for binocular vision. Binocular vision requires the separate images in the right and left eyes be fused to give a single view. Fusion of the images of an object is possible only if the images fall on corresponding parts of the right and left retinae. If they do not, a double view of the object results.

1. Hold one arm outstretched, with the index finger upright and in line with some distant object, such as a clock on a far wall. Look at the finger and keep it in focus, but concentrate all attention on the distant object. Note that the distant object is seen doubled – there are two images, side by side.

2. Cover the right eye. Note the right image of the distant object disappears.

3. With both eyes open, look at the distant object. Note your finger is seen doubled.

4. Cover the right eye. Note the left image of your finger disappears.

5. Ask a volunteer to look first at a distant object, and then at an object held close up, about 15 cm from the face. Note the volunteer’s eyes are turned inwards when looking at a close object.

6. Record all your observations in the Data Notebook.

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Sensory Physiology Student Protocol

Exercise 2: AccommodationIn this exercise, you will learn how the eye can accommodate for far or near vision by varying the shape of the lens.

1. Cover or close one eye, and hold a pin about 15 cm in front of the other eye, in line with some distant object. Look at the distant object and note the pin appears blurred and dim as it is out of focus.

2. Now look at the pin. Note that the distant object becomes dim and indistinct. Note also that accommodation for the near object (the pin) is accompanied by a feeling of effort.

3. Cover one eye and hold the pin at arm’s length. While looking at the point of the pin, slowly bring it toward the face until it becomes blurred. The shortest distance at which the pin can be kept in focus is the near point.

4. Record all your observations in the Data Notebook.

Exercise 3: The Blind SpotIn this exercise, you will locate your blind spot, which is the part of the retina with no photoreceptors.

1. Obtain a pen that writes with black ink but has a white barrel. Alternatively, wrap some white paper around the barrel of a black pen, leaving only the black writing tip exposed.

2. Mark a small cross on a piece of white paper. Close the left eye and look steadily at the cross, at a distance of about 25 cm. For the rest of this exercise, keep the head completely still and continue to look at the cross.

3. Slowly move the pen to the right of the cross. At a certain distance the tip will become invisible. Mark this place with a spot on the paper.

4. Carry the pen further to the right, until it becomes visible again. Mark this place with another spot.

5. Similarly, mark the upper and lower limits of the blind spot.

6. Record all your observations and the sheet depicting your blind spot in the Data Notebook.

Exercise 4: Mechanical Stimulation of the RetinaIn this exercise, you will mechanically stimulate the retina by placing pressure on the eyeball. The eye has properties similar to those of a camera, in that the image formed on the retina is inverted. Mechanical stimulation also gives a visual response that is inverted. The main visual response to stimulation is a bright circle or disc on the opposite side of the visual field from the site of stimulation.

1. Turn your gaze to the left and shut both eyes. Keep looking to the left. With a fingertip, press gently on the right side of the right eyeball, at the corner of the eye. Note the visual effect.

2. Slide a finger up and down, and note the direction of movement of the visual response.

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Sensory Physiology Student Protocol

3. Turn your gaze to the right, and similarly press on the left side of the right eyeball, at the corner of the eye. Again, note the visual effect.

4. Record all your observations in the Data Notebook.

Exercise 5: The Positive AfterimageIn this exercise, you will create a positive afterimage. Retinal photoreceptors have a surprisingly long and slow response to light. A brief visual stimulus gives rise to a response that outlasts the stimulus long enough to give an afterimage.

1. Face a bright scene, such as a sunlit window or a strongly illuminated bench top.

2. Close both eyes and cover them with your hands. Wait for 30 seconds.

3. Remove your hands. Open the eyes for the shortest possible time and close them again.

4. Note the afterimage. Bright features of the scene remain visible for an appreciable time, about one fraction of a second.

5. Record all your observations in the Data Notebook.

Exercise 6: The Negative AfterimageIn this exercise, you will create a negative afterimage. The sensitivity of retinal photoreceptors decreases gradually while they are being stimulated by light and increases while they are not. This adaptation to light and dark allows visual function over a very wide range of light intensities. It has the side effect of giving rise to negative afterimages.

1. Place a black object on a piece of white paper or draw a black square on the paper.

2. Look fixedly at the black object for 30 seconds. You may blink, but should take care to keep their gaze fixed.

3. Shift the gaze to a piece of plain white paper, and note the afterimage of the black object. The image lasts for many seconds. The image is inverted in contrast, which is why it is called a negative afterimage.

4. Repeat with a colored object of one color, and note the color change in the afterimage. For example, a red object gives a green afterimage (Figure 5).

5. Record all your observations in the Data Notebook.

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Figure 5. Negative Afterimages

Exercise 7: Retinal Blood VesselsIn this exercise, you will use a small flashlight to view the retinal blood vessels in the eye. The blood vessels of the retina lie in front of the neural and photosensitive layers. Absorption and scattering of light by the retinal vessels would be expected to give rise to an image. This image, however, is normally suppressed. Altering the direction of illumination of the vessels temporarily makes the image visible.

1. Shut both eyes and direct the gaze to the left. Shine the beam of a small flashlight on the eyelid at the right side of the right eye. Hold the flashlight close to the eye, so that it forms a small bright illuminated spot (Figure 6).

2. A lacy network pattern should be visible for a short time, although it fades. Moving the flashlight slightly restores the pattern. Continuous rhythmic movement of the flashlight keeps the image visible.

Note: The vessels may be seen more clearly if the eyes are open and the flashlight is directed at the sclera, or the white part of the eye, directly. This requires, however, either that the experiment is done in a darkened room or that the subject looks at a featureless scene, such as a plain dark-colored wall.

With patience, the branching zigzag pattern of the vessels can be seen clearly. The center of the visual field has no blood vessels passing across it. Vessels approach, in a radial direction, from outer parts of the field. Most people can see the origin of the branching pattern of vessels, in a small region to the right of center. This is the optic disk, which was explored in Exercise 3.

3. Record all your observations in the Data Notebook.

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Figure 6. Exercise Procedure

Exercise 8: Two-point DiscriminationIn this exercise, you will learn the density of tactile receptors in the skin differs greatly in different parts of the body.

1. Take a metal paperclip and unfold it. Bend it into a U shape, with the wire points about 10 mm apart.

2. Touch the two points gently on the palm of a volunteer’s outstretched hand, and ask if one point or two is felt. With a separation of 10 mm, the double stimulus from the two points can be easily felt.

3. Ask the volunteer to close both eyes. Bend the paperclip so as to bring the points closer together. By repeated trials with different point separations, find the smallest separation that the volunteer can distinguish as two points. Test the truthfulness of the volunteer’s responses, from time to time, by turning the paperclip slightly, and pressing only one of the points down (Figure 7).

4. Measure the separation of the points with a ruler.

5. Repeat steps 3 and 4 with trials on different parts of the body, such as a finger tip, the back of the hand, and the back of the forearm.

6. Record all your observations in the Data Notebook.

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Figure 7. Two-point Discrimination

Exercise 9: A Tactile IllusionIn this exercise, you will create a tactile illusion.

1. Cross two adjacent fingers over, so that the fingernails lie side by side, but in a position reversed from the normal. Most people find it easiest to cross the middle finger over the index finger.

2. Place a small object, such as a pen, in the V-shaped gap between the two fingernails, and gently move it back and forth (Figure 8). Note the different sensations you feel.

3. Record all your observations in the Data Notebook.

Figure 8. Tactile Illusion

Exercise 10: A Thermal IllusionIn this exercise, you will learn about receptor adaptation and the perception of temperature. Temperature receptors in the skin adapt, and thermal sensations of warmth or cold are determined more by changes in temperature than by the temperature itself.

1. Obtain three small buckets. Fill one container with hot, but not painfully hot, water. Fill another with cold water, and fill the third with lukewarm water.

2. Place one hand in hot water and the other hand in cold water. Leave them there for 30 seconds. Now place both hands in the lukewarm water.

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3. Record all your observations in the Data Notebook.

Exercise 11: Taste and SmellIn this exercise, you will learn about the importance of smell when tasting food. A large component of taste is actually due to olfaction.

1. Ask a volunteer to close their eyes and to pinch their nostrils together, preventing airflow through their nose.

2. Place a small piece of apple in the volunteer’s mouth, and ask them to try to identify it by taste.

3. Repeat with a piece of raw potato and then with a piece of raw onion. Identification is difficult.

4. Repeat steps 2 and 3, but this time allow the volunteer to breathe through their nose. Identification is now easy.

5. Record all your observations in the Data Notebook.

Exercise 12: Distribution of Taste BudsIn this exercise, you will determine the distribution of taste buds in your mouth. Taste buds are found principally on the tongue but also on the palate and pharynx.

1. Obtain small beakers of the following solutions: table sugar and water, table salt and water, and citric acid and water.

2. Dip a small piece of clean cotton wool or the end of a cotton bud in the sugar solution and shake off the excess solution.

3. Apply the cotton to the back of a volunteer’s tongue, and ask the volunteer to report the sensation (Figure 9). Discard the cotton wool or bud.

Figure 9. Locations to Test the Solutions4. Using a fresh piece of cotton wool or new cotton bud, test the sensitivity of one side of

the tongue. Similarly, test the tip of the tongue.

5. Repeat steps 2 to 4, but with the salt solution. Note the distribution of salt sensitivity.

6. Repeat steps 2 to 4, but with the citric acid solution. Note the distribution of sour sensitivity.

7. Record all your observations and draw a map of the taste bud distribution in the Data Notebook.

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Exercise 13: The Joint Position SenseIn this exercise, you will learn the effectiveness of proprioception, or joint position sense.

1. Ask a volunteer to hold out one hand with the palm facing up and the fingers stretched out.

2. Hold the volunteer’s index finger by placing your thumb on one side and your index finger on the other. Do not hold the volunteer’s finger by the front and back. That could give cues about movements, deriving from the force of lifting or pulling down (Figure 10).

3. Bend the volunteer’s finger up while saying, “This is up.” Then pull the finger down to the original extended position while saying, “This is down.”

4. With the volunteer’s eyes shut, test their ability to identify the direction of a series of finger movements. Try both large and small movements.

5. Record all your observations in the Data Notebook.

Figure 10. How to Hold the Volunteer’s Hand

Exercise 14: The Semicircular CanalsIn this exercise, you will use a swivel chair to detect rotatory movements of the head. The semicircular canals are liquid-filled channels in the temporal bone of the skull and form part of the inner ear. They detect rotatory movements of the head in three axes, but they do not signal the body’s position.

1. Have a volunteer sit on the swivel chair, with both feet in the air, and close both eyes.

2. Ask the volunteer to say when a rotation is detected and to indicate in which direction. Test the volunteer’s ability to sense rotary motion, by rotating the chair at various speeds and for various durations. Very slight movements are reliably detected.

3. Record all your observations in the Data Notebook.

Take care that the volunteer does NOT stand immediately after spinning. The volunteer should be assisted out of the chair after a few minutes of recovery.

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Data Notebook

Exercise 1: Convergence of Gaze

Exercise 2: Accommodation

Exercise 3: The Blind Spot

Exercise 4: Mechanical Stimulation of the Retina

Exercise 5: The Positive Afterimage

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Exercise 6: The Negative Afterimage

Exercise 7: Retinal Blood Vessels

Exercise 8: Two-point Discrimination

Exercise 9: A Tactile Illusion

Exercise 10: A Thermal Illusion

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Exercise 11: Taste and Smell

Exercise 12: Distribution of Taste Buds

Exercise 13: The Joint Position Sense

Exercise 14: The Semicircular Canals

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Study Questions

1. Explain the process of activating a sensory neuron.

2. What is the anatomical basis for the blind spot?

3. Where was your tactile discrimination the best? What can you say about the density of tactile receptors on your arm?

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4. What happened when you placed your hands from the hot and cold water to the lukewarm water? Why do you think this happened?

5. What can you say about the distribution of the different types of taste receptors?

6. What is the primary difference between fast pain and slow pain?

Copyright © 2010 ADInstruments Pty Ltd. All rights reserved.

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PowerLab® and LabChart® are registered trademarks of ADInstruments Pty Ltd. The names of specific recording units, such as PowerLab 8/30, are trademarks of ADInstruments Pty Ltd. Chart and Scope (application programs) are trademarks of ADInstruments Pty Ltd.

www.ADInstruments.com

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