Lab 5_Sensory Receptors

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Jan 9, 2024

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Bio 240L Intro Human Physiology Lab LAB 5: SENSORY RECEPTORS Specialized cells called sensory receptors allow an individual to perceive stimuli relating to the five senses: vision, audition, taste, olfaction (smell), and touch. The region that detects the stimulus due to the presence of sensory receptors is called the receptive field . Along with the brain and its neural pathways, sensory receptors form what are known as sensory systems. Sensory systems function in response to stimuli. There are four aspects to keep in mind when referring to a stimulus: modality (type of stimulus; Examples: smells and flavors/tastes), intensity , location , and duration . Not surprisingly, even within a given sensory system, different receptors respond to different types of stimuli. In all cases, the stimulus causes depolarization of a nerve ending and an action potential is initiated. General classes of sensory receptors:  Chemoreceptors o Responsive to chemical stimuli; e.g. tastes associated with food we ingest  Mechanoreceptors o Responsive to physical deformation of a cell caused by vibration, touch, pressure, stretch or tension; e.g.: hair cells in the cochlea are sensitive to vibrations created as air passes across them and convert those waves into sound  Nociceptors o Responsive to pain; for example, if you touch a hot pot on a stove, nociceptors will sense the heat as abnormally high and the brain will instruct you to withdraw your hand from the source of danger  Photoreceptors o Responsive to light; convert light stimuli hitting the retina to visual elements that we perceive as shapes and colors  Thermoreceptors o Responsive to temperature allowing us to perceive when the temperature of our environment is hot or cold I. The Somatosensory System The somatosensory system consists of the receptors and transmitters (pathways) that perceive the cutaneous senses touch or pressure, temperature, and pain (nociception). In addition, posture, movement, and facial expression (proprioception) are also detected. The skin is the largest organ of the body and has many receptor sites for the cutaneous sensations listed above. The sensitivity to a given sensory stimulus is dependent on the density of the cutaneous receptors and the size of the receptive field (area in which a sensory neuron may be activated) in the area that is stimulated. Receptive fields may be small and numerous or alternatively, may be large and overlapping. Areas of the body such as the tongue, lips, back of the neck, and fingertips contain higher numbers of receptors per unit area than the back, arms, palm, thigh, and calves. As a result, the former can sense stimuli more acutely, i.e., are more sensitive. Types of cutaneous senses  Light touch and texture: activation of Merkel (tonic=slow adaptation; epidermis) and Meissner corpuscles (phasic=fast adaptation; most abundant fingertips, palm)  Heavy touch: Ruffini corpuscles, tonic receptors in the dermis and joint capsules  Krause receptors: located in mucosa membrane of lips, tongue, anterior of the eye 1

Bio 240L Intro Human Physiology Lab  Deep touch (pressure, stretch, vibration): lamellar pacinian corpuscles activated, found in bones, viscera, deep in the dermis of palms and feet, genitals  Thermoreception: free nerve endings or bare dendrites activated in response to heat or cold; the body has 3-10 times more cold receptors than warm receptors.  Majority are tonic receptors that adapt slowly and generate signals more steady Proprioception  One’s ability to perceive the relative positions of various body parts at any given time  Entails an awareness of different factors including posture, movement, facial expression  This perception is necessary for the maintenance of balance, for example, while walking  These are tonic receptors that adapt very slowly and generate signals more steady (the brain must always be aware of the body position The term kinesthesia is often used interchangeably with proprioception but there is a key distinction – kinesthesia refers specifically to the ability to move (e.g., walking), not one’s ability to maintain balance while doing so. Vestibulo-ocular/Occulovestibular Reflex  Maintains image stability when one’s head is moved  Does so by moving the eyes in the opposite direction of where the head was moved thereby allowing the eyes to remain trained on the center of the visual field Two-Point Discrimination Two simultaneous touches are distinguished only if they stimulate two sensory receptors. In areas with many small sensory receptors (high density), the ability to distinguish different simultaneous stimuli is very sensitive (stimuli as little as 1 mm apart may be distinguished as separate). Conversely, in areas with a low density of receptors or overlapping receptors such as the back, simultaneous stimuli as far as 5-6 cm apart are only perceived as one stimulus because they are populated by few, large sensory receptors. Therefore, areas with many small receptors have much smaller two-point thresholds (minimum distance at which simultaneous stimuli can be perceived as separate) whereas areas with few large receptors have much larger two-point thresholds. II. The Visual System (photoreceptors) This is the component of the nervous system responsible for an organism’s ability to see. It interprets information from visible light to construct a three- dimensional representation.  Comprised of: o Eye (retina) o Optic nerve o Optic chiasm o Optic tract o Lateral geniculate body o Optic radiation o Visual cortex 2

Bio 240L Intro Human Physiology Lab Blind Spot The retina of the eye contains two types of light receptors, rods and cones. Rods are responsible for black and white and night vision. Cones are responsible for color and daylight vision. There is one area of the retina that contains neither rods nor cones because the optic nerve exits from this area to carry stimuli to the brain. This area is called the blind spot as images projected there are not perceived by the eye (and brain) due to the absence of receptors. Afterimage As a photoreceptor is stimulated, its visual pigment is used up and must be replaced before sensitivity is restored. For example, the adjustment period experienced when going from bright light to darkness (or vice versa) is the time required to resynthesize the necessary pigment. The same is true after looking at a brightly colored object, such as the piece of brightly colored paper used in the lab, for an extended period of time. Over time, the pigments in the cones are degraded and when the color is removed, the area of the retina previously stimulated by the color is unable to respond to the less intense color contained in the white light of the underlying paper. During this time, the eyes will be able to respond to every wavelength of light except the one normally absorbed by the depleted pigment. As a result, an afterimage is formed from the remaining colors of the spectrum. Visual Accommodation The human eye can focus on objects at very close or far distances. This is due to changes in the shape of the lens that allow the lens to adapt to these distances and project clear images. III. The Auditory System (mechanoreceptors)  The outer and middle ear are designed to amplify and transmit sound waves to the inner ear  Hair cells within the cochlea are then stimulated and generate nerve impulses which are transmitted via the acoustovestibular nerve (cranial nerve VIII)  The signal is eventually delivered to the primary auditory cortex IV. The Gustatory System (taste; gustatory receptors) 3

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Bio 240L Intro Human Physiology Lab The sense of taste is a type of chemoreception. It is involved the identification of flavor associated with substances ingested. Chemicals must be dissolved in saliva or other solvent to stimulate the taste buds (gustatory receptor cells). Initially, it was thought that there are only four individual taste responses: sweet (in response to hydroxyl groups of a carbohydrates), sour (in response to hydrogen ions in acids), salty (in response to inorganic metal ions such as Na + and K + ), and bitter (in response to alkaloids). We now know that taste buds are able to detect additional flavors including umami (savory or meaty; not pictured) produced by amino acids such as aspartic and glutamic acids. The sense of taste is not uniform for all these categories. For example, there is a higher sensitivity to bitter tastes than to sweet which is probably a safety mechanism since bitter taste buds are located at the back of the tongue and may help to trigger involuntary vomiting when a potentially harmful substance is ingested. Some substances can be tasted by certain individuals and not others (genetically controlled). In addition, the sense of taste is coupled with the sense of smell (olfaction). V. The Olfactory System (smell; olfactory/odor receptors) The ability to smell is also a type of chemoreception. There are numerous smell responses to many types of chemicals. The olfactory epithelium which is located in the upper portion of the nasal cavity is responsible for the ability to sense smells. It consists of olfactory receptor cells in hair-like cilia that are covered in a mucus layer. Odor molecules must first dissolve in this mucus layer and bind to the olfactory receptor cell membranes. Impulses are then transported from the olfactory epithelium within the nasal cavity to the olfactory bulb of the brain. These receptors adapt very quickly because they fire fast when first stimulated and then reduce or stop signaling. The sense of smell is closely linked to one’s ability to taste flavors. For example, flavors are less intense (food tastes bland) when suffering from a stuffy nose attributed to a cold or sinus infection. This is because the sense of taste comes from the integration of taste, smell, and chemical irritation (how something fells in one’s mouth – carbonation, spiciness, or coolness, for example) stimuli by the brain. The sense of smell is also closely linked to the limbic system which may explain why sometimes smells are able to evoke certain memories. 4

Bio 240L Intro Human Physiology Lab Name ___________________________________ Carolina TM Human Senses BioKit ® INSTRUCTIONS : During this lab you will be learning about sensory receptors. Perform the following after completing the exercises in the handout you are given in lab. Work in groups. You will need a partner for most of the procedures. Dispose of all material that has come in contact with saliva in the biohazard bag (orange bag in white containers), NOT in the sharps containers (red boxes). Part 1: Cutaneous Senses During this activity, you will measure the skin’s sensitivity to touch, pressure, and temperature. Perform the following tests on the back of the hand. Procedure A. Light Touch 1. Ink the grid stamp. Stamp the back of the test subject’s hand; wait a few minutes to dry. 2. Place the test subject’s hand palm-down on the desk. Instruct the test subject to close his or her eyes and say “yes” upon feeling a touch on the area being tested. 3. The experimenter should lightly touch a grid square with a bristle and wait for a response. a. If no response is given, proceed and touch the next square 4. If the subject senses the touch, the data recorder should fill in the corresponding square on the Data Sheet. Continue until all squares in the grid have been tested. B. Deep Touch (Pressure) 1. Follow the instructions in “Light touch” above, using the same grids but testing for deep touch (pressure) responses. Instead of a bristle, use the head (not the point!) of a straight pin and instruct the test subject to say “Yes” upon feeling pressure–not just a touch on the area being tested. 2. As before, if the subject senses the pressure, the data recorder should fill in the corresponding square on the Data Sheet. Continue until all squares in the grid have been tested. C. Thermoreception ( Hot and Cold) 1. Follow the instructions in “Light touch” above, using the same grid but testing for responses to hot and cold stimuli. Instruct the test subject to say “yes” upon feeling warmth on the area being tested. Instead of a bristle, use the head of a pin that has been heated to 55-60° C for 10-15 seconds. Touch a square within the given grid and then quickly remove the pin. 2. Reheat the pin and touch another point on the grid. Repeat and continue testing the grid according to your teacher’s instructions. Record all positive responses on the Data Sheet. 3. Now, test for cold responses. Use the same procedure, but dip the head of the pin in ice before each test. Record all positive responses by filling in the corresponding circles on the Data Sheet grid. Questions: Were there more warm or cold receptor sites on your hand? _________________________ 5

Bio 240L Intro Human Physiology Lab Did some areas sense both heat and cold? ______________ Why do you think this it is important to sense both heat and cold? (1 point) _____________________________________________________________________ Data Sheet Carolina ™ Human Senses BioKit ® Part 1: Cutaneous Senses Apply to the back of the hand: A. Light Touch B. Pressure (Deep touch) B. Thermoreception (Heat) C. Thermoreception (Cold) 6 x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x X xx x x x X x x x x x x X x x x x x X x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x x

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Bio 240L Intro Human Physiology Lab D. Two-Point Discrimination 1. Place the test subject’s hand palm-up and motionless on the desk. Instruct the test subject to close his or her eyes and to say “one” or “two” depending upon the number of points of contact felt on the area being tested. 2. Start with the two points of the calipers closed, with the pointer at 0mm. Begin the experiment by touching the subject’s fingertip. The subject should keep his or her eyes closed, and should feel only one point. 3. Create a 1 or 2 mm gap between the two points of the probe by moving the indicator to the 1-mm or 2-mm mark and touch the subject’s fingertip with the caliper point again. 4. Repeat this process, increasing the distance between points 1 or 2 mm each time, until the test subject reports feeling two separate points. This minimum distance is the “two- point threshold.” Record this two-point threshold in the “Fingertip/ Trial 1” cell on the Data Sheet. 5. Repeat steps 2-4 for the following parts of the body: the back of the neck, the mid-calf, and the palm of the hand. Record each result in the “Trial 1” column of the table on the Data Sheet. 6. Conduct two additional trials at each location. Record the two-point threshold values in the appropriate rows and columns of the table. 7. Determine the subject’s average two-point threshold value for each location. Are some locations more sensitive than others? __________________________ D. Two-point Discrimination Location Trial 1 Trial 2 Trial 3 Average Fingertip 2 mm 4 mm 6 mm 4 mm Back of Neck 20 mm 26 mm 28 mm 25 mm Mid-Calf 36 mm 38 mm 40 mm 38 mm Palm 8 mm 10 mm 12 mm 10 mm Question 1. Which one, the fingertips, back of the neck, mid-calf, or palm, is most sensitive? (1 point) ______Fingertips_________________________________ On the basis of the two-point discrimination data for these four areas of the skin, what inferences can you make regarding receptor densities and area sensitivity? (Hint: in order to distinguish that there are two points, each point of the probe must touch a different skin receptor.) 2 points 7

Bio 240L Intro Human Physiology Lab ______The greater the density the more sensitive.___________________________________________________________________ _ __________________________________________________________________________ Part 2: Chemical Senses A. Taste During this activity, you will map the areas of the tongue that respond to the four basic taste stimuli: sour, sweet, bitter, and salty. Throughout the activity, the experimenter should conceal the identity of the solutions from the test subject. Procedure 1. Fill the bottom of a cup with one of the four test solutions, sour, sweet, bitter, or salty. 2. Have the subject sit with mouth open and tongue extended. 3. Dip a sterile swab in the solution in the cup. Carefully place the swab on one of the zones of the tongue shown on the tongue in the diagram below, and then withdraw the swab. Instruct the subject to say “yes” if he or she tastes the solution and “no “if not. The data recorder should mark this as a positive (+) or negative (-) response in the corresponding area of the tongue diagram below. 4. Dip the swab in the solution again and place it on a different zone. Again, record the response given by the test subject. Continue until all seven zones in the diagram have been tested for that test solution. Discard the swab in the biohazard waste container, the excess solution down the drain and the cup in the garbage. 5. Have the subject rinse his or her mouth with water. At the same time, prepare a cup containing a different test solution. Test all seven areas of the tongue with the second solution and then record the responses of the test subject on the Data Sheet below. 6. Repeat the taste test experiment testing the remaining solutions. Have the subject rinse his or her mouth with water between each experiment. As before, record the responses for each of the seven zones on the tongue diagrams and discard the swab, solution, and cup after each test. 8

Bio 240L Intro Human Physiology Lab Questions 1. How many tastes were detected in the center of the tongue? (0.5 point) 2. How many tastes should you have been able to detect in the center of the tongue? (0.5 point) 3. Where did the tongue detect tastes that are a. Bitter? ____________________________________________________ b. Sour? ____________________________________________________ c. Sweet? ____________________________________________________ d. Salty? ____________________________________________________ B. Smell In this exercise, you will explore how long you retain the ability to smell a particular odor until it begins to fade or becomes nonexistent. This phenomenon is termed “olfactory fatigue.” Follow the instructions below, observe the outcome, and then answer the questions that follow. Caution: Use the “wafting method” to sniff each odor. Procedure 1. Use the dropper to add clove oil to the cotton tip of a sterile swab. 2. The lab partner should hold the swab approximately 30 cm (1ft) away from the nose of the test subject. 3. Have the test subject close his or her left nostril by pressing with the left index finger and use his or her other hand to waft the scent towards the open (right) nostril, inhaling thorough the right nostril, then exhaling through the mouth; the data recorder should note the start time. 4. Have the subject continue to sniff the odor and breathe out though the mouth at a normal rate until the smell is no longer detectable or has greatly diminished; at this point, the data recorder should make a note of the amount of time that has elapsed since the start time and record in the table below. 5. Repeat this process using the same nostril , but this time, the test subject will sniff a new swab with peppermint oil. As before, the data recorder should make a note of the start time and the amount of time elapsed until the subject no longer smells the scent; record this time in the table below. 6. Repeat the steps above on the left nostril. B. Smell Fragrance Right Nostril Fatigue Left Nostril Fatigue Clove Oil minute(s) minute(s) Peppermint Oil minute(s) minute(s) 9 1 point total

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Bio 240L Intro Human Physiology Lab Questions 1. Could the test subject smell the peppermint oil immediately after the diminished odor for the clove oil? (0.5 point) 2. What does this say about the nose’s ability to detect new or different odors? (1 point) Part 3: Photoreception Senses (Vision) A. Blind Spot The photoreceptors at the back of the eye (retina) are composed of cones for color vision and rods for black-and-white vision. The rods are located on the outer edges of the retina and the cones are located in the center of the retina. There is a hole in any image formed on the retina at the location of the optic nerve near the center of the cone area. This region is called the “blind spot” because the brain receives no information about the images there. Why? Because the optic nerve is located there, there are no photoreceptors in that spot. Procedure 1. Locate the blind spot indicator sheet of paper (the + and the •). Cover one eye. 2. Hold the paper approximately two feet in front of your face with the plus sign directly in front of your open eye and the dot to the outside of your face. 3. Stare directly at the plus sign. Without changing your focus, move the paper toward your open eye slowly; the dot should disappear and reappear. At the distance where the dot disappears, its image is being focused upon your blind spot, the location of your optic nerve. 4. Repeat the procedure with your other eye. B. Visual compensation Does your brain compensate for the loss of vision from the blind spot? In this activity, you will solve this mystery by describing the presence or absence of a white background when the dot disappears and by observing a line that extends through and beyond the blind spot. Procedure 10

Bio 240L Intro Human Physiology Lab 1. Repeat Activity A above but this time, describe the color of the area where the dot disappears compared to the color of the rest of the sheet of paper. Is there an absence of white where the dot was or does the brain automatically fill in the white background when the dot disappears? 2. Now, use the visual compensation indicator sheet (with the + and the ). Cover one eye and hold the sheet approximately two feet in front of your face with the plus sign directly in front of your open eye (with the dot and vertical line to the outside of your face). 3. Stare directly at the plus sign. Without changing your focus, move the sheet toward your open eye. Question 1. What happens to the vertical line drawn through the dot as the dot disappears into the blind spot? Does the line that passes through the dot disappear too? The vertical line disappears with the circle. __________________________________________________________________________ __________________________________________________________________________ C. After Images In this activity, you will investigate how green-sensitive and orange-sensitive cones of the retina can be fatigued, by viewing these intense colors against a white background. Over time, the pigments in the cones are degraded and, when the color is removed, the area of the retina previously stimulated by the color is unable to respond to the less intense color contained in the white light. The retina responds to all wavelengths of the spectrum other than the intense color (green or orange) stimulus. The result is an afterimage formed from the remaining colors of the spectrum. Procedure 1. Place an orange card on a sheet of white paper. 2. Have the test subject stare at the card for 30 seconds without moving his or her eyes, then remove the card. 3. The test subject should see a rectangular image where the card was. What color is the after image? Record the color on the next page. 11

Bio 240L Intro Human Physiology Lab 4. Repeat this procedure with a green card. What color is the afterimage? Record the color below. C. Afterimages Orange card afterimage color: ______blue__________________ (0.5 point) Green card afterimage color: _______pink_________________ (0.5 point) D. Visual Accommodation The closest point at which an eye can focus is termed the near point (or minimum focal point) of vision. This can be measured by focusing on a sharply defined point and bringing it closer to the eye until it becomes out of focus (blurred or doubled). The distance from the eye to the object is the near point. Procedure 1. The test subject should hold a pin oriented point-upward (not inward) at eye level and at arm’s length in front of the right eye. 2. He or she should then cover the left eye and focus on the point of the pin with the right eye 3. Slowly bring the pin closer to the right eye until the point becomes out of focus. 4. The lab partner should measure the distance in cm from the test subject’s eye to the pin and record this distance below. 5. Repeat the procedure for the left eye and record the results below. D. Visual Accommodation Near point for right eye: ___________9_________ cm (0.5 point) Near point for left eye: ____________8_________ cm (0.5 point) IS VISUAL INPUT NEEDED FOR PROPIOCEPTION ? Muscle spindle organs and Golgi tendon organs (and joint kenesthetic receptors) detect proprioception (the position of your limbs in space). You will test whether visual input is necessary to correct the signal or if muscle and tendon stretch are enough. 1. Walk to the blackboard and write an X on the board. 2. Repeat twice, but write the two Xs over the first one. 3. You should have three Xs on top of each other so you cannot distinguish them. 4. Repeat Steps 1 – 3 with your eyes closed. 5. Open your eyes and compare the Xs you drew with your eyes open to the ones you drew with your eyes closed. 6. Do you need visual input for propioception? Yes or No ? _______________ (0.5 pt) 12

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