Laboratory 1 Nerve Function

Transcription

1 Bio 104: Comparative Animal Physiology LABORATORY MANUAL Fall 2001

2 Laboratory 1 Nerve Function RESTING MEMBRANE POTENTIAL All cells in the body have a resting membrane potential that is dependent on the balance of the different ion concentrations inside and outside the cell as well as the permeability of the cell's membrane to these different ions. This laboratory is designed to help you understand the basis for determining the resting membrane potential of a cell. In addition, you can explore some of the potential consequences of changes in either membrane permeability or ion concentrations. This lab utilizes a computer simulation of neuron. There are several advantages to using a computer instead of recording from an actual neuron. First of all, it is much easier from a technical standpoint. It also bypasses the need to use animals, and it is infinitely less expensive (the average electrophysiological recording setup today costs around $30,000 to $60,000 whereas this program was much less than that...). For this lab you will want to split up into four groups. You can open the program that you need (C-CLAMP) by clicking on REST.CCS in the LabFiles folder on the desktop. Begin the simulation by selecting Begin from the Run menu. This program simulates the resting membrane potential of a giant squid axon using the Goldman-Hodgkin-Katz equation. You should get a straight line at -65mV. I. Conductance. This model assumes that the membrane is "leaky" to two ions, Na + and K +. The "leak" of a membrane to an ion is not altered by changes in membrane voltage, but is due to channels in the membrane which are always open, as well as different membrane pumps that transport ions across the cell membrane. An increase in the "leakiness" of the membrane means there are more paths for that particular ion to cross the membrane. To see the effects of changes in membrane permeability on the resting membrane potential, open Conductances under the Parameters menu. 1. Select Begin from the Run menu to begin the simulation. Now change the pnaleak to 0 and select Overlay from the Run menu. Now you can see both traces. What effect does changing pnaleak have on the resting membrane potential? Explain why this happens. 2. Now set the pnaleak to 10. What happens? Why? Return the pnaleak back to.06 and move on to the next section. 2

3 II. Ion Concentrations. The other major determinant of the resting membrane potential is the concentration of ions inside and outside the cell. The direction an ion will move is determined by the concentration gradient and the electrical charge of that ion. When these two forces are in balance, the ion is at equilibrium and the voltage across the membrane is termed the equilibrium potential for that ion. Mathematically, this equilibrium potential can be calculated using the Nernst Equation: Ex = [(RT)/(zF)] ln[x o /X i ] where R = volts. coulombs/(t. mol) {gas constant} F = x 10 4 coulombs/mol {Faraday's constant} T = temperature ( o C) {absolute temperature} z = charge of the ion {ie., +1 for sodium, -1 for chloride} 1. Using the initial concentrations for K + and Na +, calculate the equilibrium potential for each ion. You can find these concentrations under the Parameters menu by choosing Ions. Show your calculations. 2. Now calculate the K + equilibrium concentration assuming that the inside and outside concentrations are both 135 mm. 3. In the Ions menu, change the [K]o to 135; pkleak to 1; and pnaleak to 0. What is the resting membrane potential? Why? To determine the resting membrane potential when the membrane is permeable to more than one ion, we use the Goldman-Hodgkin-Katz equation. This equation takes into account both the ion concentrations and the membrane permeability: E = (RT)/(F) ln (P K [K] o + P Na [Na] o + P Cl [Cl] i ) / (P K [K] i + P Na [Na] i +P Cl [Cl] o ) 4. Use the Goldman-Hodgkin-Katz equation to determine the resting membrane potential when: P K = 0.5 P Na = 0.03 [K] o = 3.1 [K] i = 135 [Na] o = 145 [Na] i = 31 (show your calculations) Use the computer program to check your answer. 5. Again using the computer, reverse the inside and outside concentrations of the ions, keeping the permeabilities the same. What happens to the resting membrane potential? Why? 6. A large increase in the extracellular K + concentration is lethal for most animals. Determine what would happen to the resting membrane potential if the extracellular K + o was increased from 3.1 to 20 (all other values remain unchanged from normal). Why might this be lethal? 3

4 ACTION POTENTIAL Neurons are able to send information rapidly over long distances using action potentials. In this laboratory, we will examine several properties of action potentials. Again, we will use a computer simulation of a neuron. The properties we will examine include threshold, the ionic basis of the action potential, the refractory periods, and the ability of neurons to code the intensity of stimuli. For this lab you will need a different parameter file. Close the REST.CCS window (DON'T save changes) and then the ACTIVE.CCS. This program allows us to vary ion concentrations (similar to the last lab) and also allows us to stimulate our neuron by injecting current under the Protocol option (in the Parameters menu). I. Threshold. We want to determine the threshold voltage for this particular cell. This is the minimum voltage change necessary to produce an action potential. Under the Protocol menu, you can select the amplitude and duration of your stimulus (referred to as the step). Begin by setting the step onset to 20 msec, the offset at 25 msec, and the injected current at 0.5 na. Run the simulation. Now increase the amplitude of the injected current by increments of 0.5 na, up to 4.0 na, using overlay each time. 1. What was the minimum current needed to produce an action potential? 2. Estimate the threshold voltage for this cell from the trace on the computer. 3. Why does the voltage overshoot 0? 4. What happened to the amplitude of the action potential as you increased the amplitude of the stimulus from 1.0 na to 4.0 na? You can also look at the effects of changing the duration of the stimulus, keeping the amplitude constant. Set the injected current to 3.0 na. Now vary the duration of stimulus from 1 msec (ie. step on at 10 msec, step off at 11 msec) in 1 msec intervals, up to a maximum duration of 7 msec. 5. What happens to the action potential as you increase the duration from 1-7 msec? II. Ionic Basis of the Action Potential. We can use a combination of voltage current recordings to study the ions that underlie the action potential and this program to determine the roles of the different ions in the action potential. To determine which ions are important in the generation of the action potential, you can change the concentration of different ions one at a time and observe the effects. Record a control AP (no changes in ion concentrations). Record the amplitude and duration of the AP in Table I. Change each of the ions as described in Table I and record the effects on the amplitude and duration of the action potential. 1. Based on your data, what are the most important ions in generating an action potential? Why do the other ions not play a significant role? When you reduced the [Na + ] o the cell was hyperpolarized, as observed in the previous section. To compensate for this, we can inject a base current (under the protocol menu) that will depolarize the cell back to about 65 mv. We can then test the ability of the cell to fire under reduced [Na + ] o conditions. To do this, follow the conditions in Table II. 4

5 2. Based on your results, explain the role of Na + in the action potential. Now we will further explore the role of K + in the action potential. First we will look at extracellular K +. Again, as observed in the previous section, reducing the [K+]o caused a hyperpolarization. Therefore we will inject a base current to bring the membrane potential back to 65 mv, and then test the cell s ability to fire an action potential with injected current. In this experiment we will also measure the hyperpolarizing potential the lowest voltage after the action potential (the recovery phase). Follow the parameters in Table III. 3. What happened when you changed the base current to 0.3 and stimulated the cell? What does this tell you about the role of K + in generating an action potential? 4. How do the hyperpolarizing potentials change during this experiment? Explain your results with respect to K +. Next we will examine the effects of intracellular K +. Close the ACTIVE.CCS file and open the K_INTRA.CCS. This file mimics the Toothpaste experiment where Baker, Hodgkin, and Shaw squeezed the axoplasm out of a squid giant axon and replaced it with artificial axoplasm with different ion concentrations. This time we will change the [K + ] i. Record your results in Table IV. 5. What happened to the resting potential upon changing the K + concentration? How do you explain this? 6. What happened to the cell after injecting a base current of 3 na. How do you explain this with respect to K + function. Re-examine the lethal effects of increased extracellular K + using the K_INTRA.CCS file. First, close the file without saving changes and then open it again. Observe a normal action potential and then increase [K + ] o to 20 mm. Observe the changes by overlaying the new trace. 7. What happens to the action potential under these conditions? How could this be lethal to the animal? III. Action Potential Frequency. Neurons can code for the intensity of a stimulus by the number of action potentials produced. We can look at this using the Neuron program. Under the Protocol menu, change the trial duration to 250 msec, the Step on to 50 msec and the Step off to 100 msec. Now we will gradually increase the amplitude of the step and record the number of action potentials produced in Table V. 1. Graph the data collected with the stimulus amplitude as the x axis, versus the number of action potentials. What might determine the maximum frequency of action potentials a cell can produce? 5

6 Results section: hand in this section with your lab report. Table I - Amplitude and duration [ion] out Control 0.1 mm Mg mm Na mm Ca mm Cl mm K + AP amplitude (mv) AP duration (msec) Comments Table II - Reduced Na + conditions [Na + ]out Base Injected (mm) current (na) current (na) Resting potential (mv) Action potential? Comments Table III - role of extracellular K + [K + ]out (mm) Base current (na) Injected current (na) Resting potential (mv) Action potential? Hyperpol. potential

7 Table IV - Toothpaste experiment [K + ] i (mm) Base current (na) Injected current (na) Resting potential (mv) Action potential? Comments Table V - Action potential frequency Amplitude # Action potentials RESTING MEMBRANE POTENTIAL I. Conductance 1. What effect does changing pnaleak have on the resting membrane potential? Explain why this happens. No effect - an ion must be able to cross the membrane in order to influence the membrane potential. 2. Now set the pnaleak to 10. What happens? Why? Na + now plays a more important role in the membrane potential than K +. Therefore, the potential approaches the equilibrium potential for Na +. II. Ion Concentrations 1. Using the initial concentrations for K + and Na +, calculate the equilibrium potential for each ion. Show your calculations. E K = [8.312 ( C)] / [+1 (9.648x10 4 )] [ln (3.1 / 135)] = mv *tell the students to convert to mv, from V. 7

8 2. Now calculate the K + equilibrium concentration assuming that the inside and outside concentrations are both 135 mm. 0 mv because there is no longer a gradient across the membrane and therefore no driving force. 3. In the Ions menu, change the [K]o to 135; pkleak to 1; and pnaleak to 0. What is the resting membrane potential? Why? 0 mv, same reason. 4. Use the Goldman-Hodgkin-Katz equation to determine the resting membrane potential (show your calculations). Should be about 65 mv. 5. Again using the computer, reverse the inside and outside concentrations of the ions, keeping the permeabilities the same. What happens to the resting membrane potential? Why? +65 mv because the gradients have been reversed. 6. A large increase in the extracellular K + concentration is lethal for most animals. Determine what would happen to the resting membrane potential if the extracellular K + was increased from 3.1 to 20 (all other values remain unchanged from normal). Why might this be lethal? Increasing the extracellular K+ causes the membrane to depolarize. If it rises far enough, it will pass threshold and the neuron will fire spontaneously. This would be catastrophic, because activity of neurons must be regulated to function correctly. (Lethality would probably actually be caused by asynchronous contractions in the heart.) ACTION POTENTIAL I. Threshold 1. What was the minimum current needed to produce an action potential? 2.5 na 2. Estimate the threshold voltage for this cell from the graph. -50 mv 3. Why does the voltage overshoot 0? Because the action potential is driven by the opening of Na + channels. When the Na + conductance increases the membrane potential will be driven by the Na + concentration gradient, and the peak of the action potential will approach the equilibrium potential for Na What happened to the amplitude of the action potential as you increased the amplitude of the stimulus from 0.5 na to 4.0 na? Nothing, action potentials are all-or-none. 8

9 II. Ionic basis of the action potential 1. Based on your data, what are the most important ions in generating an action potential? Why do the other ions not play a significant role? Na + and K +. The conductance of the other ions is too low to have a measurable effect. 2. Based on these results, explain the role of Na + in the action potential. Na + is critical for production of an action potential. Without extracellular Na +, there is no ion to carry the positive charge into the neuron. 3. What happened when you changed the base current to 3 and stimulated the cell? What does this tell you about the role of K + in generating an action potential? This experiment shows that extracellular K+ is not important for generating the action potential (except indirectly through the resting potential). 4. How do the hyperpolarizing potentials change during this experiment. Explain your results with respect to K +. The potential becomes more hyperpolarized as it approaches E K +. K + ions remove positive charge from the inside of the neuron during recovery. If the concentration gradient for K+ is increased (by decreasing extracellular K + ), than E K + will become more hyperpolarized. 5. What happened to the resting potential upon changing the K + concentration? How do you explain this? It increased to ~100 mv. There is no longer a strong concentration gradient for K +, so Na + will become more important in determining the resting potential. 6. What happened to the cell after injecting a base current of 3 na. How do you explain this with respect to K + function. The resting membrane potential returned to normal, but upon the first stimulus, the potential depolarized to ~ 90 mv and stayed there. This shows, again, that K + is not needed to generate an action potential, but it is needed to recover. 7. What happens to the action potential under these conditions? How could this be lethal to the animal? The cell fires a spontaneous action potential (the action potential occurs before the stimulus) and cannot recover. Therefore, neuron activity would no longer be controlled this would very quickly become lethal because control of the cardiovascular and respiratory systems (for example) would not be regulated. III. Action potential frequency 1. Graph the data collected with the stimulus amplitude as the x axis, versus the number of action potentials. What might determine the maximum frequency of action potentials a cell can produce? 9

10 The absolute and relative refractory periods. 10

11 Laboratory 2 Sensory Receptors Introduction: Virtually all animals access information about their environment in order to perform such important tasks as finding food and mates and avoiding becoming the food of others. However, for an animal to respond to an environmental stimulus, it must first be able to sense that stimulus. Thus, our ability to respond to our environment is critically dependent upon the types and sensitivities of our sensory receptors. In the following exercises, we will explore some properties of receptor systems for taste, vision, and touch. Keep in mind that these are not the only sensory receptor systems, but that we can only explore a few in the laboratory setting. For these experiments you may find it most convenient to break up into groups of 3. One person can be the subject, one the experimenter, and one the recorder. Then you can switch places. For many of the experiments listed below we will compare results across all of the students in the class. When indicated, add your results to the board, as described by your TA. We will tally all of these results and make them available in lecture on Friday. For your lab write-up, answer all of the questions and append the tables and figures from this manual to the back of your report. You do not need to hand in a copy of the class results. I. VISUAL SYSTEM A. Near Point Discrimination Vertebrates have single lens eyes in which the image must be focused on the retina for it to be perceived clearly. To do this, the lens is bent by the ciliary muscles. In this exercise we will determine the minimum distance away that an object can be seen and still be clearly focused on the retina. 1) Place the beginning of a centimeter scale against your cheek just below your right eye. 2) Have a partner cover your left eye with an index card. 3) Hold a pin against the scale and move it along the scale until you find the minimum distance at which you can keep the pin point focused. 4) Record this as the near point for your right eye in Table I. Repeat for left eye. 5) Determine your average near point distance for your two eyes. Add your average to the class results on the board. Questions: 1. As you get older your lens becomes less elastic and the ciliary muscles cannot bend the lens as much to focus it. How will this affect your near point distance? 2. Most of the students in the class are between the ages of Do the class results show that the near point distance was the same for everyone? What might this mean? B. Visual Acuity The amount of detail the eye can distinguish is its visual acuity. Visual acuity is dependent upon several external factors such as the brightness of the stimulus, the length of exposure to the stimulus and the amount of contrast between the stimulus and the background. It is also dependent on factors internal to the eye such as the density and state of receptors. The Snellen Chart uses lines of letters of successively smaller size to measure visual acuity. The subject stands 20 feet away from the chart and covers one eye. The observer points to each line of the chart and pauses while the subject reads the letters. This continues until a line is reached which the subject cannot read. The value of the last line the subject could read correctly is recorded as the visual acuity. The result is a fraction with 20 as the numerator. The denominator indicates the distance at which a normal person can read the smallest line that you could read at 20 feet. Thus, if you have 20/200 vision, a normal person can read the line at 200 feet that you 11

12 could read at 20 feet (i.e. your vision is not as good as normal ). If you have 20/15 vision your vision is better than normal. 1) Stand 20 feet from the Snellen Chart and have someone verify your reading of the chart. 2) Record your visual acuity for each eye in Table II of the Results section. Questions: 3. How do these numbers compare with what a normal person can read? 4. Give an explanation for possible differences in visual acuity, with respect to the structure of the eye. C. Blind Spot The optic nerve is the final output of the retina. The nerve leaves the retina in a region slightly off of center and travel to the superior colliculus (in the brain) where visual processing begins. The region where the axons exit has no rods or cones, therefore light that falls on this region is not perceived; it is called the blind spot. 1) To locate the blind spot, hold the strip of paper containing the cross and the dot at arm's length. Make sure that the cross is right in front of your nose and the dot off to the right side. 2) Close your left eye, Focus on the cross and slowly move the paper toward your face until the dot disappears (do not shift your eyes to see the dot!). Have one of your partners measure the distance from the paper to your eye. 3) Repeat with your other eye and record all of your results in Table III. 4) Add your measurements to the class results on the board. Questions: 5. Look at the class results - were the results more or less clustered than the results for near point discrimination? Give a possible explanation for this. 6. Since you have a blind spot in the retina of both eyes, why don't you have a blind spot in your visual field? D. Visual Dominance Although most people can see with both eyes, we rely on one eye more than the other. This is the dominant eye, and it is similar to "handedness". 1) Roll up a piece of paper (this manual will do) to a diameter of about 4 cm. 2) Hold the paper at arms length and look at an object in the distance (e.g., a piece of equipment at the other end of the hall). 3) Without moving your eyes or the paper tube, close first one eye and then the other. With one eye closed your should still be able to see the object; this is your dominant eye. With the other eye closed the object should be lost. Remember, we are testing for the ability to see the particular object. You will still be able to see something with either eye closed! 4) Record your results in the Results section and to the class results on the board. 12

13 Questions: 7. Examine the class results. Was vision dominant on the same side as "handedness" for most people? What might this mean for processing of sensory and motor information in the brain? E. Peripheral Vision In this exercise we will demonstrate that the position of an object in the visual field determines which receptors are stimulated by light from that object. An object held directly ahead of us stimulates the center of our retina while one held off to the side stimulates the side of the retina. There are two types of visual receptors in the retina of the eye: 1) cones, which mediate daytime vision and allow for color vision, are found in their highest density in the center of the retina and 2) rods, which mediate night vision but respond in black and white, are found in their highest density in the outer part of the retina. Thus an object held directly ahead of us stimulates a high density of color receptors while one held to the side stimulates a high density of black and white receptors. 1. The subject should sit, and hold the visor up against his/her forehead, steadying the visor by resting elbows on the bench or the back of a chair. For the first series of experiments, the subject should focus on the triangle hanging down from the front of the visor. 2. One partner will move the sliding arm behind the subject's head, on the right side, and insert a black card. The other partner should stand in front of the subject (or on the other side of the bench, if necessary) and watch that the subject's eyes remain focussed straight ahead. The second partner should also record the results. 3. When the card is in place, slide the arm slowly toward the front of the visor. The subjects eyes must remain focussed completely on the triangle, or this won't work! The subject should say "now" when he/she first sees the card. The partner moving the arm should call out the angle of the sliding arm, and the recorder should then write down these results in Table IV. 4. Repeat the above steps with the red and blue cards. Record when the subject can first see the card, and when the subject can first identify the color of the card. 5. Repeat the above steps with the letter cards. Record when the subject can first see the card, and when the subject can first read the letters on the card. Add these results to the class results on the board. 6. Repeat step 5 on the left side. 7. Repeat step 5 on either side, but this time allow your eyes to move so that you can see and read the card as soon as possible. 7. Repeat the above steps with each member of the team as the subject. Questions: 8. How do you explain the difference between the results with the colored cards versus the black cards? 9. Look at the class results. What are the average fields of vision for seeing the card and for being able to read the card. The angles at which you can read the card are the region where your vision is best. What is the structural basis for this? 10. Another difference between rods and cones is that they have different sensitivities to light. If you wish to see an object in very low light how might you look at it to make the most of the light that does exist? 11. Given all of the results above, compare the roles of peripheral and "central" vision. 13

14 II. SOMATOSENSORY SYSTEM A. Touch vs. Pain This exercise demonstrates the discrete nature of cutaneous sensitivity. A stimulus can be sensed only if it excites the appropriate receptor. The receptors in the skin can distinguish between a light touch and a potentially dangerous stimulus. It is not necessary for the pain stimulus to be severe - a light touch with a pin is sufficient. 1) Place the subjects hand on the bench, palm side up, and draw a 4 x 5 grid of about 4 cm square total on the wrist near the palm. The subject should close his/her eyes. 2) Touch the skin lightly with a bristle and record whether the subject felt the touch. Use just enough pressure to bend the bristle and touch about 20 spots in a 4 x 5 grid. The recorder should write down the results on Table V using a "+" sign for a positive response (felt the touch) and a "-" sign for no response. 3) Repeat the experiment with a straight pin. Press just enough to indent the skin slightly. The subject must determine whether the stimulus elicits pain or just awareness of touch. Respond by saying "touch" or "ouch". Add up the number of touches that were painful and add the number to the class results on the board. B. Thermal sensitivity Similar to pain sensation, cold can only be detected if it activates a cold receptor. Thus, if you use a stimulating probe which is small enough, you can determine spots on your skin which are particularly sensitive to cold and spots which are less sensitive. The number of cool spots a person has in a given area of skin is related to the number of cold receptors in that area. 1) Place the subjects hand on the bench, palm down, and draw a 4 x 5 grid on the middle phalange of the index finger. 2) Remove one metal probe from the ice water and dry it quickly. 3) Place the metal probe briefly (about 2 seconds) on one box in the grid. The subject should respond by saying "touch" or "cold". 4) The recorder should place the results in Table VI. 5) Place the metal probe back in the ice water and choose another probe. 6) Repeat the test for all 20 spots in the grid. 7) Repeat steps 1-6 for each member of the team. 8) Add your results (the number of cold spots) to the class results on the board. 9) When you are finished, be sure to replace all of the metal probes in the cold water, for the next group. Questions: 12. Were you able to feel touch or pain more frequently? What does this mean in terms of receptors? 13. According to the class results, are there more receptors for pain or for cold in the skin (we will ignore for the time being that the number of receptors is also dependent upon which part of the body is begin tested)? What might these results mean in terms of an animal's survival? C. Two-Point Discrimination This experiment evaluates tactile acuity for the sense of touch. You will be determining how much separation between two points of touch is needed before they can be perceived as distinct points. The more densely packed the touch receptors are, the smaller this needed separation will be. You will determine the two point threshold on different sites of the body in order to demonstrate the extent of regional variability. 1) The subject should be seated with their eyes closed and palm up. 14

15 2) The experimenter should pick a spot on a finger tip, set the tips of two pins as close as possible and apply a stimulus by pushing the device down on the skin of 3 seconds. 3) Repeat this process with slowly increasing amounts of space between the two points. 4) At each distance, the subject should report if they feel one or two points of contact. 5) Record the smallest distance at which the subject feels two distinct points in Table VII. 6) Now start with the pins at a wider distance than was determined by the ascending series and repeat the process with decreasing distances. 7) Note the minimum distance at which the subject can distinguish two distinct points. Average the minimum two point distances for the ascending and descending series. 8) Repeat this entire process using the palm of the hand. 9) Now repeat this entire process using the back of the neck. Questions: 14. Which region of the regions that you tested had the best two-point discrimination? Which had the least? What significance might this have to the health/well-being of the animal? III. OLFACTORY SYSTEM Olfaction is the sense of choice for detecting chemicals at a distance. The smell receptors are located in the mucous membrane in the upper portions of the nasal cavity. The sense of smell is an important part of taste. You smell the food you are eating both before you put the food in your mouth and while chewing (because the nasal passages open to the throat). When you have cold, the sense of smell is deadened and this is responsible for a perceived loss of your sense of taste. The intensity of an odor depends upon the concentration of the odor molecules. To test this for yourself, sniff both of the bottles with the blue dots. These bottles contain the same solution at different concentrations. Choose which bottle has the highest concentration and check your answer: the bottle with the star (*) on the bottom is strongest. A. Adaptation Adaptation is a decrease in responsiveness that occurs during periods of continuous stimulation. Adaptation to odor stimuli is very common and usually occurs after entering an area with a distinct odor (e.g., a new car or freshly painted room). 1) Close one nostril and smell the oil of cloves by holding the vial about 1.5 cm from the open nostril. Inhale through your nose at ~1 second intervals, exhaling through your mouth (keep your mouth closed while inhaling). Make sure to keep the one nostril closed and try to keep the air that you inhale from entering your throat. If your nose is congested, obtain these results from someone else. 2) Note the time that it takes until the odor of cloves is detectably lessened (you will probably not lose the ability to smell the odor entirely). Continue to smell the cloves every 10 seconds and record how long it takes your olfactory system to recover (you should keep your other nostril closed the whole time). Record your results in Table VIII. 3) Let your olfactory system rest (while someone else performs the test) and then repeat steps 1 and 2 with the oil of peppermint. 4) Fatigue your olfactory receptors with oil of cloves and then immediately smell the oil of peppermint. Record what happens in the results section. 15

16 Questions: 15. What was the average time for adaptation to cloves and peppermint. What reasons could there be for a difference between these two answers. 16. How quickly did your olfactory system recover from adaptation to these two odors? 17. Why could you smell the peppermint after your nose had adapted to cloves? 18. What benefits might there be to olfactory adaptation (i.e., how could this help an animal to survive)? 19. Draw a voltage trace that would describe the response characteristics of the olfactory neurons that respond to cloves. B. Detection, recognition, and identification of odors We can detect a large number of different odors. Sometimes an odor is recognized as familiar, and yet is difficult to name. To test the difference between detection, recognition, and identification, sniff the contents of each of the 10 vials and fill out Table IX in the following way: 1) If you can smell an odor, place a check mark in Column 1. 2) If the odor is familiar, place a check mark in Column 2. 3) If you can name the odor, place the name in Column 3. Otherwise leave it blank. You may find that some of the odors were familiar to you, and perhaps bring to mind a mental image, but that you could not name them. Complete this test on your own - do not compare answers with your partners! 4) After you have completed all 10 odors, open the envelope and take out the Odor Identification List. Repeat step 3, but use the list to help make your selections, and place your answers in Column 4. You will probably find that you were able to nearly all of the odors, once you had a selection to choose from. 5) Check you answers with the TA (this is just for your information; the number that you get right will not reflect on your grade). Questions: 20. Many people will find that most of the odors were familiar, but that not all of the odors could be named. What reasons might there be for this? 21. Why do you think that so many smells stir up a memory or mental picture? IV. GUSTATORY SYSTEM A. Taste Mapping The sense of taste requires physical contact between the substance and the tongue. In this respect it differs from the sense of smell. The actual taste receptors are located in the taste buds which line the tongue, the upper palate, and the upper esophagus. There are about 10,000 taste buds. Four types of taste receptors have been described: sweet, sour, bitter, and salty. The sweet receptors respond to sugars, the saltiness receptors to inorganic ions, the sourness receptors to acids, and the bitterness receptors to alkaloids. The receptors are unevenly distributed over the surface of the tongue, and we are going to map your taste receptors. 1) Pour a small amount of one of the taste solutions into a clean dish (just enough to cover the bottom). 2) Dip an applicator into the liquid and drain off the excess. 16

17 3) Touch the applicator to the subject's tongue in the regions outlined on the map in Figure 1. Have the subject tell the recorder whether or not he can taste the solution. If he can taste it, put a plus on the corresponding region of the map. If he can't taste it, place a minus sign on the map. 4) Rinse your mouth out with water, and rinse the dish. 5) Repeat the procedure with each taste solution. 6) After each solution has been tested, exchange roles and repeat the test. 7) When you have finished, add your results to the class results on the board. 8) Rinse our dishes and place them in the beaker by the sink. Throw away all of the applicators that you used. Questions: 23. Consider your taste map and the class taste map. Were everyone's maps the same? What might be the advantage of taste receptors being clustered on the tongue? Why should it not matter if the maps differ slightly? 24. Why is it more important for us to be much more sensitive to bitter and sour, and least sensitive to sweet? B. The Role of Smell in Taste Sensation. As noted above, the odor of foods plays an important role in taste. We generally smell foods before we taste them, and this allows us to avoid dangerous or spoiled food. In this section we will explore this relationship. 1) Have one member of the team close his/her eyes and hold his/her nose. 2) Using forceps, place either a piece of apple or potato on your partner's tongue. 3) Have your partner try to identify by taste alone. 4) Repeat this a total of 10 times, choosing either apple or potato randomly 5) Record your results in Table X using a plus sign for correct answers and a minus sign for incorrect answers. 6) Repeat the test with eyes closed but nose open. 7) Switch positions until each member of the group has tried the taste test. Questions: 25. Briefly discuss your results. 26. What other factors might influence taste other than olfactory and gustatory sensation? 17

18 V. RESULTS SECTION Visual System A. Near Point Discrimination Table 1 Right eye Left eye Average B. Visual acuity Table II Right eye Left eye C. Blind Spot Table III Right eye Left eye D. Visual Dominance Right eye/left eye Right hand/left hand (circle) E. Peripheral Vision Table IV Conditions Right eye - black card Right eye - red card Right eye - blue card Right eye - letters Detect Identify Left eye - letters Detect Identify Eyes moving - letters Detect Identify Angle 18

19 Somatosensory System A. Touch vs. Pain Table V Touch Pain B. Thermal sensitivity Table VI Cold C. Two-Point Discrimination Table VII Palm Index Finger Back of neck Increasing Decreasing Average 19

20 Olfactory System A. Adaptation Table VIII Adaptation Recovery Oil of Cloves Oil of Peppermint Peppermint following cloves: B. Detection, recognition, and identification of odors Table IX Vial # Column 1 (Detection) Totals Column 2 (Recognition) Column 3 (Identification) Column 4 (Selection) C. Trigeminal Sensation 20

21 Gustatory System A. Taste Mapping Figure 1 21

22 B. The Role of Smell Table X Trial Nose plugged Total right Apple or Potato? Correct? Trial With smell Apple or Potato? Correct? 22