The Senses. Lab Exercise 30. Objectives. Introduction

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1 Lab Exercise The Senses Objectives - Become familiar with some of the sensory cells found in the skin - Learn about the sensory structures used for smell and taste. - Be able to determine the areas of the tongue most sensitive to the four tastes. - Be able to identify the principal eye structures. - Be able to describe the role of the iris and photoreceptors in the eye. - Define accommodation. - Be able to identify the principal ear structures. - Understand the functions of the Rinne s and Weber s tests. Introduction You have already looked at a simple neuron and learned a little about the conduction of sensory and motor impulses. However, in the previous lab exercise we saw only one type of neuron. Actually, the sensory neurons that detect environmental stimuli can be quite diverse and specialized. In your skin you have several types of sensory receptors to detect pressure, heat, cold or pain. We have specialized neurons for detecting chemicals, both in the air we breath (the olfactory receptors) and in food we eat (the taste receptors). We have specialized receptors for detecting various wavelengths of light (the rods and cones in the eye) and special receptors for sound (within the organ of Corti of the ear). In this series of activities, you will observe many of these sensory receptors and be able to conduct several experiments to see how these sensory systems work. To begin this exercise, go to the Senses lab simulation within the Mammalian Systems section of the BiologyOne DVD. 1

2 Activity.1 Touch A number of sensory receptors are found in the skin. When stimulated, these allow us to determine if a surface or environment is rough or smooth, hot or cold, or hard or soft. Intense stimulation of any of these receptors will be interpreted by your body as pain. Free dendritic nerve endings are found in the upper epidermis layer of the skin. These chiefly provide sensations of pain and temperature. Just below the epidermis layer, encapsulated nerve endings called Meissner s corpuscles provide the sensation of light touch. Also found just below the epidermis layer are structures called Merkel discs. These appear to aid the sense of light touch or pressure. Deeper in the dermal layer of the skin are the Pacinian corpuscles and the Ruffini s corpuscles. These respond to deep pressure and stretch. Nerves are also found to wrap around the base of skin hairs. These nerves, called hair root plexuses, respond to movement of the hair. Optional Experiment: This experiment tests the sensitivity of your skin a different locations, an indication of the density of sensory receptors within the skin. For this experiment you will need a pair of clean scissors and a ruler. This experiment works best if you have someone to help you. Begin the test with the points of the scissors together. Touch the point of the scissors to your forearm. You should, of course, feel the contact at just a single point. Open the scissors slightly. Again touch the forearm. Does it feel as though you are being touched at a single point or at two points. Repeat this, widening the scissors each time. When you can first detect the contact as two points rather than just one, measure the distance and record this in the Results Section. In the lab simulation, observe the illustration showing some of the sensor receptors found in the skin. Then, observe the microscopic structure of the Meissner corpuscles and Pacinian corpuscles. In the Results Section, label the illustration of the sensory receptors found in the skin. Repeat the activity on the palm of the hand, the back of the neck, the tip of the index finger, and the cheek of the face. Determine which of the parts of the body you tested have the most receptors for touch, and which part have the fewest. Record your observations in the Results Section. 2

3 Activity.2 Smell Activity.3 Taste Smell is probably the least understood of the senses. The sensation of smell occurs when specialize chemoreceptors respond to molecules in the air we breath. These chemoreceptors are located in an area of the roof of the nasal cavity called the olfactory epithelium. Here, olfactory receptor cells are stimulated by certain molecules in the air and send nervous impulses through nerves that extend up through the bone to an olfactory nerve tract and on to the brain. In the lab simulation, observe the location of the olfactory epithelium and the appearance of the olfactory receptor cells. The taste receptors on the tongue are able to distinguish four basic qualities; sweet, sour, bitter and salty. Sweet receptors respond to sugars and some amino acids. Sour receptors respond to acidic compounds. Bitter receptors respond to alkaloid substances and salty receptors respond to metallic ions. Each of these receptors are concentrated in different locations on the tongue allowing one to map where the tongue is most sensitive to these different tastes. The first experiment describe below allows you to map these taste sensations. The full experience of the taste of food and drink relies heavily on stimulation of the olfactory receptors. As you bring food to your mouth you will inhale some of the molecules from the food and as you chew, molecules will travel up the back of your throat into the nasal cavities, stimulating the olfactory receptors. The second experiment describe below tests this association between smell and taste. Areas of the Tongue to Test In the simulation, observe the illustration of the tongue s structure and examine the tongue s microscopic structure noting the papillae and taste buds. Optional Experiments: Experiment 1: For this experiment you ll need to make four solutions. One is to be sweet. For this solution you can add a teaspoon of sugar to a cup of water. The second solution is to be salty. Make this solution by adding 1/2 to one teaspoon of salt to a cup of water. The third solution is to be sour. You can make this solution by adding 1/2 to one teaspoon vinegar to a cup of water. The fourth solution is to be bitter. Tonic water, which contains quinine, can be used for a bitter solution. areas to test Dip a clean cotton swab (a Q-tip can be used) into one of the solutions and remove the excess fluid. Touch one of the areas of the tongue indicated in the illustration. It s easiest if you have someone to help you with this. Record whether you can taste the solution in the Results Section. Repeat this with all four solutions. Use a new cotton swab and rinse your mouth out with water between each test. 3

4 Activity.4 Sight Experiment 2: Collect several small pieces of candy of various flavors (jelly beans work well) or as an alternative, cut small pieces of apple and potato. While holding your nose, place one of the pieces of candy (or potato/apple) into your mouth. If you have someone to help you, close your eyes or blindfold yourself so you don t know which you are tasting. Attempt to identify the flavor without chewing or swallowing. Repeat this experiment several time, rinsing you mouth out between trials. Repeat this experiment with your nostrils open. Record your observations in the Results Section. Testing the Effect of Smell on Taste Advance in the lab simulation to view the structures associated with the sense of sight. Examine the eye structures visible from outside the body. Examine the illustration of the dissected eye. Be sure you are able to identify the cornea, pupil, iris, lens, retina, fovea, and optic disc. Observe the microscopic structure of the retina. Test your ability to identify these structures by labeling the eye diagram in the Results Section. Optional Experiments: Experiment 1: Papillary Reflex The rods and cones are the light detectors of the eye. The rods can respond to as little as one photon of light while the cones are less sensitive and are operational in bright light. We use the rods for night vision, and the cones allow us to see the rainbow of colors in daylight. Too much light can cause bleaching due to the quick breakdown of the photopigments. This leaves us blinded, as when you go from a dark room to the bright outdoors. The rods and cones can become adapted. And, there is another very simple way your eye controls the amount of light that reaches the retina. The iris, made of smooth muscle, can adjust to allow more or less light to enter the pupil. Place a heavy cardboard strip or paper between your eyes so they can be exposed to different light conditions. Have a helper shine a penlight on your left eye only. Record the response of both eyes. Repeat the activity with your right eye. Answer questions in the Results Section. 4

5 Experiment 2: Accommodation The lens of your eye is like an onion. It has many layers, and as we get older, new layers composed of lens fibers are added. As new layers are added, the older one becomes more tightly compressed which reduces the flexibility of the structure. If the lens cannot easily change shapes, we lose the ability to focus on near objects. This ability is called accommodation. To test your level of accommodation, you will need a helper, a long ruler such as a yardstick or meter stick and a stiff piece of paper (such as an index card) with a small letter A printed on it. Sit so your chin is resting on a table. Lay a ruler on the table extending away from your chin. Have your helper slowly move the card from the end of the ruler toward you. Tell them to stop when you can no longer clearly focus on the letter. Record this distance in the Results Section. Below is a table of average near points for different ages. Experiment 3: Blind Spot At the back of the retina, there is one spot where there are no photoreceptors. This is the area where the nerve fibers from the rods and the cones leave the retina and form the optic nerve. The area is the optic disc and is also referred to as the blind spot, because when light is focused on this spot, we see nothing. On a piece of white paper, make a small black dot. Focus your vision on some distant object. Close or cover one eye. While retaining focus on the distant object, move the paper with the dot into you field of vision. Slowly move the paper from side to side and up and down keeping your focus on the distant object while doing this. You should see the dot on the paper disappear and reappear as its image moves onto and off your eye s blind spot. Accommodation and Blind Spots Tests Typical Accommodation Distances Age Distance cm cm cm cm cm cm 5

6 Activity.5 Hearing Experiment 4: Integration We don t really see with our eyes; we see with our brain. The eyes detect light, but the brain interprets the light images and creates the picture. As the image is being transmitted to the brain, information from both the left and the right eye cross the optic chiasma, and the information is integrated. Perform the following experiment to experience an example of this information integration. Advance in the Senses simulation to study the structures associated with hearing. Become familiar with the structures of the ear and observe the microscopic structure of the cochlea. When finished, be sure you can identify these. Test your ability to identify these structures by labeling the ear diagram in the Results Section. Either make a tube from a piece of paper or use the center tube from a roll of paper towels or from wrapping paper. Focus your vision on an object across the room. Hold the tube up to your right eye and with both eyes open, stare at the selected object. Now, place your left hand beside the tube and slide your hand from the far end of the tube toward your face. Continue to stare at the selected object, not your hand. Optional Experiments: The Weber and Rinne tests are used to indicate whether an individual suffers from a conductive deafness or a sensorineural deafness. Conductive deafness results from an impairment of the external or middle ear mechanisms to conduct sound to the cochlea. Sensorineural deafness results from an impairment of the cochlea or the nerve leading from the cochlea. These tests require a tuning fork and a quiet environment. Describe what happens in the Results Section. Integration Test Weber s test: Holding a tuning fork near its base, strike the forked end on your arm or hand. Do not strike the fork on the table edge as this is likely to damage its function. 6

7 Hold the fork near your ear so that you become familiar with the tone you are expected to hear. Strike the fork again and press the base of the tuning fork on the middle of your forehead. Determine if the sound can be heard equally in both ears or only in one. Record your results in the Results Section and compare with the diagnosis table. Rinne s test: Strike the fork again and place its base on the mastoid process on the side head. (The mastoid process is the knob of bone just behind the ear.) It may take more than one try to locate this. You should be able to hear the tone of the fork. When you can no longer detect the sound of the fork, move the fork so that the tines are next to the auditory canal of the same ear. Determine whether you can hear the sound again or not. Record your results in the Results Section and compare with the diagnosis table. Simulated Conductive Deafness: You can simulate conductive deafness by plugging your ear with cotton. Plug one ear and repeat the Weber s and Rinne s test. Are the results different? The Rinne's Test 7

8 Lab Exercise Name Results Section Activity.1 Touch 1. free nerve endings epidermis subaceous gland dermis 2. subcutaneous 3. layer hair root plexus sweat gland nerve Results from Optional Experiment measurement of two point discrimination (mm) forearm palm of hand back of neck tip of index finger cheek Of the areas tested, where are touch receptors most concentrated? Where are they least concentrated? 8

9 Activity.3 Taste Results from Optional Experiment 1 Rate the sensitivity of each part of the tongue from 1 to 5 where 1 = least sensitive and 5 = most sensitive. Tip of Sides of Back of Tongue Tongue Tongue Sweet Sour Bitter Salty Which area of the tongue is most sensitive to sweet? to sour? to bitter? to salty? Results from Optional Experiment 2 Relationship of Taste and Smell Correct Incorrect Comments on Response % Response % Speed of Response Nostrils Closed Nostrils Open 9

10 Activity.4 Sight Results from Optional Experiment 1 Is contraction of the iris an independent even or do both irises contract together to low and high light conditions? If you left eye was blind, do you think its iris would responde to light hitting the right eye? Explain. 10

11 Results from Optional Experiment 2 What is your accommodation distance? How do you compare to the norms for your age? Results from Optional Experiment 3 Describe your observations from the blind spot experiment. Results from Optional Experiment 4 Describe your observations from the integration experiment. 11

12 Activity.5 Hearing Results from Optional Experiments Your Results with Weber's Test: Your Results with Rinne's Test: Conclusion: Weber's Test Rinne's Test Tone is heard equally Tone is heard through Normal Hearing in both ears auditory canal longer than through mastoid Tone is heard in ear Tone is louder or heard Conductive with conductive equally when fork held deafness in one ear deafness to mastoid process Tone is heard equally Sensorineural Tone is heard in ear when fork is held next deafness without deafness to ear or on mastoid process 12