Lab #11 - Nervous System II Senses

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1 Page1 Nervous System II Lab #11 - Nervous System II Senses Objectives: Dissect a sheep eye and identify the structures Observe a human eye model and identify the structures Observe a human ear model and identify the structures Perform simple tests that demonstrate the physiology of vision, hearing, and olfaction Equipment: Bring gloves, goggles, dissecting kit, photographic atlas, and proper shoes. I. The Senses A. Introduction The senses detect stimuli that originate from the internal as well as the external environment. Sensory receptors react to specific types of stimuli by sending an impulse to the CNS for processing. Receptors for the general senses are dispersed throughout the skin, organs, muscles, and joints. Some of the general senses are touch, pressure, vibration, pain, temperature, and stretch. The receptors for the special senses are concentrated in the head, and each type is associated with a sensory organ. The special senses include vision, hearing, smell, taste, and balance. In this lab exercise, we will focus primarily on vision, hearing, and taste. II. Anatomy of Sense Organs A. Observation and dissection of sheep eye 1. Preparation a. Wear goggles, gloves, and an apron for the entire time that you are working with preserved specimens. b. Obtain a dissecting tray and a sheep eye. c. Have scissors, a scalpel, and the photographic atlas available. d. At each step, refer to the photographs in the atlas for orientation and structure identification. 2. Exterior Observations a. Instructions Please note that there may be much variability in the exterior structures of the eyeballs. If the following descriptions do not match your specimen, observe the structures on another group s specimen. Clear away excess adipose tissue and expose the four extrinsic muscles (humans have six). On the anterior surface, identify the cornea. Due to the preservative, the cornea is opaque, not transparent as in a living eye. Therefore the pupil and iris cannot be visualized externally. The tough, white outer layer of the eyeball is the sclera. On the posterior surface, opposite the cornea, use your finger to locate

2 Page2 Nervous System I the cut end of the optic nerve you will be able to feel the firm protrusion within the spongy fat layer. Clear away the rest of the adipose and muscle tissue over the eyeball entire surface. b. Exterior structures sclera cornea iris optic nerve extrinsic eye muscles 3. Interior Observations a. Dissection instructions To separate the anterior and posterior halves of the eyeball, place the eyeball in the dissecting tray and hold it firmly. Insert the tip of the scissors or scalpel through the sclera halfway between cornea and the optic nerve. Firm pressure is necessary to puncture the tough sclera. Cut the eye in half by carefully snipping around the entire eyeball with scissors. The gelatinous vitreous humor with the lens attached may ooze out of the opening. Part of the black, feathery ciliary body may also be adhered to the vitreous humor and lens. The thin, off-white epithelial layer that covers the posterior section of the eye is the retina. Notice the fragile nature of the retina, especially how its shape is easily lost when detached. Beneath the retina, is the dark bluish-black choroid which absorbs light. Notice that the retina is firmly attached to one point on the choroid. This is the optic disc, or blind spot, where the retinal neurons exit the eye to form the optic nerve. The choroid in sheep contains an iridescent surface, the tapetum lucidum. This structure allows for better vision in dim light. The human choroid does not contain a tapetum. If the vitreous humor remains attached to the anterior surface, carefully scrape it away to reveal the lens attached to the iris via ligaments and the ciliary body. Pull off the lens to reveal a horizontal oval-shaped pupil surrounded by the iris. In humans, the iris is round. To fully visualize the pupil and iris, cut around the cornea on the exterior surface and remove it. Dispose of eye tissues in the biohazard containers as directed by your instructor. b. Interior structures lens retina choroid ciliary body pupil iris vitreous humor B. Observation of eye model Locate all of the structures listed under sheep eye dissection on the human eye model.

3 Page3 Nervous System I C. Observation of ear model Locate the structures listed below on the human ear model. Then identify the structures labeled on ON THE EAR MODEL in your lab report. pinna/auricle incus external auditory stapes canal cochlea tympanic membrane semicircular canals malleus Eustachian tube/auditory tube III. Testing the Senses Vision A. Visual Acuity 1. Introduction - Visual acuity is the capacity of the visual system to distinguish between two closely spaced objects. At a distance of 10 m, a normal eye can distinguish between two objects that are 1mm apart. This ability of the eyes to see fine detail can be evaluated with a Snellen chart (Figure 1). Using a Snellen chart, the visual acuity of the subject is determined by the ratios written to the right of each line. A person with normal vision can clearly see the line marked 20/20 (line 8) which is considered 100% visual efficiency. If, for example, the subject can read accurately only to line 5 (marked 20/40) this means that a person with normal acuity can read the same letters from 40 feet away. Figure 1. Snellen visual acuity test chart 2. Procedure Place the subject 20 ft from the Snellen chart hanging at eye level (20 ft will be marked on the floor with tape). With the left eye covered, the subject should read each line of the chart out loud beginning at the top line. The test stops when the letters can no longer be read accurately. Record the ratio of the last line that was read correctly in the lab report. Repeat with the left eye. If the subject wears glasses, repeat the entire procedure with glasses in place. B. Astigmatism 1. Introduction Astigmatism is the blurred or distorted vision caused by abnormalities in the shape of the cornea or the lens of the eye. 2. Procedure Place the subject 10 ft from the astigmatism chart hanging at eye level (10 ft will be marked on the floor with tape). Instruct the subject to gaze at the center of the radiating lines on the charts with the left eye covered. If no astigmatism is present, the lines should appear sharp and of equal thickness. If the lines appear to be wavering or of different thicknesses, record the presence of astigmatism in your lab report. Repeat with the left eye. If the subject wears glasses, repeat the entire procedure with glasses in place.

4 Page4 C. Macular Degeneration 1. Introduction The macula is an area in the center of the retina at the back of the eye. This area contains the largest concentration of cones and so is the area of sharpest vision (greatest visual acuity). The macula is critical to central vision, and is vitally important for activities such as reading, driving, and facial recognition. Macular degeneration is an age-related eye disease that results in the deterioration of the macula, causing objects in the center of vision to appear blurry, distorted, or dark. 2. Procedure- If you normally wear glasses or contacts, you should wear them for this test. Cover your left eye and view the Amsler grid (Figure 2) provided at your lab bench using your right eye. The grid should be at about the same distance from your eyes that is normal for reading your textbook or a newspaper. Focus on the dot in the center. Check for wavy, blurred, or distorted lines. All lines should be straight, all intersections should form right angles, and all the squares should be the same size. Also check for missing areas or dark areas on the grid. Figure 2 illustrates what a person with normal vision should see. Figure 3 shows the type of distortion that macular degeneration might produce. Repeat the test for your left eye. Record the results in your lab report. Figure 2. Amsler grid viewed with normal vision Figure 3. Amsler grid viewed with macular degeneration D. Color Deficiency 1. Introduction Color deficiency, often erroneously called color blindness, is a decreased ability to perceive some colors in ambient light. Within the retina, the cones are the color receptors. There are three types of cones which absorb light at different wavelengths. These wavelengths correspond to the detection of red, blue, and green light. There are several categories of color deficiency based on the types of cones that are dysfunctional. The most common type of color deficiency causes difficulty in distinguishing yellow, red, and green from each other. This disorder, known as red-green color blindness, is found in 8% of males but only 0.5% of females. 2. Procedure Obtain an Ishihara color vision test book. Follow the directions and record your results in the lab report.

5 Page5 E. Blind Spot 1. Introduction There are no visual receptor cells at the point where the optic nerve connects with the retina. This is the optic disc which creates the blind spot (Figure 4). 2. Procedure Use a small piece of paper imprinted with a dot and a cross (Figure 5) to test for the blind spot. lens optic disc optic nerve Figure 5. Proper orientation of paper with dot and cross for the RIGHT EYE retina Figure 4. Cross-section of eyeball Position the paper so that the dot is to your right and the cross is directly in front of the right eye at a distance of 90 cm. Cover your left eye. Move the paper slowly toward the right eye until a point is reached where the cross is still seen, but the dot is no longer visible. Stop moving the paper, and the evaluator will use a ruler to measure the distance from the paper to the right eye in cm. Flip the paper to repeat the same procedure for the left eye, now holding the paper in the left hand with the dot on the left side, and the right eye covered. Wait for the dot to disappear. Record the measurements in the lab report. F. Stereoscopic Vision 1. Introduction Stereoscopic (three-dimensional) vision is possible due to the brain s ability to fuse the two slightly different images received by each eye (Figure 6). Stereoscopic vision provides greater depth perception. 2. Procedure Obtain a Stereoscopic vision test booklet. Read each line of numbers (A-F). For each line, record the number that appears as 3D in the lab report. G. Near Point of Vision Figure 6. Stereoscopic vision 1. Introduction When viewing objects less than 20 feet away (near vision), the lens of the eye must change shape to keep the light rays entering the eye properly focused on the retina. This change in the shape of the lens is called accommodation. However, there is a limit to the amount of accommodation possible for the lens. The point at which the lens has reached its maximum possible accommodation is called the near point of vision. 2. Procedure To determine near point of vision, the subject should hold a pencil at arm s length with the eraser at eye level. With both eyes focused on the eraser, move the eraser closer to the nose. Stop at the point where the eraser goes out of focus. The evaluator will use a ruler to measure the distance from the eraser to either eye in cm. This distance marks the near point of vision (maximum accommodation of the lens). Record this measurement in the lab report.

6 Page6 H. Field of Vision and Reading Field of Vision 1. Introduction Humans can see objects outside of the central focus range, especially if the objects are moving. Peripheral vision refers to vision around the perimeter of the eye. However, color and acute vision is limited in the periphery. The cones and most of the rods are concentrated in the center of the retina. Peripheral vision can be tested using a HAP vision disk. 2. Procedure a. Field of vision, right eye 1. Prepare the vision disk (Figure 7) by sliding the transparent arm to the 120 degree mark on the right side. Insert a letter card into the slot without allowing the subject to see the letters. Fold down the focus marker at the zero degree mark. 2. Place the subject in a seated position without glasses. For testing the right eye, the handles of the vision disk are held by the left hand with the disk held level against the forehead and both eyes always focused on the focus marker (Figure 7). 3. The evaluator stands behind the subject and slowly moves the arm toward the center of the disk. 4. The subject should signal when the card can first be seen. Record the degree indicated within the triangle opening in the arm in the lab report. b. Reading Field of Vision, right eye 1. Continue moving the arm toward the center of the disk. 2. The subject should then signal when the letter on the card can first be read. Record the degree indicated in the lab report. c. Field of Vision and Reading Field of Vision, left eye Repeat procedures a. and b. on the left side. Figure 7. Using the Vision Disk IV. Testing the Senses - Hearing A. Introduction Hearing level can be assessed using an audiometer (Figure 8). This instrument produces tones of varying frequency and intensity. Frequency is perceived as pitch and it is measured in Hertz (Hz). Human hearing ranges from about 20 to 20,000 Hz. Intensity is perceived as loudness and it is measured in decibels (db). Typically hearing is tested at the faintest sound that can be heard in frequencies that range from 500 to 4,000 Hz. Figure 8. AudioScope 3 audiometer

7 Page7 B. Procedure The AudioScope 3 is used for the hearing test (Figure 8). Examine the unit and locate the ON/decibel level switch, the START button, and the indicator lights (tone, pre-tone, ready, and low battery). Slide the decibel switch to the correct db level (see chart below) to turn the audiometer on. The unit is properly charged when the green READY light is on. Check that the lens is centered (Figure 9). Choose the appropriate decibel level for the subject: 20 db - children aged 16 years and younger 25 db - adults aged 17 to db - adults aged 65 and older 1. Clean all three sizes of ear speculum tips with an alcohol wipe and choose the largest size that can fit comfortably in the ear. A snug fit will help block out ambient sound. Twist the speculum clockwise onto the audiometer. 2. Take the subject to a quiet area. 3. Instruct the subject that they will hear a loud tone and then fainter tones. The subject is to respond every time a tone is heard by raising a finger. 4. Retract the subject s pinna with the thumb and index finger. Gently pull it slightly up and back (Figure 10). 5. Insert the speculum tip into the ear and position it so that the tympanic membrane can be visualized. 6. Press the START button. The green light will go out and then tone indicators will show in sequence as the tone is presented to the subject. 7. Repeat on the opposite ear. 8. Record the results in Table 1 of the lab report. 9. Turn off the unit and replace it properly in the charging station (refer to Figure 8). The green charging light should be ON. 10. Remove the speculum tip and clean it with an alcohol wipe. V. Testing the Senses - Olfaction Figure 9. Audiometer details Figure 10. Audiometer testing procedure A. Introduction Both smell (olfaction) and taste (gustation) are examples of chemoreception. This means specific chemical substances can be detected by the olfactory or gustatory receptors. In olfaction, air-borne compounds called odorants enter the nasal passageways and dissolve in the mucus produced by the epithelium lining the passages. The odorants bind to the olfactory receptors on the dendrites of the olfactory sensory neurons. Humans can distinguish more than 10,000 different smells that are difficult to categorize. Figure 11. Olfactory structures

8 Page8 B. Procedure Students who are hypersensitive to smells should not perform this exercise. 1. Olfactory Discrimination Olfactory discrimination is the ability to recognize and distinguish different types of odors. a. Obtain five different scent vials. b. The evaluator should open each vial and waft the scent toward the subject s nose. c. For each vial, record: i. If smell is detected ii. If the smell is strong or weak iii. Whether the scent is pleasant or unpleasant iv. If the smell is perceived as edible v. Identify the specific smell if possible d. After both partners have completed the exercise, check the scent answer key. e. Record the result in Table 2 of the lab report. 2. Olfactory Adaptation Olfactory adaptation (also known as olfactory fatigue) is a temporary inablility to distinguish a particular odor after a prolonged exposure to that odor. a. Choose one scent that was reported to be strong and pleasant. b. The subject should press one nostril closed with the index finger of one hand. c. Hold the vial under the open nostril with the other hand until the smell is no longer detected. Record the time to adaptation in the lab report. d. Release the closed nostril and record in the lab report whether or not the smell is again detectable.

9 Page9 Lab Report # 11 Nervous System II Name: II. B. Sense Organs The Eye Label the structures indicated on the human eye diagram. A. B. C. D. E. F. G. H. I. I C. The Ear - Identify labels ON THE EAR MODEL (not the diagram) by filling in the blanks below. A. B. C. D. E. F. G. H. I. III. Testing the Senses Vision A. Visual Acuity (Snellen eye chart) Record the distance ratio (20 / X). Without glasses With glasses Right eye Left eye Right eye Left eye 1. What is visual acuity? 2. Who has the best visual acuity? A. 20 / 20 C. 20 / 40 Circle the correct answer. B. 20 / 15 D. 20 / 60

10 Page10 B. Astigmatism Record a Y if astigmatism is present and N if astigmatism is absent. Without glasses With glasses Right eye Left eye Right eye Left eye What causes astigmatism? C. Macular Degeneration Right eye Left Eye 1. What area of the retina produces the greatest visual acuity? 2. What part of your visual field is primarily affected by macular degeneration? D. Color Deficiency Record the results of the test. Normal (Y/N) Type of deficiency 1. What causes color deficiency? 2. What is the most common type of color deficiency? E. Blind Spot Record the distance from the eye at which the blind spot was detected. Right eye (cm) Left eye (cm) 1. Give the anatomical reason for the presence of a blind spot. 2. Explain why you are normally not aware of your blind spot. F. Stereoscopic Vision Record the results of the tests. Report the number on each line (A-F) that appeared to be 3D. A. C. B. D. E. F. 1. What does stereoscopic vision mean? 2. Give a specific example of how stereoscopic vision would be helpful in everyday life.

11 Page11 G. Near Point of Vision Record the distance at which focus was lost (cm) 1. What is accommodation? 2. What is near point of vision? 3. How does age affect your near point of vision? J. Field of Vision and Reading Field of Vision Record the degree at which the card could be seen or the letter read. Field of Vision Right eye Left eye Reading Field of Vision Right eye Left eye What specific area of your visual field is tested when using the vision disk? IV. Testing the Senses Hearing Record a Y if tone was heard; a N if it was not heard. Table 1. Results of audiometer testing 500 Hz 1000 Hz 2000 Hz 4000 Hz Right ear Left ear What is the normal range for human hearing in Hertz? V. Testing the Senses Olfaction Vial # Smell present (Y/N) Table 2. Results of Olfactory Discrimination Strong (S) or Weak (W) Pleasant (P) or not (N) Edible (E) or not (N) Identification Correct ID (Y/N) Olfactory Adaptation Time until scent was no longer detected (sec) Was the scent detected when the closed nostril was released? (Y/N) 1. Give an everyday example of olfactory discrimination. 2. Give an everyday example of olfactory adaptation.