BIOLOGY 211: HUMAN ANATOMY & PHYSIOLOGY **************************************************************************************************** EYES AND VISION **************************************************************************************************** Reference: Saladin, KS: Anatomy & Physiology, The Unity of Form and Function; 8th ed. (2018) Be sure you have read and understand Chapter 16 before beginning this lab. INTRODUCTION: The eye is probably the most complex of all of our sensory organs. It not only includes receptors which are stimulated when light strikes them and send electrical signals to the brain, but it is also able to modify how much these receptors are stimulated by allowing more or less light to enter the eye and to focus it specifically on the receptor cells. Unlike any other sensory organ in the body, the eye is also able to begin processing information from the receptor cells before it is sent to the central nervous system. Vision is one of the five special senses which humans possess. The receptor cells of the eye are classified as exteroceptors since they detect changes in the external environment. Specifically, they are photoreceptors which detect light (a form of electromagnetic radiation) within a rather narrow range of frequencies. These receptors are located in the posterior part of the eye; other structures in the anterior part of the eye regulate how much light is allowed to strike them and focus the light specifically on them. Like other receptor cells in the body, they generate an electrical signal, or action potential, in response to being stimulated. However, rather than passing information directly to the central nervous system, these photoreceptor cells pass it through a series of other neurons, from which it is eventually relayed to the brain. The photoreceptor cells, the neurons through which the information is passed (and partially processed), and the neurons whose axons carry the signal toward the brain form the retina. In addition to structures directly dealing with detecting light and sending information to the brain, the eye also has structures which provide it with structural strength. This is necessary not only because of its exposed position, but also because it is constantly being pulled around within the orbit by six extrinsic or extraocular muscles. These muscles need strong points of attachment to the eye itself, and the eye must be able to withstand their pull without deforming. Thus, in today s lab we will be dealing with structures which carry out a wide variety of functions in providing us with vision.
THE ORBIT: 1: Get a skull and examine the orbits. Be careful: do not use anything sharp or rigid. Notice that each orbit lies lateral to the nasal cavity, separated from it by parts of the lacrimal, ethmoid, and maxillary bones (consult Figures 8.7, 8.12, and 8.14 in your Saladin text for review). Notice how each orbit lies inferior to the anterior cranial fossa and is separated from it by parts of the frontal and sphenoid bones. Notice how the lateral wall of each orbit is formed by parts of the frontal, zygomatic and (very posteriorly) sphenoid bones. On the skull, identify the: Optic canal Superior orbital fissure Inferior orbital fissure Supraorbital margin Notice how the entire anterior rim of the orbit is thickened to help protect the eye from trauma. ANATOMY OF THE EYE: 2: Identify the following structures on your lab partner s eye, using Figures 16.23, 16.24 and the text associated with it in your Saladin text: Eyebrow over the supraorbital margin of the frontal bone Superior and inferior palpebrae (eyelids) Eyelashes (both upper and lower) Medial commissure Lateral commissure Palpebral fissure Conjunctiva Lacrimal caruncle Sclera Iris 3: Identify the following eye structures on Figures 16.24 and 16.26 of your text, and also on a model of the eye: Cornea Iris Pupil Lens Ciliary body Suspensory ligaments Vitreous chamber or body Retina Fovea centralis Macula lutea Choroid Sclera Optic nerve and optic disc Anterior chamber or anterior portion of the aqueous chamber Posterior chamber or posterior portion of the aqueous chamber
4. Examine the microscopic appearance of the retina on Figure 16.35 of your Saladin text. You should understand the relative positions of the rods and cones, the bipolar neurons, and the ganglion cells. Be sure you understand the direction in which light is traveling as it passes through the different layers of the retina, and what other parts of the eye it as traveled through to get there. - Without using your book, explain to other members of your lab group: In what order does light pass through these three layers of cells in the retina? Which neurons send their axons through the optic nerve toward the brain? All members of your group: It is your responsibility to be sure these explanations are correct (If time permits, your lab instructor may demonstrate the examination of the retina of a member of the class through an ophthalmoscope) TRANSMISSION, REGULATION, AND REFRACTION OF LIGHT: For the photoreceptor cells (rods and cones) in the retina to function correctly, the appropriate amount of light must be accurately focused on them. For this to happen: a) There must be a clear path for the light from the time it enters the eye until it strikes the photoreceptor cells. This includes the cornea, aqueous humor in the anterior and posterior chambers, pupil, lens, and vitreous humor in the vitreous chamber or body. If any one of these becomes non-transparent (opaque), light will not be able to reach the retina. b) The eye must be able to regulate how much light reaches the retina. More light must be allowed through from darker surroundings, and less light must be allowed through from brightly lit surroundings; the result is that a relatively constant amount of light strikes the retina regardless of changes in the light intensity. This is the job of the iris surrounding the pupil. It contains smooth muscle which can either increase (in dim light) or decrease (in bright light) the diameter of the pupil. Light striking the iris is blocked by the pigments it contains (that s why your eyes have color ) so only the light passing though the hole in its middle - the pupil - can reach the retina. c) In order to focus it on the retina, the light must be bent, or refracted, as it enters the eye and passes through anterior structures in the eye. The curvature of your cornea provides most of this refraction but can not be changed. The curvature of your lens, which CAN be changed, provides the fine focus and allows you to change the focus when shifting between near and far vision.
5: Trace on Figure 16.26 of your Saladin text and on the model of the eye the pathway which light must follow from the time it enters the cornea until it reaches the retina. Be sure you understand where the aqueous and vitreous humors are located, and why these two fluids must remain clear. -. Explain to your lab partners what would happen to your vision if your corneas were damaged and became opaque so they could no longer allow light through. What are one or two things which might cause this? - Explain to your lab partners what would happen if your aqueous humor became cloudy and could no longer transmit light. What are one or two things which might cause this? - Explain to your lab partners what would happen if your lens became cloudy and could not transmit light. What are one or two things which might cause this? - Explain to your lab partners what would happen if bleeding occurred into your vitreous humor. What are one or two things which might cause this? All members of your group: It is your responsibility to be sure these explanations are correct 6: Carefully watch one of your lab partner s pupils for 20 or 30 seconds and observe the frequent, tiny changes in its size as it constantly adjusts itself to regulate the amount of light it lets through. As you continue to watch one pupil, use your hand to shade light from entering both eyes (you will see later in this exercise why it needs to be both eyes rather than just one), then remove this shading. You should see the pupil adjust in response. - Have your ever stepped directly from a relatively dark room into bright sunlight, or from sunlight into a dark room? You had trouble seeing for a couple of seconds. Explain to your lab partners what is happening to your pupil during those few seconds. All members of your group: It is your responsibility to be sure this explanation is correct
7. Observe one of your lab partner s corneas from the side and observe that it is curved rather than flat. Use Figure 16.31 and the text accompanying it ( Refraction and The Near Response ) to be sure you understand how the cornea and lens refract (bend) light to focus it on the retina because they are curved. If you wear eyeglasses, notice that the lenses are curved - they are doing part of the refraction which your cornea and lens can no longer do adequately. - Explain to your lab partners how would it affect your vision if your lens became inflexible and could no longer change its shape? (you might want to look up presbyopia in your medical dictionary) All members of your group: It is your responsibility to be sure this explanation is correct 8. Examine the structure of the eye again as shown in Figure 16.26 and on the model of the eye. Identify the fovea centralis ( central pit ) in the center of the macular lutea ( yellow spot ). This small area of the retina contains no rod receptors but has a high concentration of cone receptors which detect color and have high resolution, so this is where you have the sharpest and most detailed vision and is the part of the retina where your cornea and lens focus the image of an object when you are looking directly at it. These cones are not very sensitive, so they require relatively bright light to stimulate them. In contrast, the more peripheral parts of the retina surrounding the macula lutea and fovea centralis have very few cones but very large numbers of rods. These receptors are much more easily stimulated by lower levels of light, but they are unable to differentiate colors. Rods do not have the high resolution of cones, but they are very good at detecting motion. This differential distribution of rods and cones in different parts of the retina is why you are able to use your central vision to focus on objects and to see colors when the cones in the fovea centralis are stimulated in bright light, and why you use your peripheral vision to see in shades of gray in dim light and to detect motion. Try this at home tonight after sunset. In the bright light of a lamp or overhead light you can easily read a book and can easily determine the colors of things in the room. Now turn off the light so the room is dark except for some light coming in the windows, give your eyes a minute or two to adjust, and try to read the book. You will not be able to do so, nor will you be able to see the colors of things around you items you know are red or yellow or blue or green will all appear as shades of gray.
EXTRINSIC (EXTRAOCULAR) MUSCLES: 9: Identify each of the following muscles on a model of the orbit and eye which shows the extrinsic muscles and using Figures 16.24 and 16.25 of your Saladin textbook as a reference: Superior rectus Medial rectus Lateral rectus Inferior rectus Superior oblique Inferior Oblique - Each of the extrinsic muscles receives its stimulus through one of the cranial nerves. List the proper nerve for each of the muscles above (hint: you may need to use an earlier chapter in your text) - Explain to your lab partners which of these muscles contract when you look to the left with both eyes? (Caution: it may not be the same muscle for both eyes) - Explain to your lab partners which of these muscles contract when you look to the right with both eyes? (see caution above) - Explain to your lab partners which of these muscles contract when you look cross-eyed? (see caution above) All members of your group: It is your responsibility to be sure these explanations are correct VISUAL PATHWAYS FROM THE EYE TO THE BRAIN: Once the photoreceptors in each eye have been stimulated and that information has been passed to other layers of the retina, it must still be transmitted to the brain before you can see anything. The axons of the ganglion cells of the retina converge at the posterior end of each eyeball and exit it to form the optic nerve. The optic nerves from the two eyes converge at the optic chiasm, located just inferior to the hypothalamus and anterior to the infundibulum of the pituitary gland. Here, axons from the medial half of each retina cross sides, left-to-right and right-to-left, while axons from the lateral half of each retina pass through the chiasm without crossing. As these axons leave the optic chiasm they form the optic tracts which lead to the lateral geniculate nucleus of the thalamus. Those axons synapse here with neurons which lead to the primary visual cortex on the occipital lobe of the cerebrum. This is diagramed in Figure 16.44 of your Saladin textbook.
10: The diagram below has two colored dots on the posterior part of each retina. These represent the cell bodies of four ganglion cell neurons: One on the lateral part of the retina of the right eye One on the medial part of the retina of the right eye One on the lateral part of the retina of the left eye One on the medial part of the retina of the left eye Close your book. From memory, draw the pathways of the axons of each of these four neurons as they pass through the optic nerve, optic chiasm, and optic tracts to reach the thalamus. Two of them should cross sides in the optic chiasm and two should not - be sure you understand why each one goes where it does. - Explain to your lab partners what effect would it have on your vision if each of the following were damaged: The right optic nerve: The left optic nerve: The right optic tract: The left optic tract: The optic chiasm: All members of your group: It is your responsibility to be sure these explanations are correct
Most, but not all, regions of your retina contains photoreceptor cells (rods and cones), and any light which strikes these will generate action potentials which can be sent to the brain. However, one region of your retina does not contain any rods or cones. This is the optic disk, where the axons of the ganglion cells of the retina gather and then leave the eye to form the optic nerve. It is also known as the blind spot. Identify its position on Figures 16.26, 16.29 and on the model of the eye - medial to the fovea centralis and macula lutea which are in the exact center of the retina. Any object whose image is focused on the optic disk can not be seen, since there are not photoreceptors there, so we also call it the blind spot of the retina. Fortunately, your brain has learned to ignore this lack of signals from one region of each retina so that we are not normally aware that we have a blind spot - otherwise we would see a dark region just lateral to whatever we were focusing on. 11: Use the diagram below to identify the blind spot in each of your eyes. a) Hold the diagram at arm=s length in front of your face. Close your left eye and focus on the circle with your right eye. You should be able to see the cross with peripheral vision. b) Continue to focus on the circle as you slowly move the diagram closer to your face until the cross disappears. The image of the cross is now hitting your blind spot. c) Continue to move the diagram closer to your face as you continue to focus on the circle. The cross will reappear as its image hits photoreceptors again. d) Repeat this exercise with the right eye covered and the left eye focused on the cross. In this eye, it is the circle which will disappear.