Lab #8: The Special Senses: Hearing, Vision, and Orientation

Transcription

1 Lab #8: The Special Senses: Hearing, Vision, and Orientation Background The special senses (vision, hearing, equilibrium, gustation, and olfaction) differ from the somatesthetic senses in two fundamental ways. First, the receptors for the special senses are all found within specific locations the head, and often within complex organs designed to modify the environmental change in a way that focuses and amplifies its effect on the receptor cells. Second, all of the sensory neurons associated with the special senses are found within cranial nerves, and therefore sensory information from these receptors travel directly into the brain without the involvement of the spinal cord. In this lab exercise, we will examine aspects of two of the special senses vision and hearing. We will also examine how different sensory inputs contribute to a person s ability to orient themselves in their surroundings. Vision The eyes consist of structures that are designed to refract (bend) light from objects in a person s surroundings. There are four structures that refract light as it travels through the eye: the cornea, the aqueous body, the lens, and the vitreous body (Fig. 8.1). As light passes into each structure, the change in density from the previous medium causes the angle that the light is traveling to change. Ultimately, the refraction of light by these various structures leads to a focused image being projected on sensory cells Cornea Pupil Suspensory ligaments Lens Vitreous body (posterior chamber) Macula lutea Fovea centralis Fig 8.1. Diagram of major structures of the eye. Aqueous body (anterior chamber) Iris Ciliary muscle Retina Optic disk Optic nerve Fig 8.2. Effect of distance on light dispersion. Light from objects at a distance enter the eye at a narrow range of angles, whereas light from near objects enters the eye at a wider ranges of angles. within the retina. A focused image enables specific photoreceptors or groups of photoreceptors in the retina to be stimulated by light eminating from a particular point on the object being viewed, and thus enables maximal visual acuity. The degree to which light from an object needs to be bent in order to focus an image depends on the distance of an object from the eye (Fig 8.2). Light eminating from a point on a distant object enters the eye at a relatively narrow range of angles. Thus, that light does not need to be bent extensively to bring light from that point into focus when projected on the retina. In contrast, light fron a point on a near object enters the eye at a much wider range of angles. Therefore, light must be bent more by the refractive structures of the eye to bring that point into focus. The degree to which light is bent by a lens (any lens, not just the anatomical structure within the eye) is referred to as its refractive power. Refractive power is quantified in units called diopters, which are calculated as Refractive power (diopters) = 1 Focal length (m) where the focal length is the distance that light passing through a lens must travel before converging on a single focal point. For convex lenses (lenses that bend light inward, Fig 8.3), the refractive power is a positive value based on

2 Convex Lens Concave Lens Fig 8.3. A comparison of convex and concave lenses. Convex lenses converge light toward a single focal point, whereas concave lenses disperse light away from a single focal point. the thickness of the lens. Thicker lenses bend light more (i.e., have greater refractive power), thus light that passes through the lens converges into a single point in a shorter distance (i.e., shorter focal length) than for a thinner lens (Fig 8.4). Concave lenses (Fig 8.3), in contrast disperse light passing through them rather than converge that light, and thus have a negative refractive power based on the degree to which they disperse light. Focal Length Focal Length Fig 8.4. Effect of lens thickness on refractive power in convex lenses. Thicker lenses bend light more, thus the focal length for the lens is less and the refractive power (in diopters) is greater Fig 8.5. Accomodation. In order for light from a distant object to be brought into focus on the retina, the ciliary muscles relax, tightening the suspensory ligaments and stretching out the lens and reducing its refractive power. To view near objects, the ciliary muscle contracts, thus slackening the suspensory ligaments and allowing the lens to recoil into a thicker shape with greater refractive power. Most lenses have a fixed refractive power, and thus an object must be a specific distance from the lens in order for light from that object to be focused by that lens. However, the refractive power of the lens of the human eye is adjustable. The circumference of the lens is attached to the ciliary muscle of the eye through a series of ligaments called suspensory ligaments (Fig 8.1). By contracting the ciliary muscle to different degrees, the tension exerted by the suspensory ligaments on the edges of the lens can be altered, allowing the lens to be stretched thin or allowed to elastically recoil into a thicker shape. Thus, as the thickness of the lens is altered, its refractive power is altered. This alteration in lens thickness can be used to focus on objects at different distances from the eye through a process called accomodation (Fig 8.5). To view distant objects, from which light enters the eye at a narrow range of angles, the ciliary muscles are relaxed. This causes this ring of muscle to become thin, which in turn pulls outward, increasing the tension on the suspensory ligaments. As these ligaments are pulled taut, they stretch out the lens, causing it to assume a thin shape with a lower refractive power. In contract, to view close objects, the ciliary muscles contract. As this ring of muscle thickens it releases tension on the suspensory ligaments, and the lens elastically recoils back to a thicker shape with a higher refractive power.

3 Near Myopia Far Normal Cornea, Frontal View Astigmatism, Frontal View Lateral View Hyperopia Near Dorsal View Far Fig 8.6. Myopia and hyperopia. In myopia, they eye has abnormally high refractive power, thus whereas near objects can be seen in focus (since refractive power would need to be increased anyway), the refractive power of the eye cannot be lowered enough to focus on distant objects, as light passing through the eye reaches its focal point in front of the retina. In hyperopia, the eye has abnormally lower refractive power. In this case, distant objects can bee seen clearly, but the refractive power of the eye cannot be increased enough to see near objects, as the focal point for light passing through the eye would be behind the retina. Many visual disorders are associated with abnormal refraction of light due to mishaped structures in the eye. Myopia, or nearsightedness, is a condition where an individual has trouble seeing distant objects, although near objects are seen clearly (Fig 8.6). Myopia results from having an unusually elongated eyeball or an unusually thick cornea or lens. In effect, the eye cannot reduce refractive power enough to view distant objects, so light from these objects comes into focus at a point in front of the retina and is back out of focus by the time it reaches the retina. Concave lenses, which spread out light into a wider array of angles before it enters the eyes, are prescribed to correct this visual disorder. Individuals with hyperopia, or farsightedness have the opposite problem of those with myopia. Having a relatively short eyeball, shallow cornea, or thin lens, individuals with hyperopia cannot focus on nearby objects (but can focus on distant objects) because the eye does not have enough refractive power to bring the image for close objects into focus by the time it reaches the retina, and instead the focal point is located somewhere behind the eye. Corrective convex lenses, which bend light Fig 8.7. Astigmatism, as illustrated with a mishapen cornea. Notice that the cornea on the right is wider than it is high. This means that light in the vertical plane will be refracted more than light in the horizontal plane. As a result, if the lens is adjusted to bring light in the vertical plane into focus, light in the horizontal plane will go out of focus. inward before it enters the eye, are used to correct for this visual disorder. Astigmatisms are visual disorders causes by a mishaping of some refractive structure in the eye such that the focal length for light entering in the eye is not the same at all angles (Fig 8.7). as a result, the lens cannot be adjusted to bring light at all angles into focus simultaneously. Corrective lenses for astigmatisms have an angular alteration in refractive power that corrects for the angle of mishapening in the eye (Fig 8.8). Fig 8.8. Optometry prescription form. Sphere indicates the overall refractive power adjustment (in diopters). The cylinder indicates refractive power change for an astigmatism, and the axis indicates the angle of the astigmantism correction. From

4 The ability of the eye to adjust its refractive power can change with age. As a person gets older, the lens becomes less flexible and attachments for the the suspensory ligaments are moved forward on the lens. As a result, the lense remains in a stretched state even when the ciliary muscles constrict. This reduces the ability of the eye to increase its refractive power to see near objects. This form of farsightedness, called presbyopia, is nearly ubiquitous among people over the age of 45 years, and is part of the overall aging process. Once light is bent correctly by the refractive structures of the eye, a clearly focused image of objects in the visual field is projected on the retina of the eye. The ability to see objects in the visual field and determine their spatial relationship to one another depends upon what part of the retina light from a point in the visual field is projected. Light that passes through the axis extending through the centers of the cornea and lens is directed toward a structure in the retina called the macula lutea (Figs 8.1 and 8.9), where most of the cone cells of the retina (those that permit color vison and high visula acuity) are located. At the center of the macula lutea is a pit-like region called the fovea centralis. This particular location has an extremely high density of cone cells, each connected with the peripheral nervous system in such a way that each cone cell stimulates its own ganglion cell (sensory neuron). This means that the receptive field for each ganglion cell is very small, and thus visual Fig 8.9. A view of the fundus of the eye, showing structures on the internal surface of the retina. Optic disk Fig Projection of light from laterally-oriented objects onto the retina. Note that more laterally postioned objects project images on more medial regions of the retina. Also note that images projected onto the optic disk (e.g., the purple circle above) cannot be seen, since therre are no photoreceptors at that location on the retina. acuity is very high in the fovea centralis. Acuity tends to decrease somewhat for light projected on more peripheral regions of the macula lutea, where each ganglion cells receives signals from multiple cone cells (creating a larger receptive field for each ganglion cell), and even more so for light falling outside of the macula lutea, where each ganglion cell is stimulated by many individual rod cells (those that have high light sensitivity but no color discrimination and low acuity). Light from objects that are positioned in the lateral parts of the visual field are projected onto the media portions of the retina (Fig 8.10). Interestingly, it is here in the media region of the retina where the optic nerve joins with the eye, and on the inner surface of the retina, a structure called the optic disk can be seen that demarcates this connection point (Figs 8.9 and 8.10). The optic disk lacks photreceptor cells, so is light from an object is projected directly on the optic disk, no image will be perceived. This creates a blind spot within the visual field, although the brain perceives a continous visual field based on sensory inputs from areas surrounding the blind spot. If a person directly faces an object, light from that object (which would be medially oriented) will tend to be projected somewhat laterally on the inner surface of the retina (Fig. 8.11). The degree to which the image is

5 8.11. Effect of object distance on lateral projection of images in stereoscopic vision. Light from near objects tends to be projected onto more lateral regions of the retina than does light from distant objects. Thus, differential stimulation of photoreceptors in both eyes simultaneously allows depth perception for objects that are relatively near to the individual (< 100 ). projected laterally is related to the distance of the object from the eyes. Recall that light from points on distant objects enter the eye at a very narrow range of angles. Light from these objects would be projected more or less towards the center of the retina for both eyes. However, light from points on near objects enter the eye at a broader range of angles, so light from those objects would tend to be projected onto more lateral regions fo the retina. This lateral projection onto the retinas of both eyes stimultaneously, coupled with the relative size of the object in relation to other objects appearing in the visual field, enables depth perception in human vision. In order for lateral projection of images on the retina to be effective, however, the image must fall into the visual field of both eyes. Thus, the stereopsis (the viewing of the same object from two slightly different angles simultaneously) provided from having two eyes postioned at the front of the head with overlapping visual fields is important component of our ability to see in three dimensions. Hearing The outer and middle regions of the ear act as a conduction and amplification system that collects sound waves in the air and amplifies these vibrations enough to generate waves of fluid pressure in the cochlea of the inner ear, where the sensory receptors (hair cells) are located. In order for hearing to take place, sound must be effectively conducted into the Fig Internal anatomy of the ear. Illustration from inner ear with enough strength to stimulate the hair cells, the hair cells must be able to respond to these vibrations by releasing enough neurotransmitter to sensory neurons to trigger an action potential, the action potentials must propagate through the auditory nerve into the central nervous system and through appropriate second-order and third-order neurons to reach the auditory cortex in the temporal lobe. Hearing impairments can be caused by a number of different conditions, but can be categorized into two different types. The first, conductive deafness, results from a condition where the conduction and amplication of vibrations between the external environment and the fluid of the cochlea. As a result, the vibrations that reach the hair cells are not strong enough to lead to action potential generation in the sensory neurons of the auditory nerve. Examples of conditions that cause conductive deafness include occlusion of the external auditory meatus (e.g., excessive earwax production), perforation of the tympanic membrane, abnormal development or damage to the audittory ossicles, or damage to the oval or round windows. Conductive deafness can often be corrected through the use of hearing aids that amplify sound entering the ear so that the vibrations reaching the inner ear are strong enough to effectively stimulate the hair cells. The other type of deafness, sensorineural deafness, results from damage to either the sensors (hair cells) for hearing, to the nerve pathways that conduct signals from the ear to the auditory cortex, or to the auditory cortex itself. In this case, the basic process of sensation

6 cannot take place because the sensory pathway Fig Damage to stereocilia resulting from excessive volume. Normal stereocilia are above, and damaged sterocilia below. Photos from is compromised. Often, sensorineural deafness affects the ability to hear specific pitches rather than a reduction in hearing at all pitches (as would happen in conductive deafness). A common cause of sensorineural deafness is trauma to the hair cells induced by exposure to excessively loud sounds (>85 db). At this volume, vibrations in the cochlea are strong enough to damage the stereocilia. Indeed, sounds in in excess of 140 db are strong enough to kill the hair cells themselves, and hair cells are not regenerated in humans or other mammals (Fig 8.13). Sensorineural deafness also occurs as part of the normal aging process. This agerelated loss of hearing, called presbycusis, involves a loss in the ability to hear high frequencies, and typically begins in the early 20 s. Treatments for sensorineural deafness include hearing aids for some conditions as well as cochlear implants, which directly stimulate the auditory nerve electrically in response to sound. The positioning of the ears on the lateral surfaces of the head enables binaural hearing, where the central nervous system not only detects vibrations in the surrounding environment, but by comparing characteristics of the sound detected by each ear, can perceive the direction from where the sound originated. To illustrate why, imaging that you have your left ear oriented toward a stereo speaker, and your right ear oriented away from the speaker. Sound eminating from the speaker will reach the left ear a fraction of a second before the right ear. Thus there will be difference in stimulation time (called the interaural time difference). In addition, the right ear will be in an acoustic shadow created by the head blocking the movement of some of the sound through the air. Thus the intensity of the soundwaves entering the right ear will be will be less than those entering the left ear. The resulting interaural intensity difference, coupled with the interaural time difference, enables directional perception of sound. Orientation and Balance A person s ability to orient themselves within their surroundings involves the input of a number of senses that enable us to ascertain the direction of objects in our surroundings and the distance between our bodies and those objects. Although some senses can provide information regarding the direction of objects in our surroundings (e.g., hearing, smell), our orientation within our surroundings and subsequently aour ability to maintain posture and balance is based primarily upon proprioception, cutaneous mechanoreception (touch and pressure), equilibrium (from the vestibular apparatus), and vision. While proprioception and equilibrium enable orientation of the body relative to itself and to the force of gravity, balance and coordination also require at least two different points of reference in the surrounding environment, which are derived from cutaneous mechanoreception and/or sight.

7 Experimental Procedures Experiment I: Vision A. Visual Acuity Testing for Myopia with the Snellen Eye Chart. Posted on the cabinets at the back of the lab are Snellen eye charts (Fig 8.14). There are strips of tape marking on the floor near the front of the room that mark a 20 distance from the charts. Standing at the 20, remove glasses if you are wearing them, cover your left eye, and read the smallest line of text you can see. If you correctly read all of the characters in the line (verified by your lab partners), record the visual acuity value associated with that line of text. Repeat with your left eye, being sure to cover your right eye in the process. If you wear glasses, put them back on, and retest your Fig The Snellen eye chart. vision in each eye with your vision corrected. Visual acuity (specifically in reference to myopia) is typically expressed in values such as 20/20, 20/40, etc. This expression gives your visual ability in comparison to what the average person should be able to see. For example, if you have 20/40 vision, you can see clearly at a maximum of 20 what the average person could see at a maximum of 40. This indicates myopia (since you cannot see distant objects as well as the average person). Often prescription glasses enable better than average acuity, thus while wearing prescription glasses it is not uncommon to have a vision of 20/15 or even 20/10. Fig An astigmatism chart. B. Visual Acuity Testing for Astigmatism. Adjacent to the Snellen eye charts are astigmatism charts (Fig 8.15), which depict a series of banded blocks and a semicircle of radiating lines. Stand 20 from the chart, remove your glasses, and cover one eye. If you have astigmatism, one of the blocks will appear sharply black and white, whereas the others will appear more grayish. Similarly, one or a few adjacent lines of the semicircle will appear particularly dark and sharp, whereas the others will appear more faded and grey. If you do not have astigmatism, all of the lines and blocks will look identically sharp. C. Visual Acuity Measuring the Near Point of Vision. Place the end of a meter stick on the bridge of your nose and hold it so that it extends outward horizontally (Fig 8.16). Close one eye, then take a pencil and hold it at arms length so the point is against the edge of the meter stick. Focus on the tip of the pencil with the open eye, and move the point along the edge of the meter stick towards you. At the point where the tip of the pencil becomes slightly blurry, stop moving the pencil toward you, and note where the pencil is located along the meter stick. The distance is your near point of vision. Fig Near point of vision measurement

8 D. Observing the Blind Spot. Obtain a strip of paper with a and a + on the two ends. Hold the slip of paper between your thumb and forefinger in your right hand in front of you at arm s length so that the is just above your thumb and the + extends laterally off to the right. Cover your left eye, and with the right eye look straight at the, noting that you can see the + out of the corner of your eye. Start to move the piece of paper towards your face, always focusing on the but keeping track of the + in the periphery. Eventually you will move the paper to a point where the + appears to disappear from the end of the paper strip, and all you will see is white. When you continue to move it even closer to your face, the + should reappear. Fig Testing depth perception. E. Stereopsis and Depth Perception. Have one person in your lab group hold a test tube up for you. Stand three steps away facing that person and hold a pencil up in an overhand fashion (Fig 8.17). With both eyes open, take three steps forward, and place the pencil in the test tube in a single smooth downward movement of your arm. Step back three steps, then cover one eye and repeat the action. Did you get the pencil into the test tube? If you did, was it easy this time as it was when you had both eyes open? Experiment II: Hearing A. Identifying Types of Deafness Rinne Test and Weber Test Conduct a Rinne test for hearing in the following manner. Strike a tuning fork and place the end of the stem on the mastoid process located just behind the ear (Fig 8.18). The subject should be able to hear the tuning fork through the vibrations conducted into the skull. When the sound dies down, move the prongs of the tuning fork over to the opening of the ear canal. The tone should return. Have the subject then simulate conductive deafness by pushing the tragus of their earlobe over the opening of the ear canal. Repeat the exercise. The subject should be able to hear the vibrations through the mastoid process just as well (if not better) than before, even though their ability to hear the vibrating prongs through the air will be impeded. If a person had real conductive deafness, they would be able to hear the sound through the mastoid process clearly, even though hearing through the ear canal would be impeded. In contrast, if sensorineural deafness accounts for the hearing impairment, then the subject would not be able to hear the tone through either means. Conduct a Weber test by striking the tuning fork and placing it on the mid-sagittal line of the skull, either directly at the top of the head or on the forehead (Fig 18.19). A tone should be heard from the vibrations being conducted through the skull. Try simulating conductive deafness in one ear. The sound should be clearer in the ear with conductive deafness. If Fig The Rinne test. Photos from Resource/ENT%20Exam/ENT%20Exam.htm

9 an individual had sensorineural deafness in the affected ear, the tone would be clearer in the unaffected ear. B Binaural Sound and Directional Perception. Pull a stool away from you lab bench so that you can easily walk around it. Have someone in your group sit on the stool and close his/her eyes. Strike a tuning fork and have the subject point to where the sound is coming from. Move the tuning fork to various positions around the subject, having them point to the tuning fork at all times. Were they able to locate the tuning fork accurately at all times? Have the subject simulate conductive deafness in one ear by pushing down on the tragus. Repeat the experiment. How did their accuracy in locating the tuning fork differ from when they had both ears open? Fig The Weber test. Photo from Experiment III: Orientation and Balance Sensory Integration Time your lab partner as he/she stands on one foot with their eyes open. Then time the same individual as he/she stands on one foot with their eyes closed. Finally time them one more time as they stand on one foot with their eyes closed but are lightly touching one finger to the top of the lab bench. Discussion Question: Would Ralph Machio have been able to pull off the crane kick so effectively if he had had his eyes closed?