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Home , Afterimage, Ceruminous gland, Choroid, Ciliary body, Ciliary muscle, Ear

The sensory function of the was explained in Chap. 6. Information about the and was not included there because these organs are more specialized and compli- 7 cated than other sense receptors. THE EYES

EYES AND The eyes assist in the monitoring function of the nervous system by receiving information about EARS conditions on the surface of the body and the re- gion surrounding it. This information is used to make proper responses that promote healthy sur- vival and maintain the quality of life. Unlike most sense receptors, the eyes can also provide information about conditions far from the body and thus enable a person to respond to harmful factors long before they affect the body. Examples include avoiding an oncoming car and seeking shelter from an approaching storm. A person can also efficiently and effectively seek out, move toward, and utilize helpful factors at some distance from the body. For example, the eyes are of inestimable value in obtaining and preparing food and in any project that involves obtaining and assembling parts. In addition, information provided by the eyes contributes greatly to learning, and much com- munication occurs through the eyes. Finally, much of the beauty of the world can be appreciated and enjoyed because of the eyes.

Image Formation and Vision

To perform these functions successfully, each must produce accurate images of objects by fo- cusing some of the light that reflects from or is given off by those objects. Focusing light involves bending it so that by the time it reaches the in the rear of the eye, the light has the exact orga- nization it had before leaving the object. For ex- ample, a group of people in a room can recognize each other because light from a lamp strikes them and reflects in all directions from each one. Some of the reflected light from each person enters the eyes of each of the other people, where it is fo- cused into accurate, recognizable images (Fig. 7.1). If enough light enters the eye, it stimulates cer- tain neurons to initiate impulses. The impulses are then processed by other neurons in the eye and by neurons in the . The vision center, the part of the cerebral at the back of each Chapter 7 - Eyes and Ears 149

FIGURE 7.1 Image formation and the pathways used for vision.

! i Vision Optic Optic Optic center radiation tract chiasma BRAIN

cerebral hemisphere, plays a major role in pro- seem to move, or may go completely unnoticed. cessing these impulses. The final result is the con- By contrast, when excess light hits the receptors, scious of the images, a phenomenon they may be overly stimulated or damaged. called vision. The third reason is that neurons in the eye and brain may be unable to properly process the im- Deterrents to Clear Vision pulses sent to them. For example, a person with normal eyes may be blinded because of a . Clear vision is not always possible for three rea- sons. First, the eye may not be able to focus light FORMING IMAGES well enough for an accurate image to be formed. This can occur if the light is too disorganized be- fore it enters the eye. For example, the water drop- lets in fog and the irregular surfaces of the glass Each eye is nearly spherical in shape. The front used in rest room windows cause this effect. In- surface consists of a smooth thin layer, the con- adequate focusing of light can also occur if parts junctiva, which covers the part of the eye exposed of the eye are improperly shaped or malfunction to air (Fig. 7.2). The conjunctiva also extends away because of nearsightedness or farsightedness. from the edge of this region to form a lining on The second reason involves having improper the inner surfaces of the upper and lower . amounts of light the receptors in the eye. It is transparent, and so it does not absorb or block With insufficient light, not enough impulses are any of the light that strikes the eye. produced for accurate perception to occur, and so The conjunctiva secretes a fluid that helps pre- errors in vision are made. For example, objects in vent eye damage from drying and lubricates the a very dimly lit area may be misidentified, may eye so that the eyelids slide over it easily. Small 150 Aging: Biological Perspectives

FIGURE 7.2 Structure of the eye.

Sclera

Choroid Canal of Schlemm Retina

---- Path of light

Anterior chamber Contain Posterior chamber ] ~queous Suspensory ligaments umor

Optic disk () Retinal blood vessels (contains vitreous humor)

blood vessels in the conjunctiva that are visible Cornea over the white part of the eye help nourish the cornea. Immediately behind the central region of the con- junctiva is another transparent structure, the cor- Age Changes Aging results in a small decrease nea (Fig. 7.2). Because the cornea is fairly thick in the smoothness of the conjunctiva, causing light and curved, it bends (refracts) the light that passes entering the eye to be slightly disorganized and through it. scattered. These changes make focusing the light into a clear image more difficult. The conjunctiva Age Changes Aging results in a gradual decrease also diminishes slightly in transparency, causing in the transparency of the cornea. This change is it to absorb and block the passage of some light. usually great enough to block a significant A more important age change is a gradual de- amount of light. Light at the blue end of the spec- cline in the amount of fluid the conjunctiva se- trum is blocked more than is light toward the red cretes. In some individuals fluid production is so end; therefore, seeing blue objects or objects lit low that the eyelids can no longer glide smoothly with blue light is reduced preferentially. The blue over the eye. The irritation and inflammation of end of the spectrum has light of shorter wave- the conjunctiva that result can be quite uncom- lengths than does the red end. fortable. Artificial tear solutions can relieve this Aging of the cornea also increases the degree of discomfort in most individuals. scattering of light that passes through the cornea. Chapter 7 - Eyes and Ears 151

Much of the scattered light still reaches the retina, available to form images. but since it strikes the retina in the wrong places Age changes in the cells and fibers in the iris and in a disorganized way, it causes the viewer may also slow the rate at which the pupil dilates to see bright areas in wrong places in the field of when changing from bright to dim light. This ef- view. This phenomenon is called glare. A similar fect, combined with age-related slowing of pupil- effect is noted when one looks at outdoor objects lary constriction, retards pupillary adaptation to on a bright, hazy day. An extreme example can be changing light intensities. created by viewing bright lights at night through a window covered with drops of water. Ciliary Body With aging, the cornea becomes flatter, reduc- ing the amount of it can cause and mak- The outer edge of the iris is attached to a thick- ing it difficult to see close objects clearly. The cur- ened ring of cells called the ciliary body (Fig. 7.2), vature of the cornea also becomes irregular so that which contains muscle cells that regulate the cur- light from certain parts of the field of view is not vature of the lens. As in the iris, these muscle cells focused properly. This condition, called astigma- are controlled by reflexes. The ciliary body also tism, decreases the clarity of images from some secretes a fluid called aqueous humor, which is parts of the field of view. Eyeglasses or contact discussed below. can often compensate for . Fi- nally, the sensitivity of the cornea to from Age Changes An important age change in the pressure on the eye diminishes, which in turn ciliary body is the slowing of its secretion of aque- decreases a person's ability to ward off eye in- ous humor. The significance of this change is dis- jury from external pressure. cussed below. Iris and Pupil Lens and Suspensory Ligaments A short distance behind the cornea is the iris (Fig. 7.2), which is shaped like a phonograph record A short distance behind the iris and the pupil is or compact disk. Pigments in the iris give the eye the transparent lens, which is nearly round (Fig. its . 7.2). However, because the lens is elastic, its shape The hole in the center of the iris is called the can be changed. The lens increases in thickness pupil. The pupil allows light to pass from the front throughout life. of the eye into the rear region. Muscle cells in the A ring of thin fibers called suspensory liga- iris regulate the size of the pupil. These cells are ments radiates outward from the lens much as controlled reflexively by autonomic motor neurons. spokes radiate from the center of a bicycle wheel. In the presence of bright light, some muscle These ligaments reach and attach to the ciliary cells in the iris constrict the pupil to reduce light body much as spokes connect to the rim of a entering the eye; this helps protect the eye from wheel. being damaged by excess light. The pupil is also Since the lens is a thick curved structure, it re- constricted when a person looks at an object close fracts the light passing through it. Unlike the cor- to the eye. This helps form a clear image by block- nea, however, the curvature of the lens can change ing out stray light. . so that the lens can refract light to a greater or When light is dim, other muscle cells in the iris lesser degree (Fig. 7.3). Alterations in refraction dilate the pupil. This allows enough light into the are important because light coming to the eye eye to adequately stimulate the neurons. from close objects must be refracted more than is light from farther objects. Therefore, to focus light Age Changes With aging, the number and from close objects, the lens becomes more strength of the muscle cells that cause dilation of rounded so that it bends the light more. The lens the pupil diminish and the thickness and stiffness becomes flatter and bends light less to help the of the fibers increase. As these processes eye form a clear image of a distant object. continue, the size of the pupil for any light inten- sity decreases with each passing year, starting at Age changes As the lens ages, it is altered in four age 20. The result is a continuous decline in light ways. One alteration is a decline in transparency J 152 Human Aging: Biological Perspectives to all of light, especially blue light. The sighted during youth may benefit from this age markedly declining transparency of the lens blocks change since their ability to see distant objects more light than does the reduction in the trans- improves. parency of any other part of the eye. This change Many individuals have lenses so flat that they begins during the third decade of life and increases cannot focus light even from far objects. This con- exponentially with age. dition may eventually begin to improve because The second alteration is the development of thickening of the lens increases its ability to re- opaque spots, which begins during the fifth de- fract light. cade. Usually these opacities are toward the pe- The fourth age change is a decrease in elastic- riphery of the lens. Lens opacities block the pas- ity, which may result partly from an increase in sage of light and cause a great increase in light the cross-linkages among collagen fibers. As the being scattered. They account for more scattering lens loses elasticity, its shape changes more slowly of light and more glare than do age changes in when it adjusts to near or distant objects. Declin- any other eye component. ing elasticity actually begins some time before age If many opacities form close to the center of 10 and continues at a steady rate until about age the lens, vision is greatly impaired and the con- 50. The decline in elasticity decreases the amount dition is called . While cataracts are - of curvature the lens can achieve, and objects must egorized as a disease, their development is part be farther away from the to be seen clearly. of aging of the lens. Everyone who lives long Thus, the smallest distance from the eye at which enough will eventually develop cataracts. an object can be seen clearly-the near point of The third alteration of the lens is a reduction accom mod ati 0n- increa ses. in its ability to refract light. This change results This increase is usually not noticed until about from accumulation of damaged plus age- age 40 because most objects used in daily living related formation of abnormal proteins. These two are located beyond the near point. After age 40, changes offset the age-related increase in lens the reserve capacity of the lens for elasticity has thickness and curvature. With diminishing refrac- dwindled sufficiently and the near point becomes tive power, it becomes increasingly difficult to see large enough to interfere with ordinary activities close objects clearly. Individuals who are near- such as reading and writing. This condition, called

FIGURE 7.3 Focusing light from (a) close objects and (b) distant objects.

Less refraction

Nearly parallel rays from distant object

- :::-~ _::--::: ------]}£

Lens is flatter

(a) (b) Chapter 7 - Eyes and Ears 153 I

, involves farsightedness caused by early in childhood. Thereafter, the liquefaction stiffening of the lens. Difficulties and limitations slowly and continuously spreads outward. caused by presbyopia can be reduced by wearing The vitreous humor is held in place by a ring eyeglasses or contact lenses. of attachment on the front edge of the retina and The increase in the near point is rapid from an attachment point at the back of the eye where about ages 40 to 50, but the rate of change slows the optic begins. during the sixth decade of life. By age 60 there is The vitreous humor produces an outward pres- usually no further increase in the near point because sure so that, like the aqueous humor, it puts ten- the lens has lost all ability to change its curvature. sion on the suspensory ligaments. The soft con- Because adjusting the curvature of the lens re- sistency of the vitreous humor allows it to pro- quires the use of the aqueous humor and vitre- tect eye structures by absorbing shock. It also ous humor, a more complete explanation of how holds the retina and layers in place by the curvature of the lens is adjusted and how this pushing against them. process is affected by aging follows the descrip- tion of those two humors. Age Changes With aging, alterations in the chemistry of the vitreous humor make it lose Aqueous Humor transparency and cause more scattering of light. Blocking and scattering of light are increased by As mentioned above, the ciliary body produces a small areas of vitreous humor that become opaque liquid called aqueous humor (Fig. 7.2). The aque- and often grow large enough to be visible in the ous humor flows forward from the ciliary body, field of view. These areas, which appear as ob- passes through the pupil, and is removed from jects of varying sizes, shapes, and textures, are the eye by a special tube located around the edge called because they are seen to move, es- of the cornea. pecially when the eye moves. Though floaters are Since aqueous humor is produced and removed not dangerous, they decrease the quality of the continuously, its steady flow delivers nutrients and images that are formed, are often distracting, and removes wastes. These services are important for can obscure parts of the field of view. Some float- II the cornea and lens, which have no blood vessels. ers are pieces of vitreous humor that have broken The aqueous humor fills the cavity between the off from the main mass and float between the vit- cornea and the lens. The fluid provides a slight reous humor and the retina. outward pressure that helps keep the cornea Chemical changes caused by aging of the vit- curved outward so that it refracts light properly. reous humor also make it decrease in size and By causing the entire eye to bulge outwardly, this shrink away from the retina. These alterations pressure also helps provide tension on the suspen- reduce the amount of support provided for the sory ligaments. The tension helps to hold the lens retina. At the same time, more of the central re- in place and pull it into a slightly flattened shape. gion is becoming liquefied, making the vitreous humor move about when the eye moves. As the Age Changes As a person ages, the rate at which vitreous humor shifts, it pulls on the retina, espe- aqueous humor is produced declines. This change cially during rapid eye or movements, and reduces the rate at which the cornea and lens are causes the person to perceive flashes of light serviced. There is also a decrease in the amount called flashers. of aqueous humor present, and this may contrib- The presence of a few flashers may be distract- ute to the flattening and irregular curvature of the ing but is of little importance. However, as the cornea that develop with aging. vitreous humor ages, the tension it places on the II retina increases. If the tension becomes great Vitreous Humor enough, the person may perceive many flashes and part of the field of view may become dark- Another eye humor, the vitreous humor, fills the ened. These symptoms constitute a warning that cavity in the eye behind the lens (Fig. 7.2). The the vitreous humor may have detached the retina transparent vitreous humor is composed of a soft from the back of the eye. If not treated and cor- that has the consistency of partially solidified rected immediately, detachment of the retina usu- gelatin. The center of the gel becomes liquefied ally causes some degree of blindness.

...... ~ 154 Human Aging: Biological Perspectives

ACCOMMODATION plished by having the ring of muscle cells in the ciliary body contract and pull the edges of the cili- Having described the aqueous and vitreous hu- ary body inward. When this happens, the normal mors, we now move to a more complete descrip- elasticity of the lens causes the lens to spring back tion of the process of adjusting the curvature of to its round shape. the lens. This process is called . To flatten the lens again to focus on distant Since the lens is normally in a slightly flattened objects, the muscle cells in the ciliary body relax, condition because of pressure from the aqueous allowing the lens to be pulled back to its slightly and vitreous humors, it is normally set for focus- flattened shape. When the amount of contraction ing light from distant objects (Fig. 7.4). For the of the ciliary body is regulated, the curvature of lens to become more rounded so that it can focus the lens can be adjusted. Then light from the ob- light from near objects, the tension on the suspen- ject a person is viewing is in focus. Other objects sory ligaments must be decreased; this is accom- at different distances appear to be less clear.

FIGURE 7.4 Accommodation to (a) distant objects and (b) near objects.

Ciliary muscle fibers relaxed

Suspensory ligaments taut

Distant object

Lens flattened for distant vision

(a)

Ciliary muscle fibers contracted

Suspensory ligaments relaxed

Nearby object

Lens rounded for close vision

(b) Chapter 7 - Eyes and Ears 155

Age Changes face region, closest to the vitreous humor, use their synapses to process these impulses. The impulses Age changes in the ciliary body and the lens are then passed to the , which begins gradually reduce and finally eliminate the ability near the back of the retina at a spot called the optic to adjust the shape of the lens. Therefore, the disk. This nerve carries the impulses to the brain. range of distances over which the eye can focus Blood vessels in the retina nourish the neurons diminishes considerably. that process impulses; these vessels pass through A person can partially compensate for dimin- the optic disk. The photoreceptors are nourished ishing accommodation by using eyeglasses with by vessels behind the retina in the choroid layer, different refractive powers for viewing objects at as is the outer layer of the retina, the pigmented different distances. Of course, it is inconvenient . to change when the distances being The photoreceptors in the sensory retina are of viewed change frequently, as occurs when a per- two main kinds. The cones are clustered together son checks prices while shopping or takes notes in a small circular region at the very back of the during a lecture. The expense of two or more sets eye. This region is in line with the center of the of eyeglasses is also a significant factor. cornea and the lens and is called the macula lutea These problems can be largely overcome by or simply the macula In the center of the macula using eyeglasses with bifocal or trifocal lenses, is a slightly depressed area that contains a very which have regions with two or three different high concentration of cones. This central area is refractive powers, respectively. Thus, one set of called the fovea centra lis or simply the fovea. glasses can be used over a wide range of distances The outer layer of the retina is called the pig- simply by looking through the appropriate region mented epithelium because is it a darkly colored of the multipowered lenses. Unfortunately, each thin layer of cells. A noncellular membrane region of the lens has a limited field of view, (Bruch's membrane) lies behind the pigmented and such lenses are sometimes considered to epithelium (Fig. 7.5). The pigmented epithelium have undesirable cosmetic effects. They are also and apparently Bruch's membrane regulate the expensive. exchange of materials between the choroid and the photoreceptors of the sensory retina. RETINA Cones We have seen that light from an object being viewed passes through the optic media (conjunc- There seem to be three types of cones, and each tiva, cornea, aqueous humor, lens, and vitreous type is most sensitive to light of a different color, humor) and the pupil so that it is focused and its either blue, , or red. If only one type of cone intensity is adjusted. In this way, clear and accu- is stimulated, the person sees the corresponding rate images are formed on the retina. The retina color, while stimulation of various combinations is a thin layer that lines the rear portion of the of cone types allows a person to see many other inner cavity of the eye. It extends back from the colors and shades of color. This principle explains edge of the ciliary body and thus partially sur- how people can see many colors on a television rounds the vitreous humor (Fig. 7.2). screen that employs patterns of red, blue, and green dots to form images. Cones initiate impulses because of specific Layers and Regions chemical reactions that occur when light strikes pigmented molecules within them. Cones are not The retina consists of two main layers of cells. The particularly sensitive to light; therefore, relatively inner layer is called the sensory retina (Fig. 7.5). high intensities of light must strike them before It is in contact with the vitreous humor and con- they initiate impulses. tains several layers of neurons. The neurons in the When a person views a scene, the image of deeper region are called photoreceptors because what is in the center of the field of view is focused they respond to light by starting impulses in the on the fovea. Since the fovea has a very high con- form of action potentials. The neurons in the sur- centration of cones, it allows the person to see the 156 Human Aging: Biological Perspectives

FIGURE 7.5 Structure of the retina and associated eye components.

Cones

Rods

Sclera Processing neurons Pigmented epithelium

Bruch's membrane

object in the center of the scene in color and with of the photoreceptors are of the second type, maximum (the amount of detail that called rods (Fig. 7.5). The concentration of rods can be seen). diminishes steadily at greater distances from the Objects immediately surrounding the center of edge of the macula, and the front edge of the the field of view are focused on the outer regions retina has few rods. Therefore, images from very of the macula. Since the cones are less concen- peripheral objects are focused on retinal areas trated there, these objects are seen with less vi- with low concentrations of photoreceptors and are sual acuity but still are seen in color. seen with little visual acuity. Rods are of only one type. Images focused on Rods rods are seen as black and white images. Though rods do not permit us to perceive color, they al- While the regions of the retina immediately sur- low us to see in dim light because they are more rounding the macula contain some cones, most sensitive to light than are cones. As in the cones, Chapter 7 - Eyes and Ears 157 the rods produce impulses when light causes spe- light); atmospheric oxygen; air pollutants; normal cific chemical reactions within them. metabolism (e.g., mitochondria); and impulse generation. Age Changes The *FR damage may be from a combination of age-related increases in *FRs and ROS in the Cones Beginning at age 40, cones decrease in eye; age-related decreases in *FR and ROS de- length and many are lost. This decrease in num- fenses in the eye; and accumulation of damage ber is greatest in the fovea. The cones that remain from *FRs and ROS due to slow turnover of some in the retina widen to fill the spaces left by de- eye components. Slow turnover seems especially generated cones. relevant in the lens and the retina. The damage Since the level of visual acuity and color vi- from *FRs and ROS may lead to cataracts, glau- sion are directly related to the concentration of coma, decreasing sensitivity of the retina to light cones in the retina, the declining number of cones and to color, and age-related . with aging contributes to a gradual drop in vi- sual acuity and a diminishing ability to distin- OTHER EYECOMPONENTS guish colors. Choroid Rods Aging causes little if any change in the number of rods. However, the rods lengthen, The choroid lies in contact with Bruch's mem- causing them to be bent into irregular shapes to brane on the outer surface of the retina (Fig. 7.2). fit within the confines of the sensory retina. These As has been mentioned, it contains many blood age changes further diminish an aging person's vessels that nourish and remove wastes from the ability to see in dim light. photoreceptor layer in the sensory retina. The choroid also contains a large amount of Processing Neurons The sensory retina loses not black pigment that absorbs light that has passed only photoreceptors but also neurons in its inner through the retina. Therefore, reflection of the layer that process impulses from the photorecep- light back through the retina is prevented. If the tors. The loss is greatest in the neurons that serve light were reflected, it would pass through the the macula. The declining number of neurons pro- vitreous humor and strike other parts of the cessing impulses probably decreases the quality retina, causing foggy or blurry images. of perceived images and the ability to interpret those images. Age Changes With advancing age, the blood vessels in the choroid become irregular, decreas- Pigmented Epithelium and Bruch's Membrane ing their ability to service the retina. Other age numbers and cell functioning also diminish changes in the choroid include thickening, weak- in the pigmented epithelium of the retina. Fur- ening, declining elasticity, and the formation of thermore, the exchange of materials between the irregularities. These changes cause irregularities sensory retina and the pigmented epithelium de- in the retina and therefore interfere with the fo- creases, resulting in diminished servicing of the cusing of light on the retina. These changes also photo receptors by the pigmented epithelium. allow the choroid to be more easily torn as a re- Age changes in Bruch's membrane seem to fur- sult of trauma. ther inhibit the exchange of materials required by the cones and rods. Changes in this membrane Sclera also cause a reduction in capillaries in the chor- oid. These capillaries normally provide nutrients The outermost layer of the eye is the sclera, which for the cones and rods and carry away wastes. is the white part (Fig. 7.2). It extends from the edge of the cornea at the front of the eye around to the Free radicals in the eye A unifying factor con- optic nerve at the back. tributing to several age-related changes in the eye The sclera consists of a tightly woven mat of is damage from free radicals (*FRs) and reactive collagen fibers. Its strength allows it to support oxygen species (ROS), especially HPz. Free radi- eye structures and protect them from trauma. The cals in the eye are produced by radiation (e.g., UV sclera also serves as a firm attachment point for 158 Human Aging: Biological Perspectives the external muscles of the eye, which allow a for distant viewing when at rest, they must be person to turn the eye in its socket. rotated inward toward the nose (converged) when a person wishes to view a close object. By con- Age Changes The structure and functioning of trast, they must be rotated outward (diverged) the sclera are virtually unaffected by aging, when the person wants to view a distant object though it becomes somewhat yellow and devel- again. ops translucent areas that appear as darkened To observe divergence and convergence, have spots. These color changes may have some cos- a volunteer hold a pencil away from his or her metic impact. face at 's length. Then watch the volunteer's eyes as he or she looks first at the pencil, then at a External Muscles distant wall, and then back at the pencil. Eye movements produced by the external eye Six external muscles attach to each eye (Fig. 7.6). muscles are controlled voluntarily at some times The arrangement of these muscles allows a per- and occur under reflex control at others. For ex- son to turn the eye in any direction. Turning the ample, a person may voluntarily contract the eyes serves several purposes. First, it permits the muscles to move the eye to the right and left. By eye to be pointed directly at a stationary object. contrast, convergence and divergence, which are Second, it allows the eye to keep a moving object caused by contraction of the same muscles, usu- centered in the field of view. Maximum visual ally occur reflexively. acuity is achieved when the eye follows the ob- ject smoothly. Third, it permits the scanning of a Age Changes The ability of the external eye wide field of view. muscles to turn the eye smoothly declines with Rotating the eyes in their sockets also allows age. This may be due to age changes in the both eyes to be aimed at the same object simulta- muscles or to alterations in the nerve pathways neously so that a person does not have double that control the muscles. vision. This goal cannot be achieved by any other The decline in the smoothness of eye move- means. For example, since the eyes are positioned ments greatly decreases visual acuity both when

FIGURE 7.6 The eye in its , with external eye muscles and fat tissue.

Fat (adipose) tissue

Superior oblique tendon

Conjunctiva

Inferior oblique muscle

Optic rectus nerve muscle Chapter 7 - Eyes and Ears 159 1

one is viewing moving objects and when one is Opening the eyelids occurs through contraction viewing stationary objects while moving. Decline of a muscle attached to the upper . Contrac- in vision becomes greater as the speed of move- tion of other can also influence the ment increases. The reduction in visual acuity position of and movement of the eyelids. caused by this age change results in difficulties Contraction of muscles affecting the eyelids oc- in activities such as participating in sports, driv- curs under voluntary control at some times and ing a vehicle, and watching motion pictures. under reflex control at others. For example, a per- son may blink any time he or she chooses, but blinking also occurs reflexively. Closing the eyes when falling asleep or sneezing is another ex- The space between the eye and the surrounding ample of reflex control of the eyelids. contains much fat tissue (adipose tissue). This tissue cushions the eye from trauma, sup- Age Changes Age changes in the of the eye- ports the eye in position, and helps maintain its lids are the same as those that occur in other parts round shape. The skull forming the eye of the facial skin. The discomfort and possible socket also protect and support the eye. injury resulting from the decrease in secretion by an aging conjunctiva were described earlier in this Age Changes The amount of adipose tissue chapter. Finally, age changes in the muscles of the around the eye decreases with aging. This allows eyelids are the same as those that occur in other the eye to sink deeper into its socket, causing a voluntary muscles. cosmetic change in the face and a decrease in the size of the field of view. The decrease in fat tissue Lacrimal Gland also reduces the support the eye receives; there- fore, the eye may become misshapen. Depending The upper outer portion of the eye socket con- on the nature of the change in eye shape, the posi- tains a lacrimal gland, which secretes lacrimal tion and functioning of several structures, includ- fluid. The eyelids spread this fluid over the front ing the cornea, vitreous humor, retina, and chor- of the eye and the fluid moves gradually toward oid may be affected. Any alteration in these struc- the corner of the eye near the nose. There it drains tures will adversely affect vision. through an opening into lacrimal ducts and is eventually carried by ducts to the nasal cavity. Eyelids

The upper and lower eyelids each consist of a fold FIGURE 7.7 Eyelids and the . of skin (Fig. 7.7). Part of the conjunctiva covers the inner surface of each eyelid. A row of is present along the edge of each eyelid. lacrimal The front of the eye must be kept moist because gland it consists of living cells. If the cells on the sur- Upper duct face of the eye were to die, as do those on the sur- face of the skin, light could not be properly fo- cused. The eyelids keep the eye moist by secret- Eyelids ing fluid from the conjunctiva, spreading the con- lower junctival and lacrimal fluids over the eye surface, duct and closing to prevent evaporation of moisture. The eyelids also protect the eye from traumatic Nasolacrimal duct injury and prevent exposure to dust, noxious chemicals, microbes, and excess light. Opening of duct into nasal cavity The functioning of the eyelids depends on their / ~.iN positioning and movement, both of which are ,;< ==::~-'.-'IY[ determined mostly by the contraction of muscles. Closing the eyelids is accomplished by contract- ~- .. ing a ring of facial muscle that surrounds the eye. , - 160 Human Aging: Biological Perspectives

Besides moistening the eye, lacrimal fluid quality of images formed in one eye becomes sub- washes away irritating objects, chemicals, and stantially different from the quality of those microbes. It also prevents infections by killing formed in the other, the brain has increasing dif- . certain bacteria through the action of a chemical ficulty comparing the impulses from the two sets called lysozyme. of images. Any alteration in the ability to aim both eyes at the same object has the same effect. The Age Changes The production of lacrimal fluid brain may even completely block its interpreta- decreases with aging. Many people produce tion of impulses from one eye and provide vision enough regardless of age, but production becomes that is equivalent to seeing with only the other so low in some individuals that they develop dry eye. Another reason for decreased and irritated eyes. The use of artificial tear solutions is aging of the retinal and brain neurons that pro- significantly reduces these problems, just as it does cess the impulses originating in the cones and rods. when secretion by the conjunctiva is inadequate. AGE CHANGES IN VISION TWO EYES Light Intensity Though each eye can perform all the functions carried out by the other, there are two advantages As people age, brighter lighting must be present to having two eyes. The first is that a person's field if they are to see as much as they did when they of view is wider when viewing with both eyes at were younger. The main reason for this is that the the same time. This feature provides more informa- eye allows less of the light striking the conjunc- tion about the environment and improves a person's tiva to reach the retina. Less light can pass through ability to respond to the surrounding conditions. the eye partly because of the declining transpar- ency of the conjunctiva, cornea, lens, and vitre- Depth Perception () ous humor. The decreasing size of the pupil also blocks the passage of a substantial amount of The second advantage of viewing with two eyes light. With less light reaching the retina, fewer is that a person can perceive the distance between cones and rods are stimulated enough to send himself or herself and the objects being viewed. impulses in response, and the photoreceptors that This is known as depth perception or binocular do respond send fewer impulses. vision. It is very important in activities where a Brighter lighting is also needed to compensate person needs to make contact or avoid making for age changes in the cones and rods. These contact with objects in the environment, as occurs changes reduce the sensitivity of the retina to when people reach out to grasp something or light, causing an additional decrease in the num- drive a vehicle. ber of impulses being produced for a given Depth perception is possible because the eyes amount of light. Aging of the processing neurons are a few inches apart. Because of this separation, of the retina may further reduce the number of the images of objects at different distances from impulses that leave the eye. Since fewer impulses the eyes are seen in slightly different positions go to the brain, it is less able to perceive clearly relative to each other. This difference can be dem- and accurately what is being viewed. Aging of onstrated by holding one out at arm's length neurons in the visual pathways and processing and looking at a distant scene first with one eye areas of the brain seems to add to the problem. and then with the other. It will be noted that the The deficit in retinal impulses is probably hand blocks out different parts of the scene when greatest in the macula region because that region the scene is viewed with each eye. During impulse contains cones. Recall that cones need bright light processing, when viewing occurs with both eyes to be stimulated and that they and the processing open, the brain interprets the differences in the neurons for the macula are more affected by ag- impulses from the eyes as depth or distance. ing than are the rods and their processing neu- rons. Therefore, aging causes a decline in the abil- Age Changes Aging causes a decrease in depth ity to distinguish colors. Since age-related de- perception. Some of this decline occurs because creases in transparency preferentially affect short aging does not affect both eyes equally. As the wavelengths, the perception of violet, blue, and

.~.~..~.--c- J '. Chapter 7 - Eyes and Ears 161 green decrease the most. changes that contribute to declining refractive The need for brighter lighting causes vision power include flattening of the cornea and lens problems in aging individuals whenever the field and the diminishing ability of the ciliary body to of view is lit dimly. This is especially apparent move. Close visual acuity is decreased to a lesser when they move from a brightly lit environment degree by reductions in the sensitivity to contrast to a dimly lit one or when the lighting consists caused by changes in the neuron pathways. Since mostly of light with shorter wavelengths, such as most of the decline in close visual acuity is caused violet and blue light. by a decrease in refractive power, the use of eye- Aging individuals also see less well when light glasses that increase refractive power can restore intensity changes rapidly, because the pupil ad- much of the lost acuity. justs to such changes more slowly. Rapidly inter- Recall that the decline in near visual acuity spersed periods of dim light may produce alter- caused by aging is usually not noticed until about nating periods of inadequate lighting and excess age 40. Most of the deterioration of close visual lighting. Such conditions may be encountered acuity is completed by age 60 because by then the during the day when one is driving through lens has become completely stiff. However, since patchy shade (e.g., wooded areas) and at night the neuronal changes and flattening of the cor- when one is driving past bright lights (e.g., on- nea and lens continue for many years, close vi- coming traffic, electric lights in roadside signs). sual acuity may continue to decline gradually af- ter age 60. By contrast, in some individuals the Quality of Light continued thickening of the lens and the result- ing increase in its refractive power may restore Vision in old age is also affected by the quality of some close visual acuity that has been lost. light. This phenomenon occurs because more of Visual acuity for distant vision also declines the light entering the eye is scattered. Age changes with aging. However, distant visual acuity does in the cornea, the vitreous humor, and especially not begin to decline until about age 45, after which the lens contribute significantly to light scattering. the decline is usually steady but slow. The dete- Since much of the scattered light strikes the rioration of distant visual acuity is so slow that retina in the wrong places, the retina produces more than 50 percent of those who reach age 80 scattered impulses, which cause glare. Depend- have fairly good distant vision as indicated by ing on the degree of aging of the eye and the con- scores of 20/40 or better on a Snellen eye chart. ditions present, the amount of glare may range Most of the decline in distant visual acuity is from being barely a nuisance to obscuring objects caused by age changes that reduce the amount of completely. light that passes through the eye. Since only a As age increases, external conditions that in- small proportion of the decline is caused by fo- tensify problems from glare include having cusing problems, eyeglasses and contact lenses brightly lit objects or bright lights against a dark can compensate for only a limited amount of the background (e.g., driving at night); having bright loss. Thus, unlike near visual acuity, distant vi- light strike the eye at an angle (e.g., multiple sual acuity declines steadily even if corrective bright lights); and viewing light with shorter lenses are used. wavelengths, such as blue light. Sunlight causes Declining close and distant visual acuity is glare because it contains much short-wavelength noticed whether a person is observing stationary light. Yellow light produces less glare. Aging also or moving objects. However, the decline for mov- slows recovery of good vision after being exposed ing objects is much greater than that for station- to glare. ary ones because viewing moving objects involves additional processes that also undergo age Visual Acuity changes. One of these processes is smooth movement Even when light is bright and glare is minimized, of the eyes to keep the object being viewed cen- there is a decrease in visual acuity with aging. tered in the field of vision. Recall that with aging, Almost all the decrease for close objects is due to the movement of the eyes becomes less smooth, stiffening of the lens, which causes a great reduc- causing the image of a moving object to move ir- tion in the refractive power of the eye. Other age regularly on the retina. The brain has difficulty r::::: - -

162 Human Aging: Biological Perspectives

interpreting the increasing complexity of the re- cause can be minimized by (1)providing adequate sulting retinal impulses. lighting, (2) reducing sources of glare, (3) enlarg- Another relevant process is the rapid recovery ing items, (4) increasing contrast, (5) positioning of each neuron every time it is stimulated. This is objects at greater distances from the eyes, (6) slow- needed because moving objects produce rapid ing the motion of moving objects, and (7) using changes in the light striking the photoreceptors. eyeglasses or contact lenses. These steps can help Since aging causes slowing of the neurons in- prevent accidents and injuries while preserving volved in vision, neuron recovery lags behind much of an aging person's activity, independence, changes in the moving images. The result is a and quality of life. lengthening of . An is a lingering perception that an DISEASES OF THE EYES image is present even though the image in the eye has changed or disappeared. For example, a light In many people, one or more diseases compound that is flickering rapidly may be perceived as the adverse effects of aging of the eyes. The inci- emitting a steady light because the individual dence of such diseases increases with age, so that images blend into one continuous image. Another the risk for those over age 75 is 2.5 times greater effect of long afterimages is the apparent blurring than the risk for those between ages 50 and 65. of an object as it moves faster. This effect is easily Some of these diseases are little more than a nui- observed by watching a fan as it begins to spin. sance, while others drastically reduce the quality As aging lengthens afterimages, blurring of of vision and in severe cases cause blindness. moving objects increases and therefore visual acu- ity for moving objects decreases. Detecting small Minor Diseases movements in nearly stationary objects also be- comes more difficult. These results adversely af- Lacrimal Fluid The production of lacrimal fluid fect a person's performance in walking and driv- may decline so far that the eyes become dry, caus- ing, recreational and occupational activities, and ing irritation. The application of wetting solutions even spectator activities. can largely relieve this problem. Other individu- als experience what seems to be an excessive pro- Depth Perception (Binocular Vision) duction of lacrimal fluid because the fluid pours out of the eye, forming tears. A person's eyes usually do not undergo age In most cases the cause of the tearing is not changes at the same rate. As the eyes become more excess production of fluid but some factor that different from each other, they send increasingly prevents the fluid from draining properly into the different impulses to the brain. These differences, nasal cavity. One common cause of such tearing together with the declining ability of the brain to is excessive weakening of the circle of muscle sur- interpret them, result in a decrease in binocular rounding the eyelids. When it is weak, the muscle vision, as was discussed previously. allows lacrimal fluid to accumulate behind the eyelid until it overflows onto the face. Tearing Field of View may also be caused by infection or other factors that block the lacrimal ducts that lead from the eyes to The width of the field of view (i.e" peripheral vi- the nasal cavity. sion) decreases gradually from 170 degrees to 140 degrees. Also, there is an age-related decline in Eye Muscles Weakening of the eye muscles may the ability to notice a specific object in a field of cause problems other than tearing, For example, view. This latter change is more evident in the inadequate contraction of the circular muscle sur- periphery of the field of view and with increas- rounding the eyelids may cause drying of the eye ing complexity of the items in the field of view. if the eyelids are not closed completely during sleep. Also, weakening of the muscle that raises Optimizing Vision the upper eyelid causes drooping of the lid. Be- sides producing the appearance of drowsiness, a Though age changes that affect vision cannot be drooping lid may obstruct part of the field of view. prevented, the limitations and disabilities they The circular eye muscle sometimes contracts Chapter 7 - Eyes and Ears 163

(excessively,causing the lower eyelids to turn in- or should undergo . ward.The edge of the lid and its then , scrapeon the eye, producing irritation. If this con- ditionis severe and is not corrected, the scar tis- suethat forms can drastically reduce vision by Age-Related blockingand scattering light. The second most common serious among the elderly is age-related macular degen- eration (AMD), also called senile macular degen- Cataractformation is the most common serious eration. It accounts for about 25 percent of visual eyedisease among the older population. How- loss among those under age 80 and about 40 per- ever,its impact in the United States has been low- cent among those over age 80. As the name indi- eredconsiderably by the high rate of success in cates, this disease causes deterioration of the treatingthis disease surgically. surgery retina. Though the cause of AMD is not known, isthemost common surgical procedure in the U.s. factors that increase the likelihood of its occur- Thereare one million surgeries per year, account- rence include being of advanced age, having high ingfor 12% of annual Medicare expenses. blood pressure, having family members with the Cataracts are the most common age-related eye disease, and having atherosclerosis. diseaseand the main cause of blindness in "third The mechanism by which AMD causes retinal world" locations. damage is not clear, but it seems to involve *FR Recallthat aging causes the formation of opaci- damage and formation of lipid peroxides (LPs). tiesin the lenses. Free radicals and are It seems that age changes in the pigmented epi- maincontributing factors. At first opacities usu- thelium and Bruch's membrane begin to occur in ally develop toward the periphery of the lens, excess near the edge of the macula. These changes wherethey scatter light and cause glare, but even- substantially reduce the passage of nutrients from tually they form toward the center of the lens. the choroid to the sensory retina, leading to de- Central opacities cause glare and decrease visual generation of the cones. There is no treatment for acuitybecause they scatter and block light. An af- the disease at this stage. fected individual is said to have cataracts when If the disease progresses no further, there will centralopacities reduce visual acuity substantially. be decreased visual acuity in the center of the field Main risk factors for cataract formation in de- of view. However, in about 10 percent of cases scending order of importance are increasing age; blood leaks from choroid vessels and passes be- exposure to UV-B light; and topical or internal tween the pigmented epithelium and the macu- corticosteroids. Other risk factors include being lar region of the sensory retina. Phagocytic cells female; having mellitus; ; fam- from the choroid also invade the retina. These ily history; low socioeconomic status; malnutri- changes cause more severe degeneration, and tion or low levels of AOXs (e.g., vit A, vit C, vit E, about 90 percent of these advanced cases result carotenoids); dehydration; and eye trauma or in- in macular blindness. ternal eye inflammation. Diabetes mellitus is a The onset of AMD is indicated by gradual de- disease involving hormone imbalances and abnor- terioration of vision in the central region of the mally high levels of a sugar called in the field of view. Sudden rapid distortion of vision blood. Opacities in the lenses of diabetics seem in this area is a warning sign that and to be caused by the conversion of glucose to an- rapid macular degeneration are occurring. Pro- other sugar, sorbitol, which accumulates in the fessional help should be sought at once. Treatment lenses. Estrogen supplements in postmenopausal with lasers can slow or stop further vessel dam- women seem to reduce their risk of developing age and bleeding in about 50 percent of advanced cataracts. cases. If treatment is not obtained or is unsuccess- When cataracts are treated by surgical removal ful, macular blindness is likely. of the affected lens, the ability of the eye to re- Since AMD affects only the macula, peripheral fract light is restored by implanting an artificial vision is not appreciably altered. When central lens or by wearing eyeglasses or contact lenses. vision is affected, activities such as driving and However, not all individuals with cataracts can reading become difficult or impossible. 164 Human Aging: Biological Perspectives

Glaucoma Secondary The remaining 10 percent of glaucoma cases involve secondary glaucoma. Glaucoma is the third leading serious eye disease This condition is characterized by elevated lOP among older people. It causes diminished vision resulting from another disorder such as diabetes, because the high pressure that develops from ac- tumor, or disease of the vessels in the eye. Each cumulation of aqueous humor inside the eye dam- type of secondary glaucoma is treated according ages the retina and optic nerve. The pressure in- to its specific cause. side the eye is called (LOP). About 9 percent of people over age 65 have one of the three types of glaucoma. Open-Angle Glaucoma About 80 percent of all Diabetic retinopathy (DR) means "disease of the retina associated with diabetes." The form of dia- patients with glaucoma have open-angle glau- betes involved is diabetes mellitus. While DR is coma. Its risk factors include being of increasing age, having relatives with glaucoma, being black, not nearly as common as cataracts, AMD, or glau- and being male. coma, it is the third leading cause of blindness in The cause of and the mechanism producing the United States and the most prevalent cause of high lOP are unknown. Pressure in both eyes is blindness among those with diabetes mellitus. affected. When the pressure remains somewhat The incidence of DR among diabetics increases high for extended periods, it slowly damages the with the length of time since the onset of diabe- retina and optic nerve. The damage may be no- tes. For example, only 7 percent of those who have ticed by the affected individuals as a gradual had diabetes for less than 10 years have devel- shrinkage in the width of the field of view. Unfor- oped DR, while more than 62 percent of those who tunately, this narrowing may go undetected until have had it for more than 15 years have devel- permanent injury has occurred. If left unchecked, oped DR. open-angle glaucoma leads to total blindness. Diabetic retinopathy seems to develop because Fairly simple procedures that are normally part some of the extra glucose in the blood is converted of a complete can detect the pres- to sorbitol, which weakens retinal capillaries so ence of open-angle glaucoma long before signifi- much that they are overly dilated by normal blood cant eye damage has occurred. These procedures pressure. Some weak spots in the capillaries bulge include measurement of lOP and visual exami- outward as aneurysms that leak and bleed. In- nation of the optic nerve using an ophthalmo- jured vessels and hemorrhaged blood can be seen scope. The combination of the two tests identifies when the inside of the eye is examined with an about 80 percent of all affected people. ophthalmoscope. Fluids and blood from the leak- Though there is no cure for open-angle glau- ing dilated capillaries injure the retina. coma, the lOP can be controlled and the effects of As more blood flows through the dilated cap- the disease can be prevented with appropriate illaries, unaffected capillaries receive less blood . flow and eventually shrink and close. The areas of the retina that were served by these shrunken Angle-Closure Glaucoma Angle-closure glau- capillaries are injured because they receive inad- coma or narrow-angle glaucoma gets its name equate blood flow. Such ischemic areas can be from the accompanying abnormally narrow space seen with an ophthalmoscope. As the number of between the lens and the cornea. It accounts for abnormally dilated and shrunken retinal vessels about 10 percent of all cases of glaucoma. increases, more of the retina is adversely affected With angle-closure glaucoma, the lOP may rise and vision deteriorates. quite high within a matter of minutes or hours. In more advanced cases of DR, retinal vessels Affected individuals often experience eye pain, near the optic nerve grow into the vitreous hu- headache, nausea and vomiting, halos around mor. Since these proliferating vessels are weak, lights, and blurred vision. If this is not treated they rupture easily, especially when the vitreous within 2 to 3 days, permanent eye damage and humor moves. Hemorrhaging near or into the vit- blindness can result. Treatment usually involves reous humor often leads to blindness from sec- surgery that reestablishes normal drainage of the ondary glaucoma or from detachment of the retina aqueous humor. from the choroid. Chapter 7 - Eyes and Ears 165

The best treatment for diabetic retinopathy is Air in the canal carries vibrations to a to maintain blood sugar levels within the normal thin flexible membrane covering the inner end range. Once vessels become dilated, the progres- of the . This membrane is called the sive destruction of the retina can be slowed by or tympanic membrane because it re- destroying the dilated vessels with laser surgery. sembles the membrane on a drum. The eardrum marks the beginning of the . Air vi- brations cause the eardrum to vibrate. THE EARS The lining of the outer two-thirds of the ear canal contains modified apocrine sweat glands Much of what was stated about the eyes at the be- called ceruminous glands. These glands secrete ginning of this chapter is also true of the ears. The a semisolid waxy material called or ears help monitor conditions and provide informa- cerumen. Hairs are also located in the outer part tion not only about the body, but also about con- of the ear canal. ditions far from the body. Their functioning helps The sticky cerumen and the hairs trap small preserve , contributes to learning, aids particulate matter and insects, preventing such in communication, and provides enjoyment. items from reaching the eardrum and injuring it The ears accomplish all this by detecting four or interfering with its vibrations. Cerumen inhib- different types of stimuli. One type consists of its infections and also keeps the eardrum pliable vibrations within a certain range of frequencies, so that it can vibrate easily. Cerumen slowly which are called vibrations. is the moves to the outer opening of the ear canal, where conscious perception of sound vibrations. The it is easily removed by wiping or washing. second type is the pull of gravity. A person's awareness of gravity is perceived as a sense of Age Changes Aging of the external ear has the position of the head and results in the person's some cosmetic impact. The pinna becomes knowing whether the head is right side up, up- thicker, longer, broader, and stiffer. Hairs on the side down, or tilted. The third type of is pinna and within the ear canal become more change in the speed of motion of the head. For visible because they thicken and lengthen. Ag- example, a person feels the speed of the head ing of the skin on the pinna is similar to aging change when a vehicle in which he or she is riding of other parts of the facial skin. speeds up or slows down. The fourth type of Each produces cerumen at stimulus is rotation of the head. For example, the same rate regardless of age. However, the a person in a vehicle when the vehicle overall rate of production decreases because the makes a turn even if the speed remains the same. number of ceruminous glands slowly de- Aging adversely affects the ability of the ears to creases. The cerumen may also become thicker detect all four types of stimuli. The perception of in consistency. Therefore, it takes longer to these stimuli is further hampered by age changes move to the end of the ear canal and becomes in the CNS that interfere with the processing of even firmer. Age-related reductions in skin impulses sent by the ears. Beginning with the sense elasticity and adipose tissue cause the ear ca- of hearing, we will examine how aging alters the nal to sag. Therefore, cerumen tends to accu- monitoring of each type of stimulus and see the mulate within the ear canal. The problem is impact these alterations have on people. increased when attempts to remove the ceru- men with cotton swabs or other objects push it HEARING deeper into the canal. A buildup of cerumen in the ear canal inhib- External Ear its the passage of vibrations to the eardrum and thus diminishes a person's ability to hear. Re- Most sound vibrations that are heard travel in air moval of accumulated cerumen, which should or, occasionally, water before reaching the ear. The be done by properly trained individuals, re- visible part of the ear, the pinna or , col- stores normal functioning of the ear canal. Ex- lects the vibrations and directs them into the ear cept for increasing the likelihood of cerumen canal, which is about 2.5 cm (1 inch) long. These retention, aging of the external ear has no ef- two components make up the external ear (Fig. 7.8). fect on hearing. ;i i: 166 !: Human Aging: Biological Perspectives .; "Ii

Ii FIGURE 7.8 Structure of the ear. 1\ I, Ji Tympanic :jll cavity - ~~ IJ1I. ii Ii,

Auditory ":1'.l nerve --r I

~- IVestibular nerve

Organ of Corti

Round window

Incus Auditory AUDITORY (eustachian) tube

11 Jl EXTERNAL EAR MIDDLE EAR

altering contraction of the small muscles attached Middle Ear to the eardrum and stirrup. These muscles are controlled reflexively. Sound vibrations causing the eardrum to vibrate The space surrounding the three bones of the are passed to a small attached to the inner middle ear is filled with air so that the bones can surface of the eardrum, the hammer (). The vibrate easily. To prevent bulging of the eardrum, vibrations then pass in turn through two other the air pressure within the middle ear must be bones, the anvil () and the stirrup (). adjusted so that it always equals the air pressure The stirrup passes the vibrations to another thin in the ear canal. This is done by allowing air to flexible membrane, the , which pass between the middle ear and the nasal cavity marks the end of the middle ear and the begin- through the . ning of the inner ear. The bones of the middle ear provide a system Age Changes As age increases, the eardrum be- of levers that amplify the sound vibrations pass- comes slightly stiffer and the between the ing through them. Amplification is adjusted by hammer and anvil and the anvil and stirrup be- come calcified and deteriorated. However, these Chapter 7 - Eyes and Ears 167

age changes do not have a significant effect on factors, noise, and atherosclerosis. Identifying hearing.Age changes in the muscles and the eusta- which factors besides aging cause each of the fol- chiantube are also of no consequence in hearing. lowing observed changes is not yet possible, but these changes will be referred to as age changes Inner Ear here because they seem to occur to some extent in all people. Thevibrations of the oval window are passed to As age increases, there is shrinkage of the mass aliquid called in the inner ear (Fig. 7.9). of small blood vessels servicing the and The vibrations travel through the perilymph in producing . The resulting decline in thepart of the inner ear that detects sound vibra- nourishment may be partly responsible for tions. This section is called the cochlea because it changes in the of Corti. The reduction in hasa spiral shape like that of a cockle or snail shell. endolymph production diminishes the passing of Vibrations in the perilymph pass through a vibrations through the cochlea, resulting in a de- flexiblemembrane () within creasing ability to hear all frequencies of sound. the cochlea and enter another liquid called en- There are decreases in the numbers of several dolymph. The vibrating endolymph causes the types of cells, including the cells in the or- vibration of another flexible membrane within the gan of Corti, the cells that support that organ, and cochlea, the . This membrane the neurons that carry impulses to the brain. The protrudes inward from the wall of the spiraling becomes flattened and distorted, cochlea much as a spiral staircase protrudes from most often at the beginning of the basilar mem- the inner wall of a building. brane. The net result is an exponential decline in Sound vibrations with the highest frequency the ability to hear; since most age changes occur at or pitch cause vibration of the beginning region the beginning of the basilar membrane, hearing of the basilar membrane. This region is compa- loss of high-frequency is usually greatest. rable to the bottom steps of a spiral staircase. Sound vibrations of lower frequency or pitch Localizing Sound cause more distant regions of the basilar mem- brane to vibrate. The lower the frequency of the While each ear can provide the same information vibrations, the farther along (higher on the spiral about sound, the use of both ears at the same time staircase) the membrane vibrates. allows a person to detect an additional feature: The basilar membrane bristles with rows of the direction of the source of the sound. neurons, called hair cells, which are sensitive to Since the ears are on opposite sides of the head, vibrations. The rows of neurons make up the or- sounds originating closer to one side of a person gan of Corti. Vibration of the basilar membrane reach the ear on that side with greater intensity agitates the hair cells of this organ, causing them and the brain receives more impulses from that to initiate impulses. ear. By comparing and interpreting the differences All the components of the inner ear may also in impulses, the brain gives the person a sense of be set into motion by vibrations reaching them the direction from which the sound originated. through the skull bones. Scratching one's head, This process is called localization of sound. chewing on crunchy food, and clicking one's teeth When the source of a sound is in line with the together are a few ways to produce skull bone center of the body, each ear receives the sound vibrations. These vibrations also initiate impulses. with equal intensity. The brain often has difficulty The impulses from the hair cells are passed to localizing such sounds because the impulses from other neurons in the ear that carry them to the both ears are similar. brain. Auditory centers in the brain process and II interpret the impulses, and the result is hearing. Age Changes Aging does not affect a person's ears equally, and the ability to hear with one ear Age Changes Several changes are known to may decline much more than does the hearing occur in the inner ear as people get older, but a ability of the other ear. The brain will receive fewer variety of other factors are also involved in pro- impulses from the cochlea of the more affected ear ducing these changes. These factors include the regardless of the source of the sound. Since the amount of fat and cholesterol in the diet, genetic localization of sound depends on comparing the

" l1li ..... 168 Human Aging: Biological Perspectives

FIGURE 7.9 Pathway of sound vibrations and structure of the cochlea.

COCHLEA

Cochlear nerve

Low-pitched sound waves Malleus

Scala vestibuli (contains perilymph)

Cochlear duct (contains endolymph)

Scala tympani (contains perilymph)

Tectorial membrane

Organ of Corti

Vestibular membrane

Hair cells

Basilar membrane

High-pitched sound waves

Auditory tube

I II EXTERNAL EAR MIDDLE EAR INNER EAR

Scala vestibuli (contains perilymph)

Vestibular membrane Nerve cells to the brain (contains endolymph)

Outer hair cells Supporting cell Basilar membrane

Inner

Scala tympani (contains perilymph) Chapter 7 - Eyes and Ears 169

differencesin impulses from the ears, greater hear- ing and communication, and reduces the enjoy- ing loss from one ear causes errors that can lead ment sound provides. Since presbycusis affects to disorientation. high-frequency sounds most, it substantially re- duces the ability to hear the quality of music called brilliance. Presbycusis makes understand- ing speech difficult primarily because many con- Agechanges in the central nervous system cause sonant sounds in words are high-frequency further hearing impairment because the ability of sounds. High-pitched voices become especially thebrain to process and interpret impulses from difficult to understand. thecochlea is adversely affected. These effects are The increasing difficulty in understanding noticed as increased difficulty understanding speech has diverse effects on people with pres- sounds that contain echoes or background noise, bycusis. For example, these people may fail to re- sounds that change quickly, and speech that is spond or respond incorrectly when others speak broken up or has syllables or words missing. to them. Other people may begin to believe that the person is becoming demented. Both the people DISORDERS IN HEARING with this condition and others may feel that they are being ignored. Individuals with presbycusis Presbycusis sometimes believe that others are trying to de- ceive them or talk about them in demeaning ways Aging causes different degrees of . and often tend to withdraw from social contacts. Individuals who have lost a great deal of hearing Depression and paranoia are not uncommon out- because of aging are said to have presbycusis, comes. which is the third leading chronic condition Presbycusis may also alter speaking ability among those over age 65. Only arthritis and high because affected individuals can no longer hear blood pressure occur more frequently among the their own voices well. People with presbycusis elderly. may become annoying because they sometimes The incidence of presbycusis increases expo- talk very loudly. nentially with age. About 12 percent of those be- tween ages 45 and 65 are affected. This increases Prevention and Compensatory Techniques to more than 24 percent for those age 65 to 74 and While presbycusis is at least partially due to ag- may reach 39 percent for those over age 75. The ing, it seems that chronic exposure to loud sounds incidence among those in institutions is consid- increases both the likelihood of developing it and erably higher, reaching 70 percent. the severity of the problems it causes. Therefore, Both the percentage of cases and the serious- avoiding exposure to loud sounds, such as loud ness of presbycusis are greater for men than for music and noise from tools and machinery, may women. It is thought that greater hearing loss help prevent presbycusis. When loud sounds are among men may be caused by noise associated unavoidable, or other protective devices with occupations traditionally held primarily by should be used. men. Presbycusis becomes progressively worse as Though there is no cure for presbycusis, people the age of an affected person increases. This con- can be helped to compensate for it. Hearing can dition tends to run in families. be increased by raising the volume of sound, such Presbycusis seems to result when the previ- as by talking louder, or by using amplifying hear- ously mentioned age changes in the cochlea and ing aids. in neurons and brain areas involved in hearing The probability that a particular person with occur to an unusually severe degree. Though hearing loss will benefit from the use of a hear- some of these changes cause hearing loss of all ing aid depends on several factors. These include sound frequencies, most cause hearing loss pre- the type of presbycusis or other condition (e.g., dominantly of higher-frequency sounds. eardrum injury, middle ear infection) that pro- duced the hearing loss, the nature and severity of Effects Presbycusis reduces the ability of the ears the hearing loss, and the ability of the person to to alert a person to desirable factors and harmful use a . The evaluation for hearing aid factors in his or her surroundings, hinders learn- use is best done by a qualified audiologist. 170 Human Aging: Biological Perspectives

Reducing background noise, echoes, and the nected to the beginning of the cochlea (Fig. 7.10). speed of speaking can also improve the under- The is connected to the . Both cham- standing of what is heard. Individuals with pres- bers are filled with endolymph. I bycusis can be trained to do lipreading and use One region of each chamber contains a patch other visual cues to increase their ability to un- of nerve cells protruding into the endolymph. derstand those who are speaking. Speakers can Each patch is called a macula, the same name used help by getting the listener's ; facing the for a patch of cones in the retina. Attached to the listener; using more gestures and facial expres- ends of the nerve cells is a gel containing heavy sions; speaking slowly and clearly; repeating; crystals called . keeping their mouths visible; and asking for feed- When gravity pulls on the otoliths, it causes back to confirm the listener's understanding. them and the gel to shift. This causes a bending of the nerve cells to which the otoliths and gel are at- tached. Speeding up or slowing down of the head also causes a shifting of the otoliths and gel, result- Tinnitus is the perception of sound when there is ing in bending of the neurons. Changes in speed no sound external to the person. Sometimes the produce this effect because the otoliths tend to keep perceived sound originates from within the body; moving at the same speed because of inertia. in other cases there is no sound whatever. Until The effects produced by gravity and inertia on 1990, tinnitus was among the ten most common the otoliths and nerve cells can be observed if a chronic conditions among the elderly. It has now is attached to the end of a flexible rod such been displaced to a lower rank because of the as a fishing pole. When the rod is pointed straight higher reported frequencies of up, it will be fairly straight. If the rod is tilted or and varicose . moved from side to side, gravity or inertia will Tinnitus can result from many causes includ- cause it to bend. ing obstruction of the ear canal; abnormality of Bending the nerve cells in the macula causes any of the parts of the middle ear, inner ear, or them to initiate impulses. Since the macula in the eustachian tube; infection in the ear or CNS; tu- saccule is at a different angle from the macula in mor in the ear or CNS; high blood pressure; ath- erosclerosis; diabetes mellitus; hormone imbal- ances; malnutrition; migraine headaches; medi- FIGURE 7.10 The inner ear. cations; and toxic chemicals. In many cases of tinnitus the sound being per- ceived is little more than a nuisance, although sometimes it becomes quite distracting. Tinnitus may alter or prevent normal sleep. Eliminating tinnitus requires elimination of the cause. Individuals who cannot be cured can some- times be helped by using low levels of other sounds as a distraction or cover for the tinnitus. Other treatments include dietary modifications (e.g., vitamin supplements, reduce caffeine), and surgery.

DETECTING OTHER STIMULI

Gravity and Changes in Speed

Recall that the second and third types of stimuli detected by the ear are gravity and changes in speed. Two chambers in the inner ear are special- Cochlear duct ized to detect these stimuli. The saccule is con- Chapter 7 - Eyes and Ears 171 the utricle, some neurons will always be bent. FIGURE 7.11 A semicircular canal and the Depending on the angle of the head or its direc- detection of rotation. tion of acceleration or deceleration, the shifting and bending occur in different directions, caus- ing different impulses to be sent. The impulses are passed to neurons that carry them to the brain, where they are processed and interpreted. The result is the perception of head position relative to gravity or the sensation of speeding up or slowing down. The person can then adjust his or her position or motion volun- tarily to suit the situation. Some ear impulses are sent to the cerebellum, Ampulla causing reflexive contraction of muscles automati- cally to maintain posture and . Maintain- ing posture and balance allows a person to inter- act with the environment effectively and helps prevent falling. Rotation

Head rotation is monitored by three curved tubes-the -which are con- nected to the utricle. These canals are filled with Endolymph endolymph. One end of each canal is enlarged, forming an ampulla (Fig. 7.11). Each ampulla con- tains a patch of inwardly protruding neurons called the (Fig. 7.11). Each of the semicircular canals is positioned at right angles to the others. Therefore, rotation of the head in any direction in three-dimensional Impulses space will cause one or more of the semicircular canals to shift relative to the endolymph, causing an apparent swirling of the endolymph. The shift- ing and apparent swirling bend the neurons in sensory cells in the saccule, the utricle, and the the crista ampullaris. This effect on the neurons three ampullae. The number of neurons that carry is like the bending of aquatic plants in a current impulses from the ear to the brain also decreases. or of tall trees or grass in a breeze. The cells supporting the sensory neurons show Bending the neurons causes them to initiate degeneration. Aging also results in a steady decrease impulses. These impulses are passed to other ear in the size and number of otoliths. neurons that send them to the brain, where they The combined effects of all these changes cause are processed and interpreted. The result is the a decreased sensitivity of the ear to gravity, perception of turning or rotation. The person can changes in speed, and rotation of the head be- then adjust his or her movements voluntarily. cause fewer impulses are sent to the brain. Also, Some impulses are sent to the cerebellum to ini- age changes reduce the ability of the brain to pro- tiate reflexive muscle contractions that maintain cess and interpret the impulses and initiate vol- balance. untary and reflexive responses. Properly responding to gravity and changes in Age Changes (Gravity, Changes in speed is hampered further because the saccule Speed, and Rotation) loses more otoliths than does the utricle. The brain may be confused when the saccule sends an unusu- With aging, there is a decrease in the number of ally low number of impulses relative to the utricle.

III 172 Human Aging: Biological Perspectives

Dizziness and by the eyes, the skin receptors, and the proprio- ceptors in muscles and joints. This information A severely diminished ability to detect and re- normally helps the ears in maintaining posture spond to gravity, changes in speed, and rotation and balance. Voluntary and reflexive movements of the head produces two types of sensations. One used to stop or slow a fall are reduced because of is dizziness, which is the sensation of instability. age changes in the nervous system, the muscles, Affected individuals feel that they are unable to and the skeletal system. Many disorders and a maintain their posture, body position, or balance. variety of medications further hamper the func- The other is vertigo, the feeling that either the tioning of all these organs and systems. Because body or the surrounding environment is spinning so many factors are involved in promoting loss when no spinning is actually occurring. of balance and falling, diagnosis of the causes of Vertigo is usually experienced under one of individual cases of falling is complicated and re- four conditions: holding the head stationary, ro- quires a broad perspective on the body. tating the head, changing head position relative Another factor that makes falls more serious to gravity (e.g., sitting up from a recumbent posi- for the elderly is thinning of the subcutaneous fat, tion), and walking. Some relief can be obtained leading to less cushioning of the body. Further- by avoiding conditions that initiate or amplify more, age-related weakening of the skin, bones, vertigo and by moving more slowly. blood vessels, and other structures make them Both dizziness and vertigo caused by age more susceptible to injury from a fall. Aging also changes in the ears are unpleasant. They can also lengthens the time needed for recovery from an be dangerous because they increase the risk of injury. losing one's balance and falling. The risks from loss of balance and falls can be Several other factors increase the incidence and reduced by providing handrails, better lighting, seriousness of injuries from falling as age in- stable floor surfaces, and uncluttered walking creases. Aging reduces the information provided areas.