7.12: Detecting other Stimuli
- Page ID
- 84035
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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 specialized to detect these stimuli. The saccule is connected to the beginning of the cochlea (Figure 7.9, Figure 7.13). The utricle is connected to the saccule. Both chambers are filled with endolymph.
One region of each chamber contains a patch of nerve cells protruding into the endolymph. Each patch is called a macula, the same name used for a patch of cones in the retina. Attached to the ends of the nerve cells is a gel containing heavy crystals called otoliths.
When gravity pulls on the otoliths, it causes them and the gel to shift. This causes a bending of the nerve cells to which the otoliths and gel are attached. Speeding up or slowing down of the head also causes a shifting of the otoliths and gel, resulting in bending of the neurons. Changes in speed produce this effect because the otoliths tend to keep moving at the same speed because of inertia. Thus, a neuron with otoliths attached moves like a flexible car antenna with a ball or other weight attached to its tip.
The effects produced by gravity and inertia on the otoliths and nerve cells can be observed if a weight is attached to the end of a flexible rod such as a fishing pole. When the rod is pointed straight up, it will be fairly straight. If the rod is tilted or moved from side to side, gravity or inertia will cause it to bend.
Bending the nerve cells in the macula causes them to initiate impulses. Since the macula in the saccule is at a different angle from the macula in the utricle, some neurons will always be bent. Depending on the angle of the head or its direction of acceleration or deceleration, the shifting and bending occur in different directions, causing 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 voluntarily to suit the situation.
Some ear impulses are sent to the cerebellum, causing reflexive contraction of muscles automatically to maintain posture and balance. Maintaining posture and balance allows a person to interact with the environment effectively and helps prevent falling.
Rotation
Head rotation is monitored by three curved tubes the semicircular canals - which are connected to the utricle. These canals are filled with endolymph. One end of each canal is enlarged, forming an ampulla (Figure 7.9, Figure 7.14). Each ampulla contains a patch of inwardly protruding neurons called the crista ampullaris (Figure 7.9, Figure 7.14).
Each of the semicircular canals is positioned at right angles to the others. Therefore, rotation of the head in any direction in three‑dimensional space will cause one or more of the semicircular canals to shift relative to the endolymph, causing an apparent swirling of the endolymph. The shifting and apparent swirling bend the neurons in the crista ampullaris. This effect on the neurons is like the bending of aquatic plants in a current or of tall trees or grass in a breeze (Figure 7.14).
Bending the neurons causes them to initiate impulses. These impulses are passed to other ear neurons that send them to the brain, where they are processed and interpreted. The result is the perception of turning or rotation. The person can then adjust his or her movements voluntarily. Some impulses are sent to the cerebellum to initiate reflexive muscle contractions that maintain balance.
Age Changes (Gravity, Changes in Speed, and Rotation)
With aging, there is a decrease in the number of sensory cells in the saccule, the utricle, and the three ampullae. The number of neurons that carry impulses from the ear to the brain also decreases. The cells supporting the sensory neurons show degeneration. Aging also results in a steady decrease in the size and number of otoliths.
The combined effects of all these changes cause a decreased sensitivity of the ear to gravity, changes in speed, and rotation of the head because fewer impulses are sent to the brain. Also, age changes reduce the ability of the brain to process and interpret the impulses and initiate voluntary and reflexive responses.
Properly responding to gravity and changes in speed is hampered further because the saccule loses more otoliths than does the utricle. The brain may be confused when the saccule sends an unusually low number of impulses relative to the utricle.
Dizziness and Vertigo
A severely diminished ability to detect and respond to gravity, changes in speed, and rotation of the head produces two types of sensations. One is dizziness, which is the sensation of instability. Affected individuals feel that they are unable to maintain their posture, body position, or balance. The other is vertigo, the feeling that either the body or the surrounding environment is spinning when no spinning is actually occurring.
Vertigo is usually experienced under one of four conditions: holding the head stationary, rotating the head, changing head position relative to gravity (e.g., sitting up from a recumbent position), and walking. Some relief can be obtained by avoiding conditions that initiate or amplify vertigo and by moving more slowly.
Both dizziness and vertigo caused by age changes in the ears are unpleasant. They can also be dangerous because they increase the risk of losing one's balance and falling.
Several other factors increase the incidence and seriousness of injuries from falling as age increases. Aging reduces the information provided by the eyes, the skin receptors, and the proprioceptors in muscles and joints. This information normally helps the ears in maintaining posture and balance. Voluntary and reflexive movements used to stop or slow a fall are reduced because of age changes in the nervous system, the muscles, and the skeletal system. Many disorders and a variety of medications further hamper the functioning of all these organs and systems. Because so many factors are involved in promoting loss of balance and falling, diagnosis of the causes of individual cases of falling is complicated and requires a broad perspective on the body.
Another factor that makes falls more serious for the elderly is thinning of the subcutaneous fat, leading to less cushioning of the body. Furthermore, age‑related weakening of the skin, bones, blood vessels, and other structures make them more susceptible to injury from a fall. Aging also lengthens the time needed for recovery from an injury.
The risks from loss of balance and falls can be reduced by providing handrails, better lighting, stable floor surfaces, and uncluttered walking areas.