EFFECTS OF AGING ON THE NERVOUS SYSTEM
As a person ages, sensory function gradually declines because of decreases in the number of sensory neurons, the function of the remaining neurons, and CNS processing. As a result of decreases in the number of skin receptors, elderly people are less conscious of anything touching or pressing on the skin and have more dif-ficulty identifying objects by touch. These changes leave elderly people more prone to skin injuries.
A decreased sense of position of the limbs and in the joints can affect balance and coordination. Information about the position, tension, and length of tendons and muscles also decreases, further reducing the senses of movement, posture, and position, as well as the control and coordination of movement.
Other sensory neurons with reduced function include those that monitor blood pressure, thirst, objects in the throat, the amount of urine in the urinary bladder, and the amount of feces in the rectum. As a result, elderly people are more prone to high blood pressure, dehydration, swallowing and choking problems, urinary incontinence, and constipation or bowel incontinence.
A general decline in the number of motor neurons also occurs. As many as 50% of the lower motor neurons in the lumbar region of the spinal cord may be lost by age 60. Muscle fibers innervated by the lost motor neurons are also lost, resulting in a general decline in muscle mass. Loss of motor units leads to more rapid fatigue as the remaining units must perform compensatory work. However, regular physical exercise can forestall and reduce these declines.
Reflexes slow as people age because both the generation and the conduction of action potentials and synaptic functions slow. The number of neurotransmitters and receptors declines. As reflexes slow, older people are less able to react automatically, quickly, and accurately to changes in internal and external conditions.
The size and weight of the brain decrease as a person ages. At least part of these changes results from the loss of neurons within the cerebrum. The remaining neurons can apparently compensate for much of this loss. Structural changes also occur in neurons. Neuron cell membranes become more rigid, the endoplasmic reticulum becomes more irregular in structure, neurofibrillar tan-gles develop in the cells, and amyloid plaques form in synapses. All these changes decrease the ability of neurons to function.
Short-term memory is decreased in most older people. This change varies greatly among individuals, but in general, such changes are slow until about age 60 and then become more rapid, especially after age 70. However, the total amount of memory loss is normally not great for most people. Older people have the most difficulty assimilating information that is unfamiliar and presented verbally and rapidly. Long-term memory appears to be unaffected or even improved in older people.
As with short-term memory, thinking, which includes problem solving, planning, and intelligence, generally declines slowly to age 60 but more rapidly thereafter. However, these changes are slight and quite variable. Many older people show no change, and about 10% show an increase in thinking ability. Many of these changes are affected by a person’s background, education, health, motivation, and experience. It appears that continued mental activ-ity can decrease the loss of mental skills with aging.
Older people tend to require more time to fall asleep andexperience more periods of waking during the night, which arealso of greater duration. Factors that can affect sleep include pain, indigestion, rhythmic leg movements, sleep apnea, decreased uri-nary bladder capacity, and poor peripheral circulation.