With advancing age, skeletal muscles lose cells (fibers) and undergo a gradual reduction in their weight more or less parallel to that of the brain. Atrophy of muscles and diminution in peak power and endurance are clinical expressions of these changes. Many processes contribute to this age-dependent loss of lean muscle mass, described as sarcopenia. These include decreased physical activity; diminished appetite associated with loss of smell and elevated levels of cholecystokinin, a satiety hormone; other endocrine changes such as diminished levels of growth hormone and androgens; and (as in the brain) the accumulation of subcellular defects such as nuclear and mitochondrial DNA mutations alluded to earlier. Moreover, with aging, the slow loss of motor neurons contributes to a component of denervation atrophy. Our own observations, with Dr. R.D. Adams, of neuropathologic material indicate that the wasting involves several processes, some principally myopathic and others relating to disuse or denervation from loss of motor neurons. In this material, denervation atrophy of the gastrocnemius muscles was found in 80 percent of individuals older than 70 years of age. The lost muscle fibers are gradually replaced by endomysial connective tissue and fat cells. The surviving fibers are generally thinner than normal (possibly because of disuse atrophy), but some enlarge, resulting in a wider-than-normal range of fiber size. Groups of fibers all at the same stage of atrophy undoubtedly relate to loss of motor innervation. The reduction in conduction velocity and decrease in amplitude of motor nerve potentials and, to a greater extent, of sensory nerves in the aged may be taken as other indices of loss of motor and sensory axons. All these changes are more marked in the legs than elsewhere. However, when Roos and colleagues examined the contractile speed and firing rates of the quadriceps muscle in young men and compared them to those of men close to 80 years old, they found little difference despite a 50 percent reduction in the maximum voluntary contraction force developed by the muscle in the older men.
It has been repeatedly observed that age is an important prognostic factor in a large number of human diseases. This effect is very evident, for example, in the markedly slower and less-complete recovery from Guillain-Barré polyneuropathy in older age groups compared with younger ones. One presumes that the structural changes of aging in peripheral nerves limit the degree of myelin regeneration and lower the threshold for failure of electrical transmission.
Gerontology is defined as the study of aging and geriatrics as the study of the disorders of aging, both of aging itself and of the age-related diseases. Geriatric neurology has emerged as a subspecialty focused on age-related disorders of the nervous system (see the review of the field by Stanton). In comparison to pediatric neurology, these disciplines have not aroused much interest, yet many neurologic patients seen in practice are elderly, especially if one includes those with vascular diseases of the brain. Furthermore, many of their diseases are preventable or therapeutically controllable such as hypertension, atrial fibrillation, and hypercholesterolemia as causes of stroke. Some of the age-related nutritional and endocrine disorders (e.g., vitamin B deficiencies, diabetes mellitus) and many of the common restricted involutional changes (e.g., presbyopia) can be corrected. And always, there is the need to counsel the elderly patient on matters pertaining to health and daily activities. This was appreciated even in the time of Cicero, who, in his De Senectute, urged the practice of moderation in exercise and giving due attention to the mind, which must be kept active or, like a lamp that is not supplied with oil, it will grow dim.