Normal aging causes no noticeable changes in brain structure. True, there is a slight loss of neurons (nerve cells) in the brain as we age. However, because the total number of neurons is so large, any losses are likely to have only a minor impact on behavior. Because the physiological basis of memory is still unknown, it cannot be assumed that the memory loss observed in the elderly is caused by neuron loss in the brain.
Neurons are extremely sensitive to a lack of oxygen. As a result, it is likely that neuron loss, as well as other abnormalities observed in aging brains, are caused by disease, such as arteriosclerosis, which reduces the oxygen available to areas of the brain by reducing blood supply.
Memory impairment and reduced cognitive ability in the elderly may also be caused by genetic and environmental factors such as exposure to certain chemicals, smoking, or a lack of exercise. Increased waist circumference and obesity later in life, for example, are linked to cerebral cortex thinning and cognitive decline; the cerebral cortex is primarily composed of neuronal cell bodies, the deterioration of which is associated with memory and cognitive impairment.
There are probably functional changes in the brain that account for the slowing of responses and memory defects that are common in the elderly, and even small changes in the connections between brain cells could serve as the basis for marked behavioral changes, but behavioral changes cannot be related to physiological or structural changes in the brain until more is known about how the brain works. Because of the slow course of aging, it is known that the nervous system can compensate and maintain adequate function even in centenarians.
Human behavior is heavily reliant on the reception and integration of information from sensory organs like the eye and ear, as well as nerve endings in skin, muscle, joints, and internal organs. However, there is no direct relationship between receptor sensitivity and behavioral adequacy because the usual level of stimulation is significantly higher than the minimum required for stimulation of the sense organs. Furthermore, an individual adapts to gradual impairments in one sensory organ by drawing on information from other sense organs. To compensate for reduced acuity in the sense organs, modern technology has also provided eyeglasses and hearing aids.
The prevalence of gross sensory impairments, many of which are caused by disease processes, rises with age. According to one survey conducted in the United States, 25.9 percent of people aged 65–74 are blind, compared to 1.3 percent of people aged 20–44. 54.7 per 1,000 people aged 65–74 were classified as functionally deaf, compared to 5.0 per 1,000 people aged 25–34.