With normal aging, the pigment lipofuscin is deposited in nerve cells, and amyloid in blood vessels. Choline O -acetyltransferase levels and dopamine levels may further decrease in Alzheimer's disease and in Parkinson's disease, respectively. Another age-related change is an increase in monoamine oxidase levels. When this increase is inhibited by monoamine oxidase inhibitors, onset of disability in patients with Parkinson's disease may be forestalled. Although blood flow in women is usually greater than in men until age 60, the subsequent rate of decrease is slightly more rapid.
Redundancy is a property whereby more nerve cells exist than are needed.
Furthermore, hydrocephalic patients, who have only a thin cerebral cortical mantle, may have normal intelligence. The number of cells required for certain functions is unknown, so the extent of redundancy is difficult to estimate. However, redundancy probably reduces the effects of age-related neuron loss. Plasticity at the nerve cell level involves compensatory lengthening and production of dendrites in remaining nerve cells to offset the age-related gradual deterioration and loss of nerve cells. New connections in the dendritic tree may compensate for the fewer nerve cells.