A number of global changes occur in the human nervous system with age. These changes affect the Autonomic Nervous System as well as other parts of the nervous system. The weight and volume of the nervous system reaches a maximum at puberty and later begins a progressive decline beginning in middle age. Some areas are relatively spared, such as the parietal lobes and brain stem. In the peripheral nerves, there is a loss in fiber number and a loss of myelination. The number of neurons is reduced with age, although the extent of loss has been difficult to assess in human.
The remaining neurons have a reduced cell volume and fewer dendrites. The most reliable marker that has been identified for aging of the nervous system is the accumulation of the fatty brown pigment lipofuscin within the neuron. Lipofuscin is absent in the neurons of newborns. Its accumulation increases progressively until senescence, when 60–70% of neurons contain inclusions.
In human autonomic ganglia, there is an age- related increase in lipofuscin accumulation and neurofibrillary tangles and a decrease in catechola-mine content, as assessed by norepinephrine fluorescence. The number of cells is preserved in ganglia, but there is a distinctive neuroaxonal dystrophy characterized by large (5–30 mm) swellings.
The density of autonomic innervation has been difficult to study in humAutonomic Nervous System, so we have to rely mainly on animal data. One human study found that sympathetic innervations of the posterior tibial artery are reduced with age. Animal studies are not in complete agreement, but suggest that the sympathetic innervations decrease in some tissues with age. For example, arteries are more affected than veins.
