MODULE SUPPLEMENT: NEUROLOGICAL SYSTEM
The Nervous System

The nervous system is composed of both central (brain and spinal cord) and peripheral components, with the latter having both somatic and autonomic (parasympathetic and sympathetic) subdivisions.

Although often difficult to separate because of their overlapping effects, changes in the nervous system can be classified as (Timeras 1994):

Structural, or the changes affecting the architecture of the nervous system, such as the numbers of neurons or synapses.

Biochemical/Metabolic, include alterations that occur, for example, in neurotransmitters, and cerebral energy metabolism.

Functional, or changes that can affect the electrical activity of the nerves or motor, sensory, cognitive, and affective processes.

As we discuss these changes it's helpful to realize that, while the nervous system shares similar changes with other systems (such as the accumulation of lipofusin and altered metabolic activity), it may also be considered unique in its complexity, with neuronal cells expressing about 50-100 times more genes than other tissues (Mattson, 1999). Further, it may be especially susceptible to oxidative damage (Halliwell, 2001), a point we will return to later.

At the same time, however, the brain has a remarkable ability to compensate functionally for neuronal loss and atrophy and  many changes that do occur do not affect normal activities until after the age of 75. As an example of this coping and reserve capacity, the striatum has to lose more than 70% of its endogenous dopamine content before abnormal motor symptoms or overt Parkinson's Disease are evidenced (Poirier & Finch, 1994).