Parkinson Disease

The basal ganglia control movements of the muscles, muscle tone, balance, coordination of groups of muscles that oppose each other, and the change of state necessary for muscles to go from rest to motion. Control from this section of the brain also enables some muscles to remain at rest while others are in action. In a healthy state, signals pass from the motor cortex of the brain to the reticular formation and spinal cord and then to the various muscles that are to undergo contraction. At the same time, other signals follow a different pathway through the basal ganglia, where the nerve signal is dampened (subdued or toned down) so that the resulting contraction does not become jerky (too sudden or quick). Dopamine, found in the basal ganglia, is the neurotransmitter responsible for the dampening effect of the motor signal. If the dampening effect should become too strong, then another neurotransmitter, acetylcholine, counteracts the effects of the dopamine, thus maintaining a balance in the force of the signals sent to the muscle.

In Parkinson disease, degeneration of the basal ganglia, along with damage to the dopamine-producing cells of the substantia nigra, hampers the proper functioning of the nerve pathway that controls movements of the muscles. The muscles become excessively tense, a condition that gives rise to tremor and a rigid joint action. The movements of the body also begin to slow down as a result of this malfunction. Drug treatments are designed to increase the level of dopamine or inhibit the release of acetylcholine, which counteracts dopamine.

Damage to the basal ganglia may be caused by an environmental factor, such as an unknown toxic chemical. Another supposition is that an early viral infection causes the degeneration. Parkinsonism may follow encephalitis or other forms of brain injury. In 1997, researchers located a specific gene defect which seems to be responsible for some familial cases of Parkinson disease.

The disease is not contagious, and while it generally affects older people, there are also cases of juvenile Parkinsonism. Cases involving younger people who had used street drugs of unknown composition and developed full-blown symptoms of the disease as a result have been reported. Some of these cases were favorably treated with implantations of dopamine-producing tissues from fetal brain cells.