Amy Jacobs, age 16, of Johnson City, Tenn., for her question:

WHAT CAUSES PARKINSON'S DISEASE?

Parkinson's disease is a slowly progressive disabling

ailment, marked by increasing stiffness of the muscles and tremor. The disease is neither contagious nor hereditary. Parkinson's disease results from degeneration of the basal ganglia, an area of nerve cells at the base of the brain. The chief carrier of nerve signals in this area is the chemical dopamine, which is grossly deficient in Parkinsonian patients. The cause of this deficiency is not known.

The disease is not fatal, but no cure exists. Introduction of a drug by doctors in the mid 1960s has led to symptomatic improvement in 60 to 70 percent of the patients.

The disease, which occurs in all parts of the woaffects more men than women and is more likely to develop after the age of 35. About 200,000 cases are recorded annually in the United States and Canada.

Symptoms of the disease include excessive salivation, poor coordination, faulty body balance, tremors and muscle rigidity. Shortening of the muscles along the front of the neck tends to bend the head and spine forward.