This disease is marked by tremor and increasing stiffness of the muscles. It affects more men than women and develops in adulthood, after the age of 35. The disease is caused by degeneration of the basal ganglia, an area of nerve cells at the base of the brain and loss of transmitter activity in the caudate nucleus. The chief carrier of nerve signals in basal ganglia is grossly deficient in parkinsonian patients. The cause of this deficiency is unknown. It, however, is possible that a by-product of a synthetic form of heroin could cause similar damage; which suggests that Parkinson's disease may have an environmental origin. Symptoms are rigid limbs, jerky movement of the muscles, excessive salivation, faulty body balance, and tremors. Shortening of muscles along the front of the neck may bend the head and spine forward.