Parkinson's disease is a movement disorder resulting from a deficiency of dopamine, a neurotransmitter involved in coordinating muscle activity as well as memory function. Its typical symptoms include shaking (tremors), stiffness (rigid limbs), and difficulty with walking, balance, coordination, and starting or stopping movement. These neuromotor impairments can either be muscle weakness, muscle tone disorders or muscle control problems.

Parkinson's symptoms may also have mental and behavioral changes, sleep problems, depression, memory difficulties, and fatigue. Mild cognitive problems are common early in the disease, and dementia occurs in 30% to 80% of Parkinson’s patients in the late stages.

Parkinson’s disease occurs when nerve cells in the basal ganglia, an area of the brain that controls movement, become impaired and/or die. Normally, these nerve cells, or neurons, produce an important brain chemical known as dopamine. When the neurons die or become impaired, they produce less dopamine, which causes the movement problems of Parkinson's. Scientists still do not know what causes cells that produce dopamine to die.

People with Parkinson's also lose the nerve endings that produce norepinephrine. The loss of norepinephrine, the main chemical messenger of the sympathetic nervous system, which controls many functions of the body, such as heart rate and blood pressure, might help explain some of the non-movement features of Parkinson's, such as fatigue, irregular blood pressure, decreased movement of food through the digestive tract, and sudden drop in blood pressure when a person stands up from a sitting or lying-down position.

It's known by now that genuine functional recovery remains elusive for these patients, and there are no pharmacological, surgical or physical interventions that can restore neuromotor function. In patients with these neurological conditions, disruption of the pathways leading from brain, through spinal cord, and to organs (e.g., muscle) results in impaired signal transmission.

Drugs that increase dopamine transmission, such as levodopa, help control the physical symptoms of Parkinson’s but don’t improve mental function.