The substantia nigra (literally meaning "black substance") is a small region in the brain stem, just above the spinal cord. It is one of the centers that help control movements. Cells within the substantia nigra (SN) produce and release a chemical called dopamine. Dopamine is a neurotransmitter that controls movement and balance and is essential to the proper functioning of the central nervous system (CNS). Dopamine assists in the effective communication (transmission) of electrochemical signals from one nerve cell (neuron) to another. Dopamine released by SN neurons lands on the surface of neurons in other brain centers, controlling their activities or "firing" and thus regulating movement. The main target regions of dopamine release from the substantia nigra are called the caudate and the putamen.
In PD, cells of the SN degenerate, and therefore can no longer produce adequate dopamine. When this occurs, neurons elsewhere in the brain are no longer well regulated and do not behave in a normal manner. This results in a loss of control of movements, leading to slowed movements, tremor, and rigidity.
A principal aim of PD therapy is to replace the brain's supply of dopamine with the drug levodopa, which the brain uses to make more dopamine. Alternatively, drugs called dopamine agonists can mimic dopamine's effects on its target cells (in the caudate and the putamen).