A structure within the basal ganglia (a collection of nerve clusters that regulate voluntary movement) that produces nearly all of the dopaminE that the brain requires for communication of neurons related to movement. The name is Latin for “black substance,” a reference to the dark pigmentation of some of the cells of this structure. A protein called neuromelanin accounts for the pigmentation; it accumulates as a result of dopamine metabolism. Although biochemically neuromelanin is very similar to the melanin that pigments skin, there is no correlation between skin pigmentation and the amount of neuromelanin in the substantia nigra.
The substantia nigra is integral to voluntary movement. It has two distinct parts, the substantia nigra pars compacta (SNc), which means “dense part,” and the substantia nigra pars reticulata (SNr), which means “networked part.” The names are references to the appearances of the different tissues. Although both the SNc and the SNr communicate with other structures of the basal ganglia, the SNc does so through dopamine and the SNr through another neurotransmitter, gamma-AMINOBUTYRIC ACID (GABA).
The SNc transmits nerve signals to and from the caudate and putamen (known collectively as the striatum) that affect motor function throughout the body. The SNr receives nerve signals from the striatum that are largely specific for movements of the head and eyes, which it then conveys to the thalamus and cerebral cortex. The SNc and the SNr do not have much interaction with each other. In the context of Parkinson’s disease, references to the substantia nigra generally indicate the SNc.
It is the SNc that deteriorates in Parkinson’s, cutting brain dopamine production. Dopamine is the key NEUROTRANSMITTER essential for modulating the basal ganglia outputs that regulate voluntary movement, and its depletion causes the motor disruptions that characterize Parkinson’s. This is the foundation of Parkinson’s disease, although scientists do not know what causes this sequence of events to begin or continue. The symptoms of Parkinson’s disease become apparent when fewer than 20 percent of the SNc’s dopaminergic neurons remain.
It is sometimes possible to detect and monitor the loss of dopaminergic neurons by using sophisticated functional imaging studies such as single PHOTON EMISSION COMPUTED TOMOGRAPHY (SPECT) or POSITRON EMISSION TOMOGRAPHY (PET). However, there is little clinical value in such studies unless the neurologist is looking for evidence of effectiveness of a regimen of anti-parkinson’s medications.