Substantia nigra

The substantia nigra (Latin for "black substance") is a portion of the midbrain thought to be involved in certain aspects of movement and attention. It consists of two subdivisions, the pars compacta and the pars reticulata.

The pars compacta contains densely-packed neurons (brain cells) which produce the neurotransmitter dopamine. Degeneration of cells in this region, whether from age related changes, viral infection (certain forms of encephalitis, the "sleepy sickness" of the 1920s described in Oliver Sacks' book Awakenings), or toxins such as MPTP (a toxic substance which can be mistakenly produced during the synthesis of heroin), results in Parkinsonism. This division connects to, and relases dopamine in, the striatum via the nigrostriatal pathway.

The pars reticulata consists primarily of neurons producing the neurotransmitter gamma-amino butyric acid (GABA). This division is one of the two primary output nuclei of the brain's basal ganglia and is largely involved in the control of eye movements. This division of the nigra connects to portions of the superior colliculus.

Pathology in the substantia nigra is believed to be predominantly associated with the neurochemical transmitter dopamine and the diseases Parkinsons disease, Schizophrenia (see the dopamine hypothesis of schizophrenia) and psychomotor retardation sometimes seen in clinical depression.