The substantia nigra (SN) is a midbrain dopaminergic nucleus which has a critical role in modulating motor movement and reward functions as part of the basal ganglia circuitry. Projections from the SN to the putamen, called the nigrostriatal pathway, are critically involved in the motor deficits observed in Parkinson disease. These dopaminergic neural projections leave the SN via the medial forebrain bundle and forming synapses on multiple neuronal populations throughout the basal ganglia, but especially in the putamen.

The substantia nigra is classically considered to be the primary input into the basal ganglia circuitry and a critical element to these functions. When these subcortical nuclei are damaged such as in stroke or during neurodegeneration, a multitude of neurological conditions can result, including Parkinson disease, Huntington disease, Tourette syndrome, schizophrenia, attention-deficit hyperactivity disorder, and obsessive-compulsive disorder.

Located within the midbrain posterior to the crus cerebri fibers of the cerebral peduncle, the substantia nigra can be functionally and morphologically divided into two regions, the pars compacta containing dopaminergic neurons and the pars reticulata with inhibitor gamma-aminobutyric acid-containing (or GABAergic) neurons.