1. Substantia nigra:

The substantia nigra, part of the extrapyramidal system, is the source of dopaminergic neurons that terminate in the neostriatum. Each dopaminergic neuron makes thousands of synaptic contacts within the neostriatum and, therefore, modulates the activity of a large number of cells. These dopaminergic projections from the substantia nigra fire tonically rather than in response to specific muscular movements or sensory input. Thus, the dopaminergic system appears to serve as a tonic, sustaining influence on motor activity rather than participating in specific movements.