The substantia nigra lies above the CEREBRAL PEDUNCLES in the MIDBRAIN ventro-lateral tegmentum, extending from the rostral border of the PONS into the SUBTHALAMIC AREA. The neurons in the cell-rich part are called collectively the pars compacta, and contain the black pigment NEUROMELANIN that is visible to the naked eye in the sectioned fresh human brain. These neurons synthesize and use DOPAMINE as their neurotransmitter and project heavily onto the dorsal striatum (see NIGROSTRIATAL DOPAMINE SYSTEM; STRIATUM). Dopaminergic neurons lying more medially (that is, between each substantia nigra) are referred to as the VENTRAL TEGMENTAL AREA dopamine neurons, but are continuous with the neurons of the substantia nigra pars compacta; they project to the ventral striatum (see MESOLIMBIC DOPAMINE SYSTEM), AMYGDALA and PREFRONTAL CORTEX. The dopaminergic neurons of the substantia nigra are among the first to degenerate in PARKINSON’S DISEASE and the resultant dopaminergic denervation of the striatum is associated with AKINESIA and a variety of cognitive deficits.
The cell-sparse part of the substantia nigra is called the pars reticulata and is found ventral and adjacent to the cell groups of the pars compacta. This region of the substantia nigra shares many, but not all, cytoarchitectonic (see CYTOARCHITECTURE) and connectional characteristics with the internal segment of the GLOBUS PALLIDUS. Both structures together represent the major output nuclei of the dorsal striatopallidal system. Pars reticulata neurons invariably contain the neurotransmitter GABA and project to the THALAMUS, SUPERIOR COLLICULUS, PEDUNCULOPONTINE TEGMENTAL NUCLEUS and the PONTINE RETICULAR FORMATION. (Note: the terms pars compacta and pars reticulata are synonymous with zona compacta and zona reticulata. Most authors use pars, but many use zona; some mix the two and refer to pars compacta and zona reticulata.)
