a large cell mass, crescentic on transverse section, extending forward over the inner surface of the crus cerebri from the rostral border of the pons into the subthalamic region. It is composed of a dorsal stratum of closely spaced pigmented (melanin-containing) cells, the pars compacta [TA]; a larger ventral region of widely scattered cells, the pars reticulata [TA]; and smaller less distinct regions, the pars lateralis [TA] and pars retrorubralis [TA]; the pars compacta in particular includes numerous cells that project forward to the striatum (caudate nucleus and putamen) and contain dopamine, which acts as the transmitter at their synaptic endings. Other, apparently nondopaminergic, cells of the substantia nigra project to a rostral part of the ventral nucleus of thalamus, to the middle layers of the superior colliculus, and to restricted parts of the reticular formation of the midbrain. The nigrostriatal projection is reciprocated by a massive striatonigral fiber system with multiple neurotransmitters, chief among which is ?-aminobutyric acid (GABA). The substantia nigra receives smaller afferent projections from the subthalamic nucleus, the lateral segment of the globus pallidus, the dorsal nucleus of the raphe, and the pedunculopontine nucleus of the midbrain. The pars reticulata forms part of the output system for the striate body. The substantia n. is involved in the metabolic disturbances associated with Parkinson disease and Huntington disease.
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