substantia nigra pars compacta

The pars compacta is the densely packed cellular region of the substantia nigra, lying more or less dorsal to the pars reticulata and extending into it in some species. Dopamine is the dominant neurotransmitter used by its neurons, which separates it functionally from the reticulata, and in many species those neurons are pigmented, the property that gives the substantia nigra its name. Its Latin designation is the compact part of the substantia nigra, and it is classified within the nervous system as a regional part of that structure. The distinction between compacta and reticulata is thus drawn on three grounds at once: cell packing density, position, and transmitter phenotype.