A dorsal group of cells (retrorubral) are loosely arranged and extend dorsolaterally, circumventing the superior cerebellar peduncle and the red nucleus. These cells merge with the immediately adjacent VTA to form a continuous mediodorsal band of cells. The dendrites of this dorsal group stretch in a mediolateral direction, and do not extend into the ventral parts of the pars compacta or into the pars reticulata. Calbindin (CaBP), a calcium binding protein, is an important phenotypic marker for both the VTA and the dorsal SNc. This group is referred to as the dorsal tier. In contrast, a ventral cell group (the ventral tier) is calbindin-negative, with dendrites oriented ventrally and extending deep into the SNr.67,116,117 The main connections of the SNc are to the striatum and cortex. In addition, there are connections with the pallidum STN, thalamus, hippocampus, amygdala, and brain stem structures. The DA synapses are in a position on the dendrites of the MSNs to modulate incoming cortical information or exert a generalized volume effect in the striatum. Therefore, with relatively few synapses, the impact of the dopaminergic innervation on corticostriatal modulation is large.
