Thus, although the striato-nigro-striatal projection system demonstrates a loose topographic and functional organization—the VTA and medial SN are associated with the limbic system, and the lateral and ventral midbrain are related to the associative and motor striatal regions, respectively—there is a complexity of the inputs and outputs that influences those functions. Specifically, the data show that the VS influences a wide range of DA neurons, but is itself influenced by a relatively limited group of DA cells. On the other hand, the dorsolateral striatum influences a limited midbrain region, but is affected by a relatively large midbrain region. Thus, the size and position of the afferent and efferent connections for each system allow information from the limbic system to reach the motor system through a series of connections.13 The dorsal tier projects to the VS. However, the VS efferent projection to the midbrain extends beyond the tight ventral striatal/dorsal tier/ventral striatal circuit, terminating lateral and ventral to the dorsal tier. This area of terminal projection does not project back to the VS. Rather, cells in this region project more dorsally,
