The Fig. 9 is a summary of the synaptic relations of various neuronal elements of the neostriatum. Striatal neurons receive convergent excitatory inputs from the cerebral cortex, the intralaminar thalamus, the substantia nigra (pars compacta) and the dorsal raphe nucleus. The striatal projection neurons are numerous and medium spiny neurons are at least one type of projection neurons which receive direct convergent excitation from extrinsic afferents. The resultant influence of this excitation is directed, in turn, not only to extra-striatal target neurons, but also extensively to cells within the neostriatum. The action of the medium spiny projection neurons is inhibitory and most likely GABA-ergic.