Spiny projection neurons (SPNs) represent ~95% of neurons in rodent striatum. Two principal categories distinguish SPN subsets. The first – based on divergent axonal projections and dopamine signaling – assigns SPNs to similarly numerous “direct” (dSPN) and “indirect” (iSPN) pathways (Albin et al., 1989). The second – based on processing limbic versus sensory/motor information – groups SPNs into spatial compartments, the so-called “patch” and “matrix” (Gerfen, 1992; Graybiel and Ragsdale, 1978). Both dSPNs and iSPNs are present in the patch and matrix.
