Figure 9.

Schematic illustration of the differential projections of IT-type and PT-type cortical neurons to the two main types of striatal projection neurons – the direct pathway type containing substance P (SP) and possessing D1-type receptors and the indirect pathway type containing enkephalin (ENK) and possessing D2-type dopamine receptors. The major functions and/or projection targets of each of the two main types of cortical pyramidal and striatal neurons is indicated. The degree of selectivity of IT-type input for direct pathway neurons, and PT-type input for indirect pathway neurons, as well as the extent of thalamic input to these two striatal neuron types is not yet known. Moreover, in our analysis, we have only considered axospinous input, and the axodendritic input to these neuron types from these three sources also needs to be determined.