The main inputs to the MSNs are derived from the cortex, thalamus, and brain stem. Cortical fibers primarily project to the dendritic spines, form asymmetric terminals, and are glutamatergic.39 Thalamic projections terminate primarily onto dendrites, and are excitatory and also glutamatergic.40,41 Cortical and thalamic inputs are distinguished by their transporter molecules and by their regulation of striatal circuitry.42-44 Extracellular physiological recordings in awake, behaving monkeys show that the MSNs are phasically active neurons (PANs). These cells have a very low spontaneous discharge rate (0.5-1 spike/s), but a relatively high firing rate (10-40 spikes/s) associated with behavioral tasks, including movement, preparation for movement, and the performance of learned tasks.45-47 Brain stem projections from the dopaminergic cells of the SNc and the VTA terminate onto the spines as well as the dendritic shafts of the MSNs.48-50 However, recent studies of the relationship between dopaminergic terminals and cortical and thalamic inputs show that all cellular structures in the striatum are within approximately 1 um of a tyrosine-hydroxlyase-positive synapse, suggesting a nonselective dopaminergic axon lattice in the striatum.44 In addition to projections from the cortex, thalamus, and brain stem, MSNs also receive input from interneurons and from local collaterals of other MSNs.35,51,52
