 The medium spiny neurons are medium-sized neurons (~15 microns in diameter, ~12–13 microns in the mouse) with large and extensive dendritic trees (~500 microns in diameter).Striatal direct pathway MSNs (dMSNs) project directly to the globus pallidus internal (GPi) and substantia nigra pars reticulata (SNpr) whereas striatal indirect pathway MSNs (iMSNs) ultimately project to these two structures via an intermediate connection to the globus pallidus external (GPe) and ventral pallidum (VP).The GPe and VP send a GABAergic projection to the subthalamic nucleus, which then sends glutamatergic projections to the GPi and SNpr.