Appearance and Location
The medium spiny neurons are medium sized neurons with large and extensive dendritic trees. Each branch of the these dendritic trees is packed with numerous small spines which receive synaptic inputs from neurons outside the striatum. The corpus striatum - consisting of nucleus caudatus, putamen, and nucleus accumbens - is the main input station of the basal ganglia. Medium spiny neurons in this structure receive cortical, thalamic and brain-stem inputs. In fact, the whole human neocortex except the primary visual and primary auditory cortex project to the striatum.
Within the striatum, there are at least two different types of medium spiny neurons. These types were first distinguished because of the different neuropeptides they contain. About half the spiny cells express Substance P, dynorphin and dopamine D1 receptors and project to the internal globus pallidus and substantia nigra pars reticulata (the direct pathways) whereas the other half express enkephalin and the dopamine D2 receptor and project to the external globus pallidus (the "indirect pathway"). With different types of immunocytochemical or histochemical staining one can identify small clusters (150-300 µm in diameter) of medium spiny neurons (called "patches" or "striosomes" - making up about 15% of the volume of the neostriatum) embedded in a surrounding "matrix" (making up about 85% of the volume of the striatum).
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