Within the striatum, there is a large neuronal matrix compartment, comprising approximately 80% of the striatum, and a smaller compartment interdigitated with the matrix and known as striosomes or patches. Both the matrix and striasomal neurons receive inputs from dopamine cells coming from the pars compacta of the substantia nigra. Both cell types send efferent projections to the GP e , and these cells all contain enkephalin. In contrast, however, the matrix population also sends efferents to the substantia nigra pars reticulata and the GP i , whereas the striasomal cells project in large part back to the pars compacta. These projection cells contain substance P and dynorphin. y
Figure 16-3 Schema of anatomical nuclei and pathways involving the basal ganglia. Black arrows represent excitation, and speckled arrows represent inhibition. Note the two primary pathways that leave the striatum-the "direct" pathway that flows monosynaptically to the GP and the "indirect" pathway that has intermediate synapses in the G and the subthalamic nucleus. GP = globus pallidus internal segment; GP = globus pallidus external segment; STN = Subthalamic nucleus; SNr = pars reticularis of the substantia nigra; SNc = pars compacta of the substantia nigra; thal = thalamus.
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