Anatomical discussions of the basal ganglia usually consider structures in afferent and efferent relationship with the striatum. The caudate and putamen complex contains several different neuronal types, the most abundant population being the medium spiny neuron which uses gamma-aminobutyric acid (GABA) as its neurotransmitter. This neuron sends its axonal projection out of the striatum and also has several recurrent axon collaterals that are distributed primarily within its own intrastriatal dendritic field. In addition, the striatum contains numerous large cholinergic interneurons, known as large aspiny neurons, as well as stomatostatin-rich cells that contain nitric oxide synthase for production of the neuromodulator nitric oxide. y
Cortical afferents to the caudate and putamen are somatotopically organized and excitatory, using glutamate as the neurotransmitter. y The brain stem input is primarily from the pars compacta substantia nigra, a dopaminergic pathway. The substantia nigra is a melanin-rich structure located dorsal to the pyramidal tracts (crus cerebri) in the midbrain. Efferent pathways from the striatum are largely directed to the internal segment of the globus pallidus and the pars reticulata of the substantia nigra. There are two distinct pathways of primary importance, named conveniently the direct and the indirect pathways. The direct pathway is inhibitory and passes monosynaptically from the striatum to these nuclei. The indirect pathway reaches the same destination but synapses first in the external segment of the globus pallidus and then in the subthalamic nucleus. The direct and indirect pathways balance one another physiologically, because the indirect pathway functions in sum as an excitatory path on the GPi , with two inhibitions and one excitation (see Fig 16-3 ). Thus, activation of the direct pathway inhibits GPi neurons, whereas activation of the indirect pathway stimulates GPi .y
Efferent pathways directed beyond the basal ganglia to the thalamus and cortex emanate primarily from the GP i and pars reticulata of the substantia nigra. This influence is a tonic inhibitory one via GABAergic cells that project to the ventral anterior and ventral lateral thalamic nuclei and, to a lesser extent, to other thalamic regions, as well as the brain stem. The various influences on GP i provide phasic modulation of the tonic inhibition on the thalamus. The final part of this loop involves the thalamocortical projections, which are excitatory, probably use glutamate as the neurotransmitter, and synapse in the motor, supplementary, and premotor cortices.y
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