The Basal Ganglia Are Extensively Interconnected

Although the circuitry of the basal ganglia appears complex at first glance, it can be simplified into input, output, and internal pathways (Fig. 5.16). Input is derived from the cerebral cortex and is directed to the striatum and the sub-thalamic nucleus. The predominant nerve cell type in the striatum is termed the medium spiny neuron, based on its cell body size and dendritic structure. This type of neuron receives input from all of the cerebral cortex except for the primary visual and auditory areas. The input is roughly so-matotopic and is via neurons that use glutamate as the neu-rotransmitter. The putamen receives the majority of the cortical input from sensorimotor areas. Input to the subthalamus is from the cortical areas concerned with motor function, including eye movement, and is also via gluta-mate-releasing neurons.

Basal ganglia output is from the internal segment of the globus pallidus (GPi) and one segment of the substantia ni-gra. The GPi output is directed to ventrolateral and ventral anterior nuclei of the thalamus, which feed back to the cortical motor areas. The output of the GPi is also directed to a region in the upper brainstem termed the midbrain extrapyramidal area. This latter area then projects to the neurons of the reticulospinal tract. The substantia nigra output arises from the pars reticulata (SNr), which is histologically

Cerebral

Cerebral

Input Substantia Nigra Pars Compacta

^FGUREIHII^ Basal ganglia nuclei and circuitry. The circuit of cerebral cortex to striatum to GPi to thalamus and back to the cortex is the main pathway for basal ganglia influence on motor control. Note the direct and indirect pathways involving the striatum, GPi, GPe, and subthalamic nucleus. GPi output is also directed to the midbrain extrapyramidal area (MBEA). The SNr to SC pathway is important in eye movements. Excitatory pathways are shown in red, inhibitory pathways are in black. GPe and GPi, globus pallidus externa and interna, SUB, subthalamic nucleus, SNc and SNr, substantia nigra pars compacta and pars reticulata, SC, superior colliculus.

^FGUREIHII^ Basal ganglia nuclei and circuitry. The circuit of cerebral cortex to striatum to GPi to thalamus and back to the cortex is the main pathway for basal ganglia influence on motor control. Note the direct and indirect pathways involving the striatum, GPi, GPe, and subthalamic nucleus. GPi output is also directed to the midbrain extrapyramidal area (MBEA). The SNr to SC pathway is important in eye movements. Excitatory pathways are shown in red, inhibitory pathways are in black. GPe and GPi, globus pallidus externa and interna, SUB, subthalamic nucleus, SNc and SNr, substantia nigra pars compacta and pars reticulata, SC, superior colliculus.

similar to the GPi. The output is directed to the superior col-liculus of the mesencephalon, which is involved in eye movement control. The GPi and SNr output is inhibitory via neurons that use GABA as the neurotransmitter.

The internal pathway circuits link the various nuclei of the basal ganglia. The globus pallidus externa (GPe), the subthalamic nucleus, and the pars compacta region of the substantia nigra (SNc) are the nuclei in these pathways. The GPe receives inhibitory input from the striatum via GABA-releasing neurons. The output of the GPe is also inhibitory via GABA release and is directed to the GPi and the sub-thalamic nucleus. The subthalamic nucleus output is excitatory and is directed to the GPi and the SNr. This striatum-GPe-subthalamic nucleus-GPi circuit has been termed the indirect pathway in contrast to the direct pathway of stria-tum to Gpi (see Fig. 5.16). The SNc receives inhibitory input from the striatum and produces output back to the stria-tum via dopamine-releasing neurons. The output can be either excitatory or inhibitory depending on the receptor type of the target neurons in the striatum. The action of the SNc may modulate cortical input to the striatum.

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