The basal ganglia, like the cerebellum, constitute another accessory motor system that functions usually not by itself but in close association with the cerebral cortex and corticospinal motor control system. In fact, the basal ganglia receive most of their input signals from the cerebral cortex itself and also return almost all their output signals back to the cortex.
Figure 56-9 shows the anatomical relations of the basal ganglia to other structures of the brain. On each side of the brain, these ganglia consist of the caudate nucleus, putamen, globus pallidus, substantia nigra, and subthalamic nucleus. They are located mainly lateral to and surrounding the thalamus, occupying a large portion of the interior regions of both cerebral hemispheres. Note also that almost all motor and sensory nerve fibers connecting the cerebral cortex and spinal cord pass through the space that lies between the major masses of the basal ganglia, the caudate nucleus and the putamen. This space is called the internal capsule of the brain. It is important for our current discussion because of the intimate association between the basal ganglia and the corticospinal system for motor control.
Neuronal Circuitry of the Basal Ganglia. The anatomical connections between the basal ganglia and the other brain elements that provide motor control are complex, as shown in Figure 56-10. To the left is shown the motor cortex, thalamus, and associated brain stem and cerebellar circuitry. To the right is the major circuitry of the basal ganglia system, showing the tremendous interconnections among the basal ganglia themselves plus extensive input and output pathways between the other motor regions of the brain and the basal ganglia.
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