The Basal Ganglia and Cerebellum Modulate the Activity of Motor System Components for the Respective Selection and Adjustment of Movement

Portions of the motor system are important for proper motor function but do not appear to be directly involved in initiat­ing movement. These structures—the basal ganglia and the cerebellum—serve primarily to modulate the activity of other motor system structures without directly producing movement.

The basal ganglia are a group of nuclei, the majority of which are deep within the cerebral hemispheres. They include the caudate nucleus and putamen (known collectively as the striatum), the globus pallidus, the substantia nigra, and the subthalamic nucleus. The internal neural circuitry of this multinuclear functional unit is extremely complex and par­ticipates in several parallel pathways running through the basal ganglia. I he basal ganglia receive input from the motor cortices and many other areas of cerebral cortex and, by way of the thalamus, project output back to the motor cortices, particularly the supplementary motor and premotor cortices. Again, these regions are important in the planning and prep­aration for movement. Some basal ganglia output projects directly to brainstem nuclei controlling movement.

Generally it is thought that the basal ganglia use the information received from the cortex, including information about the movement plan and the context of the situation, to help select the appropriate movement pattern while suppressing less appropriate, competing patterns. Two principal circuits within the basal ganglia play an important role in this process. One circuit acts to facilitate inhibitory output of the basal gan­glia, presumably acting to suppress the inappropriate, com­peting movement pattern. Ihe other circuit acts to reduce inhibitory output of the basal ganglia, presumably “remov­ing the brakes” from the appropriate movement pattern. Dopamine-Containing neurons that project from the substantia nigra of the basal ganglia to the striatum of the basal ganglia play an important role in regulating these two circuits.

The structure and function of the cerebellum and its role in motor control are discussed in Chapter 12 and are mentioned only briefly here. The cerebellum’s importance in motor control is indicated by the earlier observation that virtually all the nuclei giving rise to the brainstem motor pathways receive output from the cerebellum. Also, the cerebellum indirectly receives input (through pontine nuclei) from the motor cortices (MI, supple­mentary motor cortex, premotor cortex). As with the basal gan­glia. the cerebellum not only receives information from the motor cortices, but indirectly sends information back to them as well. Importantly, the cerebellum receives much sensory infor­mation from the skin, joints, muscles, vestibular apparatus, and even the visual system. Therefore the cerebellum receives infor­mation about the planning and initiation of movement, as well as continuous sensory feedback about the progress of the move­ment. The cerebellum in turn can influence activity in the motor cortices and in the brainstem motor pathways to the spinal cord.

I hrough this organization, it is thought that the cere­bellum acts to compare information about the movement plan with information about how the movement is actually being carried out. It can then presumably make adjustments to the ongoing movement itself or even adjust the movement plan. Within this framework, the cerebellum appears particularly concerned with gathering sensory feedback about, and with adjusting the control of, the timing of movement. Both experi­mental and clinical studies have shown that cerebellar damage produces significant deficits in the coordination and smooth­ness of complex movements. These deficits presumably arise because of problems in the timing of muscle contraction com­ponents of the movement. If the muscle contraction com­ponents are not properly timed, the movement can appear jerky and uncoordinated, may exhibit improper force, and may not stop at the appropriate time.