The
brainstem contains a number of nuclei that project to the extrapyramidal tract, including the reticular formation, vestibular nuclei, and the red nuclei. In general, the brainstem appears to be involved with involuntary control of posture. The nuclei listed above are incapable of originating a voluntary movement. Our muscles are always adjusting to changes in posture. The brainstem uses sensory inputs from the skin, proprioceptors, and vestibular receptors to inform the brainstem about the body's position and the forces acting upon it. The brainstem also receives inputs from the auditory and visual system.
The
cerebellum acts as a guidance system by comparing the actual movements with the plan of movement that was generated, making corrections in force and direction of movement. In addition, the cerebellum appears to play a role in the maintenance of muscle tone (constant level of tension). The cerebellum may store memories of previous movements so that the movement can be further refined. The cerebellum receives information from the cortex, spinal cord, brainstem, basal nuclei, and sensory information from all areas of the body. It then sends signals back to the cortex (via the thalamus) so that the movement can be adjusted. People who have damaged their cerebellum can carry out voluntary movements but the movements are clumsy, misdirected, and are often made with inappropriate force. A characteristic sign of cerebellar damage is intention tremors that are a sometimes violent shaking that occurs when a person makes a voluntary movement.
The
basal nuclei are thought to provide feedback to the cortex for the development of motor strategies and to smooth out movements. There is evidence that the basal nuclei are necessary for automatic performance of learned repetitive motions. The basal nuclei receive input from the
cortex and output goes back to the cortex via the thalamus. There are two disease processes that have expanded our knowledge of the basal nuclei: Huntington's chorea and Parkinson's disease. Both are
neurodegenerative diseases that appear to exert their primary action on the basal nuclei. Huntington's chorea appears to originate from damage that disrupts the transmission of information from the basal nuclei to the thalamus. This causes several motor pathways to become overactive, thereby exerting an inhibitory influence over motor control. Parkinson's appears to originate as a disruption of communication from the
dopaminergic cells of the substantia nigra. Parkinson's is characterized by involuntary tremors, a stooped shuffling gait, and movements that are generally slow and stiff. There also appears to be difficulty in originating movement.
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