The cerebellum is behind the brainstem (see Figure 12-2a). It influences posture and movement indirectly by means of input to brainstem nuclei and (by way of the thalamus) to regions of the sensorimotor cortex that give rise to pathways that descend to the motor neurons. The cerebellum receives information both from the sensorimotor cortex (relayed via brainstem nuclei) and from the vestibular system, eyes, ears, skin, muscles, joints, and tendons—that is, from the very receptors that are affected by movement.
The cerebellum's role in motor functioning includes providing timing signals to the cerebral cortex and spinal cord for precise execution of the different phases of a motor program, in particular the timing of the agonist/antagonist components of a movement. It also helps coordinate movements that involve several joints and stores the memories of them so they can be achieved more easily the next time they are tried.
The cerebellum also participates in planning movements—integrating information about the nature of an intended movement with information about the space outside the person into which the movement will be made. The cerebellum then provides this as a "feedforward" signal to the brain areas responsible for refining the motor program.
Moreover, during the course of the movement, the cerebellum compares information about what the muscles should be doing with information about what they actually are doing. If there is a discrepancy between the intended movement and the actual one, the cerebellum sends an error signal to the motor cortex and subcortical centers to correct the ongoing program.
The role of the cerebellum in programming movements can best be appreciated when seeing the absence of this function in individuals with cerebellar disease, who cannot perform limb or eye movements smoothly but move with a tremor—a so-called intention tremor that increases as the course of the movement nears its final destination. (Note that this is different from parkinsonian patients, who have a tremor while at rest.) People with cerebellar disease also cannot start or stop movements quickly or easily, and cannot combine the movements of several joints into a single smooth, coordinated motion. The role of the cerebellum in the precision and timing of movements can be understood when one considers, for example, a tennis player who, upon seeing a ball fly over the net, anticipates its curve of flight, runs to the spot on the court where, if one swings the racquet, one can intercept the ball. People with cerebellar damage cannot achieve this level of coordinated, precise, learned movement.
Unstable posture and awkward gait are two other symptoms characteristic of cerebellar dysfunction. For example, persons with cerebellar damage walk with the feet well apart, and they have such difficulty maintaining balance that their gait appears drunken. A final symptom involves difficulty in learning new motor skills, and individuals with cerebellar dysfunction
Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition
Control of Body Movement CHAPTER TWELVE
find it hard to modify movements in response to new situations. Unlike damage to areas of sensorimotor cortex, cerebellar damage does not cause paralysis or weakness.
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