Movements can be divided into three main classes: voluntary activity, rhythmic motor patterns, and reflex responses. The highest-order activity is voluntary movement which allows for expression of the will and a purposeful response to the environment (e.g. reading, speaking, and performing calculations). Such activity is goal directed and largely learned, and improves with practice. In rhythmic motor patterns, the initiation and termination may be voluntary, but the rhythmic activity itself does not require conscious participation (e.g. chewing, walking, and running). Reflex responses are simple stereotyped responses that do not involve voluntary control (e.g. deep tendon reflexes or withdrawal of a limb from a hot flame).
Motor control is carried out in a hierarchical yet parallel fashion in the cerebral cortex, the brainstem, and the spinal cord ( CD Figure. . . . 5). Modulating influences are provided by the basal ganglia and cerebellum through the thalamus.
CD Figure 5. The organization of the motor system, showing the hierarchical and parallel control of the cerebral cortex motor areas by the cerebullum and basal ganglia through relay nuclei in the thalamus. From the cortex, pathways travel directly to the spinal cord motor neurons, or first through brainstem centers. Muscle sensory receptors provide feedback to spinal cord segments allowing reflex adjustments. (Adapted with permission from Kandel et al.(1991).
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