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The complexity of motor activity is almost beyond imagination. Scratching one’s shoulder has been estimated to recruit about 75 muscles. And what must be involved in playing a piano concerto? Over a century ago Hughlings Jackson commented that “There are, we shall say, over thirty muscles in the hand; these are represented in the nervous centers in thousands of different combinations, that is, as very many movements; it is just as many chords, musical expressions, and tunes can be made out of a few notes.” Based on clinical observations, Jackson surmised that the motor system is organized hierarchically in three levels. According to his framework, the spinal and brainstem neurons represent the lowest, simplest, and most organized motor centers; the motor neurons of the posterior frontal region represent a more complex and less closely organized second motor center; and the prefrontal parts of the cerebrum are the third and highest motor center. This scheme is still regarded as being essentially correct, although since Jackson’s time the importance of the parietal lobe and basal ganglia in motor control has also been recognized.

More recently, functional imaging has analyzed motor organization and found it to be remarkably complex. The cooperative activities of motor and sensory cortices, under the critical influence of the basal ganglia, cerebellum, and reticular formation of the brainstem, are continuously integrated and modulated by feedback mechanisms from the sensory and spinal motor neurons (Faglioni and Basso). Motor and sensory systems, although separated for practical clinical purposes, are not independent entities; without sensory feedback, motor control is ineffective. Furthermore, at the higher cortical levels of motor control, motivation, planning, and other frontal lobe activities that subserve volitional movement are preceded and modulated by activity in the parietal sensory cortex.

Physiologic studies, cast in their simplest terms, indicate that the following parts of the nervous system are engaged primarily in the control of movement, yielding characteristic derangements in the course of the disease. The large motor neurons in the anterior horns of the spinal cord and the motor nuclei of the brainstem innervate the skeletal muscles. These nerve cells and their axons constitute the lower motor neurons, constituting the final common pathway by which all neural impulses are transmitted to muscle. The motor neurons in the precentral gyrus of the frontal cortex connect with the spinal motor neurons by a system of fibers known as the corticospinal tract, or, alternatively, as the upper motor neurons. The portion of these fibers that travel through the ventral medulla is designated the pyramidal tract, because of the shape of the fasciculus in that location. In addition, several brainstem nuclei that govern motor control project to the spinal cord, notably the pontine and medullary reticular nuclei, vestibular nuclei, and red nuclei. These nuclei and their descending fibers primarily subserve movement is highly automatic and repetitive, particularly posture and gait. Two subcortical systems modulate the activity of ...

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