DISORDERS OF MOTILITY: INTRODUCTION
The control of motor function, to which much of the human nervous system is committed, is accomplished through the integrated action of a vast array of segmental and suprasegmental motor neurons. As originally conceived by Hughlings Jackson in 1858, purely on the basis of clinical observations, the motor system is organized hierarchically in three levels, each higher level controlling the one below. It was Jackson’s concept that the spinal and brainstem neurons represent the lowest, simplest, and most highly organized motor centers; that the motor neurons of the posterior frontal region represent a more complex and less closely organized second motor center; and that 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 been recognized.
More recently, functional imaging has analyzed motor organization and found it to be remarkably complex. Motor and sensory systems, although separated for practical clinical purposes, are not independent entities but are closely integrated. 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 and, in the course of disease, yield a number of characteristic derangements. The large motor neurons in the anterior horns of the spinal cord and the motor nuclei of the brainstem, the axons of which comprise the anterior spinal roots, the spinal nerves, and the cranial nerves innervate the skeletal muscles. These nerve cells and their axons constitute the lower motor neurons, complete lesions of which result in a loss of all movement—voluntary, automatic, postural, and reflex. The lower motor neurons are the final common pathway by which all neural impulses are transmitted to muscle. The motor neurons in the frontal cortex adjacent to the rolandic fissure (motor strip) connect with the spinal motor neurons by a system of fibers known, because of the shape of its fasciculus in the medulla, as the pyramidal tract. Because the motor fibers that extend from the cerebral cortex to the spinal cord are not confined to the pyramidal tract, they are more accurately designated as the corticospinal tract, or, alternatively, as the upper motor neurons, to distinguish them from the lower motor neurons. Several brainstem nuclei project to the spinal cord, notably the pontine and medullary reticular nuclei, vestibular nuclei, and red nuclei. These nuclei and their descending fibers subserve the neural mechanisms of posture and movement, particularly when movement is highly automatic and repetitive. Two subcortical systems modulate the activity of all movement; these are the basal ganglia (striatum, pallidum, ...