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Adams and Victor's Principles of Neurology, 10e >

Disorders of Motility

Disorders of MotilityThe control of motor function, to which much of the human nervous system is committed, is accomplished throughthe integrated action of a vast array of segmental and suprasegmental motor neurons. As originally conceived byHughlings Jackson in 1858, purely on the basis of clinical observations, the motor system is organizedhierarchically in three levels, each higher level controlling the one below. It was Jackson's concept that the spinaland brainstem neurons represent the lowest, simplest, and most highly organized motor centers; that the motorneurons of the posterior frontal region represent a more complex and less closely organized second motorcenter; and that the prefrontal parts of the cerebrum are the third and highest motor center. This scheme is stillregarded as being essentially correct, although Jackson failed to recognize the importance of the parietal lobeand basal ganglia in motor control.

Since Jackson's time, physiologists, and more recently, experts in functional imaging, have repeatedly analyzedthese three levels of motor organization and found them to be valid but to have remarkably complex relationships.Motor and sensory systems, although separated for practical clinical purposes, are not independent entities butare closely integrated. Without sensory feedback, motor control is ineffective. And at the higher cortical levels ofmotor control, motivation, planning, and other frontal lobe activities that subserve volitional movement arepreceded and modulated by activity in the parietal sensory cortex.

Motor activities include not only those that alter the position of a limb or other part of the body (isotoniccontraction) but also those that stabilize posture (isometric contraction). Movements that are performed slowly arecalled ramp movements. Very rapid movements, which are too fast for sensory control, are called ballistic.Physiologic studies, cast in their simplest terms, indicate that the following parts of the nervous system areengaged primarily in the control of movement and, in the course of disease, yield a number of characteristicderangements.

1. The large motor neurons in the anterior horns of the spinal cord and the motor nuclei of the brainstem. Theaxons of these nerve cells comprise the anterior spinal roots, the spinal nerves, and the cranial nerves, andthey 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 movementvoluntary, automatic, postural, and reflex. Thelower motor neurons are the final common pathway by which all neural impulses are transmitted to muscle.

2. The motor neurons in the frontal cortex adjacent to the rolandic fissure (motor strip) connect with the spinalmotor neurons by a system of fibers known, because of the shape of its fasciculus in the medulla, as thepyramidal tract. Because the motor fibers that extend from the cerebral cortex to the spinal cord are notconfined 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.

3. Several brainstem nuclei that 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 ofposture and movement, particularly when movement is highly automatic and repetitive. Certain of thesebrainstem nuclei are influenced by the motor or premotor regions of the cortex, e.g., via corticoreticulospinalrelays.

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4. Two subcortical systems, the basal ganglia (striatum, pallidum, and related structures, including thesubstantia nigra and subthalamic nucleus) and the cerebellum. Each of these systems plays an important rolein the control of muscle tone, posture, and coordination.

These are the subjects of the following chapters.

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