Disorders of Motility

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 Jackson failed to recognize the importance of the parietal lobe and basal ganglia in motor control.

Since Jackson's time, physiologists, and more recently, experts in functional imaging, have repeatedly analyzed these 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 but are closely integrated. Without sensory feedback, motor control is ineffective. And 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.

Motor activities include not only those that alter the position of a limb or other part of the body (isotonic contraction) but also those that stabilize posture (isometric contraction). Movements that are performed slowly are called 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 are engaged primarily in the control of movement and, in the course of disease, yield a number of characteristic derangements.

1. The large motor neurons in the anterior horns of the spinal cord and the motor nuclei of the brainstem. The axons of these nerve cells comprise the anterior spinal roots, the spinal nerves, and the cranial nerves, and they 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.

2. 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.

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 of posture and movement, particularly when movement is highly automatic and repetitive. Certain of these brainstem nuclei are influenced by the motor or premotor regions of the cortex, e.g., via corticoreticulospinal relays.

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

These are the subjects of the following chapters.