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, 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 structures and their disorders are the subjects of the following four chapters.
Paralysis means loss of voluntary movement as a result of interruption of one of the motor pathways at any point from the cerebrum to the muscle fiber. A lesser degree of weakness is spoken of as paresis. The word plegia comes from a Greek word meaning “to strike,” and the word palsy is from an old French word that has the same meaning as paralysis. One generally uses paralysis or plegia for severe or complete loss of motor function and paresis for partial loss.
Each spinal and cranial motor nerve cell, through the extensive arborization of the terminal part of its efferent fiber, comes into contact with a variable number of muscle fibers, ranging from only a few to 1,000 or more; together, the nerve cell, its axons, and the muscle fibers they subserve constitute the motor unit. All variations in the force, range, rate, and type of movement are determined by the number and size of motor units called into action and the frequency and sequence of firing of each motor unit. Much of the sequence and coordination of firing is modulated by subcortical structures or the basal ganglia and cerebellum. Smaller movements involve relatively few motor units; powerful movements recruit many more units that accumulate to an increasing size.
The motor nerve fibers emanating from a group of anterior horn cells in one segment of the spinal cord constitute the ventral spinal root. These roots intermingle with neighboring ones to form plexuses and then form the peripheral nerves. Although the muscles are innervated in patterns largely corresponding to segments of the spinal cord (a myotome), each large muscle is usually supplied by two or more roots. In contrast, a single peripheral nerve usually provides the complete motor innervation of a muscle or group of muscles. For this reason, paralysis caused by disease of the anterior horn cells or anterior roots has a different topographic pattern than paralysis following interruption of a peripheral nerve. These patterns follow the distribution shown in Table 43-1. For example, section of the L5 motor root causes paralysis of the extensors of the foot with a foot drop and weakness of inversion of the foot, whereas a lesion of the peroneal nerve, which also causes foot drop, does not affect the invertors of the foot since they are supplied by L5 but via the tibial nerve.