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INTRODUCTION

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Definitions

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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.

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THE LOWER MOTOR NEURON

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Anatomic and Physiologic Considerations

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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.

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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.

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All motor activity, even the most elementary reflex type, requires the synchronous activity of many muscles. Analysis of a relatively simple movement, such as clenching the fist, conveys some idea of the complexity of the underlying neuromuscular arrangements. In this act the primary movement is a contraction of the flexor muscles of the fingers, the flexor digitorum sublimis and profundus, the flexor pollicis longus and brevis, and ...

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