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When applied to motor function, paralysis means loss of voluntary movement because of interruption of one of the motor pathways at any point from the cerebrum to the muscle fiber. A lesser degree of paralysis is spoken of as paresis. The word plegia comes from a Greek word meaning “to strike,” and the word palsy from an old French word that has the same meaning as paralysis. All these words are used interchangeably, although generally one uses paralysis or plegia for severe or complete loss of motor function and paresis for partial loss.

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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 only a few or up to 1,000 or more muscle fibers; 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. Feeble movements involve relatively few small motor units; powerful movements recruit many more units of increasing size.

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Within a few days after interruption of a motor nerve, the individual denervated muscle fibers begin to contract spontaneously. This isolated activity of individual muscle fibers is called fibrillation. Inability of the isolated fiber to maintain a stable membrane potential is the likely explanation. Fibrillation is so fine that it cannot be seen through the intact skin, but it can be recorded as a small, repetitive, short-duration potential in the electromyogram (EMG) (Chap. 45). When a motor neuron becomes diseased, as in progressive spinal muscular atrophy, it may manifest increased irritability, i.e., the axon is unstable and capable of ectopic impulse generation, and all the muscle fibers that it controls may discharge sporadically, in isolation from other units. The result of contraction of one or several such motor units is a visible twitch, or fasciculation, that appears in the EMG as a large spontaneous muscle action potential. Simultaneous or sequential spontaneous contractions of multiple motor units cause a rippling of muscle, a condition known as myokymia. If the motor neuron is destroyed, all the muscle fibers that it innervates undergo profound atrophy—termed denervation atrophy.

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The motor nerve fibers of each ventral root intermingle with those of neighboring roots to form plexuses, and although the muscles are innervated roughly according to segments of the spinal cord, each large muscle comes to be 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 pattern than paralysis following interruption of a peripheral nerve. These patterns follow the distribution shown ...

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