Paralysis refers to loss of voluntary movement as a result of interruption of the motor pathways at any point from the cerebrum to the muscle fiber. The term plegia refers to severe weakness, coming from a Greek word meaning “to strike,” and the word palsy stems from a French word that has the same meaning as paralysis. A lesser degree of weakness is described as paresis.
Anatomic and Physiologic Considerations
Each motor neuron, through 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 motor neuron, its axon, and the muscle fibers they innervate 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. 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 a ventral spinal root, which joins a dorsal sensory root to form the spinal nerves that intermingle to form plexuses and then form the peripheral nerves. A myotome refers to the group of muscles that receive innervation from a single spinal nerve root, though most large muscles are usually supplied by two or more roots that have contributed to a given peripheral nerve. In contrast, a single peripheral nerve usually provides the complete motor innervation of a muscle. For this reason, paralysis caused by disease of the anterior horn cells or ventral roots has a different topographic pattern than paralysis following the interruption of a peripheral nerve (see Table 43–1). For example, a lesion of the L5 root causes weakness of foot dorsiflexion and inversion, whereas a lesion of the peroneal nerve causes weakness of foot dorsiflexion but spares foot inversion, since the invertors of the foot are supplied by L5 but via the tibial nerve.
All motor activity, even the most elementary reflex, 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 the abductor pollicis brevis. In the terminology of Beevor, these muscles act as agonists, or prime movers. For flexion to be smooth and forceful, the extensor muscles (antagonists) must relax at the same rate as the flexors contract (reciprocal innervation...