ne or two nerve branches and are contacted by single nerve terminals (5:122 123). Sensory nerve endings include those on muscle spindles (sensitive to length), in the Golgi tendon organs (sensitive to tension), and a variety of free nerve endings in the muscle tissue, some of which are involved in sensations of pain (5:122).
Motor innervation of muscle occurs either through gamma or alpha motoneurons (5:122). In mammals the gamma motoneurons provide a separate motor nerve supply for the muscle fibers of the muscle spindles (5:122). Alpha motoneurons, however, supply the bulk of the muscle fibers (5:122). Each alpha motoneuron innervates a number of fibers (5:122). While this number may vary, the complex of one motoneuron plus the fibers which it innervates is called a motor unit (5:122). The muscle fibers of a each motor unit all contract at the same time (5:122).
Within muscle fibers, the contractile proteins are organized into cylindrical or ribbon shaped myofibrils which are about 1 5 microns in diameter and very long (12:3). The
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