Skeletal muscle fibers are richly innervated by motor neurons that originate in the spinal cord or brainstem. The axons of the neurons branch as they near the muscle, giving rise to twigs or terminal branches that end on individual muscle fibers (Fig. 11.9).
FIGURE 11.9 • Photomicrograph of neuromuscular junction.
This silver preparation shows a motor nerve and its final branches that lead to the neuromuscular junctions (motor end plates). The skeletal muscle fibers are oriented horizontally in the field and are crossed perpendicularly by the motor nerve fibers. Note that these fibers distally lose their myelin sheath and divide extensively into small swellings, forming a cluster of neuromuscular junctions. 620.
The neuromuscular junction is the contact made by the terminal branches of the axon with the muscle fiber.
At the neuromuscular junction (motor end plate), the myelin sheath of the axon ends, and the terminal portion of the axon is covered by only a thin portion of the neurilemmal (Schwann) cell with its external lamina. The end of the axon ramifies into a number of end branches, each of which lies in a shallow depression on the surface of the muscle fiber, the receptor region (Fig. 11.10). The axon ending is a typical presynaptic structure and contains numerous mitochondria and synaptic vesicles that contain the neurotransmitter acetylcholine (ACh).
Release of acetylcholine into the synaptic cleft initiates depolarization of the plasma membrane, which leads to muscle cell contraction.
The muscle fiber plasma membrane that underlies the synaptic cleft has many deep junctional folds (subneural folds). Specific cholinergic receptors for ACh are limited to the plasma membrane immediately bordering the cleft and at the  FOLDER 11.3
Functional Considerations: The Sliding
The sliding filament model postulates that the ratchetlike movements of the myosin heads bound to actin produce the movement of the thin filaments relative to the thick fil-aments, which in turn causes the sarcomere to shorten. Although the sliding filament model can explain contrac-tion in a single sarcomere, it cannot adequately explain the shortening of a myofibril of a muscle fiber. Obviously, if the activity just described were to occur simultaneously in adjacent sarcomeres, no contraction could occur. Equal and opposite forces would be exerted on either side of the Z line, and the contraction of any given sar-comere would be prevented by the contraction of its two immediate serial neighbors. Recent studies with ultrahigh-speed photography have demonstrated that an extremely small temporal delay occurs between the contraction of adjacent sarcomeres, so that a wavelike contraction actu-ally occurs in each muscle fibril and, consequently, in each muscle fiber.