Figure 108

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. x620.

of end branches, each of which lies in a shallow depression on the surface of the muscle fiber, the receptor region (Fig. 10.9). The axon ending is a typical presynaptic structure and contains numerous mitochondria and synaptic vesicles that contain the neurotransmitter acetylcholine.

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 acetylcholine receptors are limited to the plasma membrane immediately bordering the cleft and at the top of the folds. The external lamina extends into the subneural folds (see Fig. 10.9). The synaptic vesicles of the axon terminal release acetylcholine into the cleft, which external lamina

Schwann cell cytoplasm axon external lamina muscle cell presynaptic vesicles synaptic cleft synaptic cleft

Schwann cell cytoplasm axon external lamina muscle cell presynaptic vesicles junctional folds of muscle cell a external lamina

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