Neuromuscular Junction

The transfer of information from the axon potentials of motor nerves to the muscle occurs at a specialized synapse, the neuromuscular junction. After entering muscle, the axon of a motor neuron divides into many thin branches or twigs. Each of these twigs makes contact with only one myofiber, approximately midway between its ends. At this

Figure 15-2 The transverse tubule system, showing the relationship of the sarcoplasmic reticulum and transverse tubular system to the myofilfFrom Bloom W, and Fawcett DW: A Textbook of Histology. Philadelphia, W. B. Saunders, 1970.)

motor endplate, the axonal twig divides further and forms a complex of approximately 50 boutons, each making synaptic contact with the muscle cell. The lower motor neuron uses acetylcholine as its chemical transmitter. When an action potential from the lower motor neuron reaches the neuromuscular junction, normally enough acetylcholine is released to produce a suprathreshold excitatory endplate potential, which produces a propagated muscle action potential and a twitch in the fiber innervated by that alpha motor neuron. This involves two major events, presynaptic release of the acetylcholine and interaction with the postsynaptic receptor.

In the presynaptic terminal, acetylcholine is synthesized from acetyl CoA and choline and is catalyzed by choline acetyltransferase. The acetylcholine is packaged in synaptic vesicles and is released into the synaptic cleft after a nerve action potential arrives by a process that requires calcium and is called stimulus-secretion coupling. The entry of calcium in the presynaptic terminal through voltage-gated channels triggers the fusion of the synaptic vesicles with the presynaptic cell membrane.^ This fusion of the vesicular and plasma membrane opens the vesicle to the synaptic cleft, releasing its contents in a process known as exocytosis. Synapsin I, a phosphoprotein localized in vesicles, may mediate translocation of vesicles to the membrane. The subsequent steps have not yet been deciphered, but after acetylcholine has been released, the presynaptic membrane is pinocytotically recaptured, and the vesicles are remade and repleted with acetylcholine. y The acetylcholine binds to a receptor on the postsynaptic membrane. If sufficient acetylcholine is released, the postsynaptic membrane is depolarized, resulting in the endplate potential. The probability that an interaction will occur between the released acetylcholine and its receptor on the postsynaptic membrane is increased by concentration of the cholinergic receptors along postjunctional folds at the neuromuscular junction. The cholinergic receptor is of the nicotinic type. The action of acetylcholine is terminated by acetylcholine esterase, which catabolizes it into its two constituents, and the choline is recaptured for reuse by low-affinity transport in the nerve terminal.

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