Neuromuscular Junction

Neuromuscular Junction

Somatic motor neurons are nerve cells that activate skeletal muscle fibers. They are found in the brain and spinal cord, but have long, thread-like extensions (axons) that are connected inside nerves to muscle cells they serve. Each axon ending forms an elliptical neuromuscular junction (end plate) with just one muscle fiber. Axon terminals and muscle fibers are separated by a space called the synaptic cleft. Inside each axon terminal are synaptic vesicles, which are membranous sacs that contain ACh. Junctional folds of each sarcolemma pro-vide large surface areas for the millions of nearby ACh receptors. Therefore, neuromuscular junctions include axon terminals, synaptic clefts, and junctional folds.

To understand more completely, the steps in which a motor neuron stimulates a skeletal muscle fiber, follow:

■■ A nerve impulse reaches the end of an axon, and the axon terminal releases ACh into the synaptic cleft. Calcium ions activate synaptic vesicles.

■■ ACh diffuses across the cleft, attaching to ACh receptors on the muscle fiber’s sarcolemma.

■■ Binding of ACh triggers electrical events that gen-erate an action potential.

The effects of ACh are quickly terminated by acetylcholinesterase, which breaks down ACh into its basic elements (acetic acid and choline). Therefore, continued and undesirable muscle fiber contraction, without additional nervous system stimulation, is prevented.