Each time a part of the body—no matter how small—moves, it is the result of a contraction of a muscle. Such a contraction is caused by binding of the neurotransmitter acetylcholine (ACh) to receptors on the muscle fibers. The ACh causes an action potential in the muscle fiber, admitting sodium and calcium ions and causing the actin filaments to slide across the myosin filaments, thus shortening the muscle. The neurons that deliver ACh to the muscle are called motor neurons, and the point at which a motor neuron and a muscle fiber meet is called a neuromuscular junction.

The axon of each motor neuron divides so that one neuron may innervate a large number of muscle fibers. The more fibers innervated, the stronger the contraction. Although the basic mechanism appears to be simple, there are many levels of...