Nerves connect with muscles at the neuromuscular junction. There, the ends of nerve fibers connect to special sites on the muscle’s membrane called motor end plates. These plates contain receptors that enable the muscle to respond to acetylcholine, a chemical messenger (neurotransmitter) released by the nerve to transmit a nerve impulse across the neuromuscular junction. After a nerve stimulates a muscle at this junction, an electrical impulse flows through the muscle, causing it to contract. After transmitting the impulse, acetylcholine is broken down so that it does not continue to stimulate the muscle.
(See also Overview of the Peripheral Nervous System.)
Disorders in which the neuromuscular junction malfunctions include
In addition, many drugs can cause the neuromuscular junction to malfunction. These drugs include very high doses of some antibiotics, certain insecticides (organophosphates), chemical-warfare agents (such as sarin gas and Novichok), and curare. Novichok was developed in Russia and has been used in assassination attempts. Curare has been used to help relax muscles during surgery and to paralyze and kill when placed on the tip of poison darts. Some of these substances prevent the normal breakdown of acetylcholine after the nerve impulse has been transmitted to the muscle.
Neuromuscular junction disorders typically decrease nerve cell activity and cause muscle weakness. But they do not affect sensation (that is, they do not cause loss of sensation or abnormal sensations, such as tingling or a pins-and-needle sensation).
Some neuromuscular junction disorders decrease the nerve's activity, causing weakness. Other disorders, such as the following, increase the nerve's activity:
Stiff-person syndrome: Antibodies produced by the body attack nerve cells in the brain and spinal cord that regulate muscle movement. As a result, muscles are stimulated continuously, causing them to become stiff.
Isaacs syndrome: Nerves repeatedly send electrical impulses to muscles. As a result, muscles are continuously overstimulated. Muscles become stiff and twitch, making exercise and some normal physical activities difficult or impossible.
Although the changes that cause stiff-person syndrome originate mainly in the brain and spinal cord (the central nervous system), they also affect the neuromuscular junction. The result is continuous stimulation and contraction of muscles.