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 (including very high doses of some antibiotics), certain insecticides (organophosphates), curare (an extract from plants formerly placed on the tip of some poison darts and used to paralyze and kill), and the nerve gases used in chemical warfare can cause the neuromuscular junction to malfunction. Some of these substances prevent the normal breakdown of acetylcholine after the nerve impulse has been transmitted to the muscle.
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 stiff-person syndrome affects mainly the brain and spinal cord (the central nervous system), it can also affect the neuromuscular junction in other parts of the body.