A neuromuscular junction is a place in the body where the axons of motor nerves meet the muscle, allowing them to transmit messages from the brain that cause the muscle to contract and relax. Every organism has thousands of these junctions that control the movements of the body and cause the heart to beat. It is only one example of many connections made between nerves and other parts of the body that result in a successfully functioning organism.
Neurons, or nerve cells, are specially designed cells that communicate using chemicals called neurotransmitters. Depending on the type of cell, specific neurotransmitters are designed to stimulate a response, assuming that a receptor is present. In addition to a body, neurons also have a long tail called an axon that terminates in dendrites, a bundle of fibers that can transmit chemicals to the next neuron. In the instance of a neuromuscular junction, one neuron can control many muscle cells, but each muscle cell only responds to one neuron.
At this junction, the motor neuron meets muscle tissue at an axion terminal. The surface of the muscle fiber forms small ridged folds for the end of axon to rest in. Inside these folds are depressions with acetylcholine receptors. The neuron forms synaptic vesicles that are filled with acetylcholine. They resemble small bulbs that will release the neurotransmitter when the muscle needs to contract.
The neuromuscular junction is crucial for life, and they begin forming early in fetal development. As motor neurons develop, a protein called agrin is made. This stimulates the formation of a muscle specific kinase, which will build receptors for acetylcholine on the surface of the muscle fiber. This is how the junction is formed, with the neuron itself emitting the needed chemical for development.
Several conditions can cause malfunctions at these sites, leading to loss of muscle control. Ultimately, lack of muscle control can cause death if it is severe, because the body's largest muscle, the heart, could stop beating. The most common such illness is myasthenia gravis, which is caused by an autoimmune reaction against acetylcholine receptors. As a result, motor neurons cannot transmit the chemical to the muscles, which will cause muscle weakness and an ultimate loss of control.
Myasthenia gravis tends to be more concentrated along skeletal muscles and is especially noticeable in the face. Additionally, some toxins, such as botulinum, will inhibit the release of acetylcholine, which will lead to muscle paralysis.