A neuromuscular junction is the junction of the axon terminal of a motoneuron with the motor end plate, the highly-excitable region of muscle fiber plasma membrane responsible for initiation of action potentials across the muscle's surface.
Mechanism of Action
Upon the arrival of an action potential at the axon terminal, calcium ions flow from the extracellular fluid into the motorneuron's cytosol and bind to particular intracellular proteins. These calcium-bound proteins cause neurotransmitter-containing vesicles to attach to the motoneuron's cell membrane and thus release acetylcholine into the synaptic cleft. Acetylcholine then binds to the nicotinic acetycholine receptors that dot the motor end plate. The receptors are ion channels, and when bound by acetylcholine, they open, allowing sodium and potassium ions to flow in and out of the muscle's cytosol, respectively. Because of the differences in electrochemical gradients across the plasma membrane, more sodium moves in than potassium out, producing a local depolarization of the motor end plate known as an end plate potential (EPP). This depolarization spreads across the surface of the muscle fiber into transverse tubules, eliciting the release of calcium from the sarcoplasmic reticulum, thus initiating muscle contraction.