Editing Motor neuron (section)

=== Neuromuscular junctions ===
A single motor neuron may innervate many [[myocyte|muscle fibres]] and a muscle fibre can undergo many [[action potentials]] in the time taken for a single [[Muscle_contraction|muscle twitch]]. As a result, if an action potential arrives before a twitch has completed, the twitches can superimpose on one another, either through [[Muscle_contraction#Gradation_of_skeletal_muscle_contractions|summation]] or a [[tetanic contraction]]. In summation, the muscle is stimulated repetitively such that additional action potentials coming from the [[somatic nervous system]] arrive before the end of the twitch. The twitches thus superimpose on one another, leading to a force greater than that of a single twitch. A tetanic contraction is caused by constant, very high frequency stimulation - the action potentials come at such a rapid rate that individual twitches are indistinguishable, and tension rises smoothly eventually reaching a plateau.<ref name="Russell 2013 946"/>

The interface between a motor neuron and muscle fiber is a specialized [[synapse]] called the [[neuromuscular junction]]. Upon adequate stimulation, the motor neuron releases a flood of acetylcholine (Ach) [[neurotransmitter]]s from synaptic vesicles bound to the plasma membrane of the axon terminals. The acetylcholine molecules bind to [[postsynaptic]] [[receptor (biochemistry)|receptor]]s found within the motor end plate. Once two acetylcholine receptors have been bound, an ion channel is opened and sodium ions are allowed to flow into the cell. The influx of sodium into the cell causes depolarization and triggers a muscle action potential. T tubules of the sarcolemma are then stimulated to elicit calcium ion release from the sarcoplasmic reticulum. It is this chemical release that causes the target muscle fiber to contract.<ref name=":3" />

In [[invertebrates]], depending on the neurotransmitter released and the type of receptor it binds, the response in the muscle fiber could be either excitatory or inhibitory. For [[vertebrates]], however, the response of a muscle fiber to a neurotransmitter can only be excitatory, in other words, contractile.  Muscle relaxation and inhibition of muscle contraction in vertebrates is obtained only by inhibition of the motor neuron itself.  This is how [[muscle relaxants]] work by acting on the motor neurons that innervate muscles (by decreasing their [[electrophysiology|electrophysiological]] activity) or on [[acetylcholine|cholinergic]] neuromuscular junctions, rather than on the muscles themselves.