[Major mechanisms involved in the synaptic transmission of the neuromuscular apparatus]

Neuromuscular transmission results from a double signal transduction from electric impulses to chemical messengers, taking place at a highly differentiated region, the neuromuscular junction (NMJ). A nerve cell responds to a specific stimulus by modifications of its plasmic membrane properties, generating an action potential (AP). This electric signal is transmitted along the axon to the NMJ, where it induces the voltage-gated calcium channels to open. Intracellular calcium entry leads to acetylcholine release in the synaptic space at the active zones but all scientists do not consider it the major release factor. Acetylcholine binding with its receptor at the muscle membrane generates an endplate potential when the induced depolarization is greater than the sodium voltage channel opening threshold. Muscle AP causes a muscle contraction involving the three phases. This paper will successively review the electrophysiological and molecular mechanisms involved at the pre-, inter- and postsynaptic levels. The last part of the article will discuss electromechanical considerations directly affecting the mechanical properties of the muscle fiber, most particularly the factors influencing the predetermined involvement of motor units, motor neuron electrical properties determining responses to synaptic inputs and finally the impact of recruited motor neurons and their electrical impulse rates on muscle contraction strength and velocity.