Muscles will contract or relax when they receive signals from the nervous system. The neuromuscular junction is the site of the signal exchange. The steps of this process in vertebrates occur as follows:
The action potential reaches the axon terminal. 
Voltage-dependent calcium gates open, allowing calcium to enter the axon terminal.
Neurotransmitter vesicles fuse with the presynaptic membrane and acetylcholine (ACh) is released into the synaptic cleft via exocytosis.
ACh binds to postsynaptic receptors on the sarcolemma.
This binding causes ion channels to open and allows sodium ions to flow across the membrane into the muscle cell.
The flow of sodium ions across the membrane into the muscle cell generates an action potential which travels to the myofibril and results in muscle contraction.
Labels:
A: Motor Neuron Axon
B: Axon Terminal
C. Synaptic Cleft
D. Muscle Cell

E. Part of a Myofibril
Own work
Elliejellybelly13