An electrical signal known as an action potential is sent down a motor neuron originating from the central nervous system. The action potential quickly reaches the synaptic knob within 1 millisecond.
The arrival of the action potential opens calcium channels in the synaptic knob.
Synaptic vesicles release the neurotransmitter acetylcholine across the synaptic cleft.
Acetylcholine binds to its receptors on the motor-end plate. This results in the opening of ligand-gated sodium (Na+) channels, so that sodium rushes into the muscle cell. The influx of positively charged sodium ions into the muscle fiber brings it to threshold (approximately +30mV).
At threshold, voltage-gated Na+ channels open and an action potential propagates down the t-tubules and towards the sarcoplasmic reticulum.
The sarcoplasmic reticulum releases a large amount of calcium that was stored within it.
Calcium binds to troponin, causing the troponin-tropomyosin complex to shift, exposing the myosin binding sites. This allows the contraction cycle to begin.
The contraction cycle immediately follows excitation. Its steps are as follows:
An energized myosin head binds to the myosin binding site.
A power stroke is initiated, the myosin head releases ADP and Pi.
A new ATP molecule binds the myosin head, detaching the myosin head from its binding site.
ATP is hydrolyzed. The energy release allows the myosin head to re-cock so the cycle may begin anew.