An action potential is propagated along the somatic motor neuron to the presynaptic axon terminal.
↓
This triggers the release of acetylcholine into the synaptic cleft.
↓
Acetylcholine binds to acetylcholine receptors (present on the motor end plate) which causes an excitatory
postsynaptic potential that spreads along the sarcolemma to the transverse tubules.
↓
This triggers calcium release channels in the terminal cisternae of the sarcoplasmic reticulum to open, and calcium
is released into the sarcoplasm.
↓
Calcium binds to the regulatory protein troponin on the actin filaments which undergoes a conformational shape
change, pulling another regulatory protein, tropomyosin, with it.
↓
The movement of tropomyosin uncovers the myosin binding sites of the actin filaments.
↓
ATP is hydrolyzed on the head of the myosin filament which then moves to bind to the actin filament, forming a
crossbridge.
↓
The myosin head pivots and moves the actin filament towards the center of the sarcomere.
↓
The myosin detaches and binds to a new ATP molecule, ready for another contraction.
↓
If stimulation stops, calcium channels in the terminal cisternae close, preventing further calcium release.
↓
Calcium is restored to the reservoirs in the sarcoplasmic reticulum by active transport pumps in its membrane.
↓
Troponin and tropomyosin return to their original positions, preventing the binding of myosin.
↓
The muscle relaxes and recoils.