Myofilament Contraction

Myofilaments are the filaments of myofibrils constructed from proteins. The principal types of muscle are striated muscle, obliquely striated muscle and smooth muscle. Various arrangements of myofilaments create different muscles. Striated muscle has transverse bands of filaments. In obliquely striated muscle, the filaments are staggered. Smooth muscle has irregular arrangements of filaments.

Muscle fiber contraction
The axon terminal of a motor neuron releases the neurotransmitter, acetylcholine.
Acetylcholine diffuses across the synaptic cleft and binds to the muscle fiber membrane.
This depolarizes the muscle fiber membrane, and the impulse travels to the muscle's sarcoplasmic reticulum via the transverse tubules.
Calcium ions are then released from the sarcoplasmic reticulum into the sarcoplasm and subsequently bind to troponin.
Troponin and the associated tropomyosin undergo a conformational change after calcium binding and expose the myosin binding sites on actin, the thin filament.
The filaments of actin and myosin then form linkages.
After binding, myosin pulls actin filaments toward each other, or inward.
Thus muscle contraction occurs, and the sarcomere shortens as this process takes place.

Muscle fiber relaxation
The enzyme acetylcholinesterase breaks down acetylcholine and this ceases muscle fiber stimulation.
Active transport moves calcium ions back into the sarcoplasmic reticulum of the muscle fiber.
ATP causes the binding between actin and myosin filaments to break.
Troponin and tropomyosin revert to their original conformation and thereby block binding sites on the actin filament.
The muscle fiber relaxes and the entire sarcomere lengthens.
The muscle fiber is now prepared for the next contraction.