Muscle Contraction - Sliding Filament Theory

Myofibrils contain myofilaments:
Thick myofilaments - myosin
Thin myofilaments - actin
Myosin and actin filaments slide over one another to make the sarcomeres (short units of the myofibril) contract.

Muscle Contraction

1. Depolarisation of the sarcoplasmic reticulum releases Ca2+ into the sarcoplasm.
2. Ca2+ binds to troponin (changing its shape) pulling tropomyosin out of the actin-myosin binding site.
3. Myosin head binds to actin (actin-myosin cross bridge).
4. Ca2+ also activates ATPase (breaking down ATP) to move the myosin head which pulls the actin myofilament along.
5. ATP also required for the removal of the myosin head from the binding site.
6. Cycle repeats - progressively shortening the sarcomere.

Muscle Relaxation

1. Ca2+ ions are actively transported back to the sarcoplasmic reticulum.
2. Troponin molecules change back to their original shape - binding sites blocked by tropomyosin.
3. No actin-myosin cross bridges.
4. Actin filaments slide back to their relaxed position - lengthening the sarcomere.