control of contractions

• 1. when the muscle is stimulated the action potential passes along the sarcolemma and then down the transverse tubules into the muscle fibre.
• 2. the action potential is carried to the sarcoplasmic reticulum which stores calcium ions, and this causes the release of calcium ions into the sarcoplasm
• 3. the calcium ions bind to the troponin, this alters the shape and pulls the tropomyosin aside. this exposes the binding sites on the actin.
• 4. myosin heads bind to the actin, forming cross bridges between the filaments
• 5. the myosin heads move, pulling the actin filament past the myosin filament
• 6. the myosin heads detach from the actin and can bind again further up the actin filament.
• Millions of cross bridges can be formed between the actin and myosin filaments. once contraction has occurred, the calcium ions are rapidly pumped back into the sarcoplasmic reticulum, allowing the muscle to relax.