5) muscle fiber is stimulated at the neuromuscular junction triggering an action potential
7) APs travel down the T-tubules to the sarcoplasmic reticulum
Released acetylcholine (due to action potential within motor neuron) from the motor neuron initiates depolarisation within the sarcolemma, which is spread through the muscle fibre via T tubules.
8) in response to an AP, calcium ions are released into the cytosol/space around the myofibrils
Depolarisation causes the sarcoplasmic reticulum to release calcium ions necessary for the contraction
3) calcium ions bind to the T and T system
1) T and T system undergoes a conformational change, exposing the myosin binding sites on actin
6) myosin head/cross-bridges bind to actin
The binding sites for the myosin heads located on actin, are covered by a blocking complex (troponin and tropomyosin) that can be unblocked by the calcium binding
2) myosin heads/cross bridges bind to actin and flex, shortening the sacromere
4) actin filaments are pulled toward the center of the sacromere, shortening the muscle fiber
As the individual sarcomeres become shorten the muscle fibres as a whole contracts
10) myosin head continues to attach, flex, release, extend, and reattach as long as calcium ions are present
9) calcium ions actively transported into the sarcoplasmic reticulum
Relaxation of a muscle fiber occurs when a Ca ions are pumped back into the sarcoplasmic reticulum .
