The various events which happen during a brief contraction, or single twitch of a vertebrate striated muscle fibre, are as follows:
1. As soon as muscle fibre or muscle is stimulated, immediately electrical events in the form of action potentials appear over the surface of muscle fibre.
2. These electrical events travel over the sarcolemma and down the numerous transverse tubules to the interior of the muscle.
3. As a result calcium ions are released from the T-sarcotubular system or cisternae of the sarcoplasmic reticulum.
4. Free liberated calcium ions bring about the sliding of the actin filaments in spaces between the myosin filaments, with the result contraction is effected in the muscle.
This process requires energy which is derived from the breakdown of the ATP to ADP+ inorganic phosphate.
The breakdown of ATP takes place in the presence of ATPase enzyme and calcium ions. During this process some heat is also produced.
5. The contraction results from the sliding of the thin actin filaments in spaces between the thick myosin filaments, so that adjacent Z-lines come closer, light band disappears, there is no change in dark or A band but H zone disappears.
The development of tension and the sliding is the result of the action of cross-bridges.
6. The ADP formed is converted back to ATP by creatine phosphate, and probably by other metabolic processes as well.
7. After contraction the sarcoplasmic reticulum begins to reabsorb the calcium ions which were released, with the result ATPase activiy and contraction cease.
8. When contraction ceases due to reabsorption of calcium ions by the sarcoplasmic reticulum, the myofibrils are stretched out again through the action of antagonistic muscles. ATP must be present.
9. Creatine formed during the contraction is rephosphorylated in the presence of ATP, some heat is produced.
10. ADP formed during the rephosphorylation of creatine into creatine phosphate, is converted back into ATP by oxidative phosphorylation in the mitochondria. During this conversion more heat is produced.
11. Any lactic acid generated during contraction is reoxidized to pyruvic acid, and the reduced nucleotide formed, is ultimately oxidized by the mitochondria to produce more ATP.