General mechanism of muscle contraction
Acetylcholine acts on a localized area of the muscle fibre membrane to open cation channels with “acetylcholine gates” through protein molecules suspended in the membrane.
The initiation and execution of muscle contraction occur in the following sequential steps.
1. An action potential travels along a motor nerve to its termination on the muscle fibre.
2. At each nerve ending, a small amount of the neurotransmitter acetylcholine is secreted.
3. Acetylcholine acts on a localized area of the muscle fibre membrane to open cation channels with “acetylcholine gates” through protein molecules suspended in the membrane.
4. The opening of the acetylcholine-gated channels allows large amounts of incoming sodium ions to diffuse into the interior of the muscle fibre membrane. This action causes a local depolarization that will lead to the opening of voltage-gated sodium channels, initiating an action potential at the membrane.
5. Action potentials travel along muscle fibre membranes in the same way that action potentials travel along nerve fibre membranes.
6. The action potential depolarizes the muscle cell membrane, and many actions potentials flow through the centre of the muscle fibre. Here it causes the sarcoplasmic reticulum to release large amounts of calcium ions that have been stored in this reticulum.
7. Calcium ions initiate an attractive force between the actin and myosin filaments, causing them to slide along each other, which is contraction.
8. After a fraction of a second, calcium ions are pumped back into the sarcoplasmic reticulum by a membrane Ca++ pump and stored in the grid until a new muscle action potential appears; Removal of calcium ions from myofibrils causes termination of muscle contraction.