To activate a muscle, the brain sends an impulse down a nerve. The nerve impulse travels down the nerve cells to the neuromuscular junction, where a nerve cell meets a muscle cell. The impulse is transferred to the nerve cell and travels down specialized canals in the sarcolemma to reach the transverse tubules. The energy in the transverse tubules causes the SR to release of the Ca2+
it has built up, flooding the cytoplasm with calcium. The Ca2+
has a special effect on the proteins associated with actin. Troponin, when not in the presence of Ca2+, will bind to tropomyosin and cause it to cover the myosin-binding sites on the actin filament. This means that without Ca2+ the muscle cell will be relaxed. When Ca2+ is introduced into the cytosol, troponin will release tropomyosin and tropomyosin will slide out of the way. This allows the myosin heads to attach to the actin filament. Once this happens, myosin can used the energy gained from ATP to crawl along the actin filament. When many sarcomeres are doing this at the same time, the entire muscle contract.
While only a small percentage of the heads are attached at any one time, the many heads and continual use of ATP ensures a smooth contraction. The myosin crawls until it reaches the Z plate, and full contraction has been obtained. The SR is continually removing Ca2+ from the cytoplasm, and once the concentration falls below a certain level troponin rebinds to tropomyosin, and the muscle releases.
While the above model is a generalized version of what happens in skeletal muscle, similar processes control the contractions of both cardiac and smooth muscle. In cardiac muscle, the impulses are in part controlled by pacemaker cells which releases impulses regularly. Smooth muscle is different from skeletal muscle in that the actin and myosin filament are not organized in convenient bundles. While they are organized differently, smooth muscle still operates on the functioning of myosin and actin. Smooth muscle can obtain a signal to contract from many sources, including the nervous system and environmental cues the cells receive from other parts of the body.