How does muscle contraction and relaxation occur?
Muscle contraction occurs by shortening of myofilaments within muscle fibers. Stimulation causes an action potential to be transmitted along the sarcolemma, then through the T-tubule system to the sarcoplasmic reticulum. This causes release of calcium into the sarcoplasm. As the calcium concentration increases, actin is released from a state of inhibition, allowing actin-myosin cross linkage and shortening of the myofilaments. The muscle fiber shortens until calcium is actively pumped back into the sarcoplasmic reticulum, which breaks the crosslinks causing the fiber to relax. ATP, electrolytes (Na, K, Ca, Mg), and three ATPase proteins contribute to normal fiber contraction and relaxation (see Chapter 72 : Metabolic and Other Genetic Mypathies).