Introduction The contractile unit of the cardiac muscle fiber is the sarcomere which contains actin, myosin, troponin and tropomyosin. Also known as the thin and thick filaments respectively, actin and myosin interact to enable the shortening and lengthening of the sarcomere. Troponin and tropomyosin are regulator proteins that allow the sarcomere to shorten in the presence of calcium ions (Ca2+) and lengthen in the absence of calcium ions . The interaction of the thin and thick filaments can occur through the sliding filament theory or the cross bridge theory. The contraction and relaxation of the cardiac fibers is a complex process involving influx and outflow of Ca2+ in the myocyte. Contraction and Relaxation of Cardiac Fibers A cell membrane …show more content…
The influx of Ca2+ leads to further increase of Ca2+ from the sarcoplasmic reticulum through calcium release channels. The T tubules have voltage gated calcium channels that undergo conformational change when the T tubules conduct action potential. This causes calcium release channels to open releasing Ca2+ into the cytoplasm. The free intracellular Ca2+ bind to troponin causing a conformational change in the troponin/tropomyosin complex. The change leads to actin interacting with myosin and the sarcomere shortens resulting in contraction …show more content…
The myofibrils shorten but both the thin and thick filaments remain the same during cardiac muscle contraction. The filaments slide past each other through asynchronous lashes of myosin cross bridges that pull the thin filaments over the thick filaments. In the cross bridges theory, the thin and thick filaments slide past each other through various cross bridges that extend from the thick filaments to the thin filaments. The cross bridges are part of myosin proteins that form globular heads where actin and ATP can bind