Answer
When the muscle is relaxed, the amount of calcium in sarcoplasm is very low, but the sarcoplasmic reticulum (SR) has a great store of calcium. The action potential (AP) opens calcium release channels in the SR, and Ca++ ions flow into the sarcoplasm around the thick and thin filaments. The troponin molecules on the thin filaments bind the Ca++ ions and this affects the conformation of the whole troponin-tropomyosin complex. This change in troponin-tropomyosin shape causes an uncovering of the myosin binding sites on the actin; this facilitates the formation of actin-myosin cross-bridges which results in filament sliding and muscle contraction.
A myosin head has an ATPase as well as an ATP binding site. After the myosin head binds an ATP, the enzyme hydrolyses it to give ADP and Pi. ; this reaction energizes the myosin and prepares its head its for attachment to actin-- the products of ATP hydrolysis meanwhile remain temporarily connected with the myosin head.
When the myosin contacts and binds to a site on an actin molecule this releases the Pi entity. After the cross bridge is formed and ADP is released. In the power stroke the cross-bridge rotates
towards the center of the sarcomere and the thin filaments slide past the thick filaments towards the M line.
The actin and the myosin molecule remain bound after the power stroke, but when the cross bridge encounters a new ATP molecule and the myosin head binds it the actin molecule is released from the cross bridge.
Work Step by Step
After the myosin head detaches from the binding site on the actin filament, the whole contraction cycle may be repeated.
