How does muscle shorten during its contraction & return to its original during relaxation.

During muscle contraction, when a neuro transmitter reaches the motor end plate (the junction between the nerve ending and muscle), an action potential or an impulse is generated which travels from the sarcolemma (plasma membrane of muscle fibre) into the sarcoplasmic reticulum through T-tubules. Sarcoplasmic reticulum releases calcium which binds to the troponin component of actin thus changing it's shape and configuration. Since tropomyosin is another component of actin attached to troponin, there is a change in the shape and configuration of tropomyosin too. This leads to the exposure of the active sites on actin. The myosin cross heads then create cross bridges with myosin which leads to shortening of the sarcomere resulting in muscle contraction. For relaxation, calcium ions are transmitted back to the sarcoplasmic reticulum in the absence of which the configuration and the shape of troponin and tropomyosin come back to it's normal form thus blocking the active sites on actin for myosin. This breaks the cross links and the muscle relaxes