 Dear students, in this module, we shall summarize the whole mechanism of muscle contraction. We shall start with a relaxed muscle fiber and discuss one by one the sequence of events that occur in it and lead to contraction and then relaxation. Dear students, the whole process starts when an action potential arrives at the neuromuscular junction and causes the depolarization of the surface membrane. Then the action potential is conducted deep into the muscle fiber along the t-tubules. The depolarization of t-tubules causes voltage-sensitive dihydropyridine receptors in these t-tubules to undergo a conformational change. These receptors make mechanical links to the rhinodine receptors in the membrane of sarcoplasmic reticulum and cause the opening of these calcium channels. Now the opening of rhinodine receptors causes the calcium ions to flow out of the sarcoplasmic reticulum. As a result, the concentration of free calcium ions increases in the cytosol. This increases from a level of minus 7 molar and increases to minus 6 molar. Now the calcium ions bind to troponin and induce conformational change in it. The change in conformation of troponin causes a change in the position of tropomyosine. As a result, the myosine binding sites on the actin filament are exposed. The exposure of actin binding sites on actin filament causes the cross bridges to form. And when these cross bridges form, myosine heads rotate and produce the force that pulls the thin filaments. The pulling of thin filaments causes the sarcomere to shorten. Now in the next step, ATP binds to the myosine head causes the myosine head to detach from the thin filament. After this, ATP hydrolyzes. And energy produced causes a conformational change in the myosine which using this energy reattaches to the next site present on the actin filament. This cycle of binding and rebinding is repeated several times until the contraction and shortening is complete. When the contraction is complete, the calcium pumps in the sarcoplasmic reticular membranes are activated and they start to actively reabsorb calcium ions from the myoplasm into the sarcoplasmic reticular. That is, now calcium sequestering activity starts. The result is that the concentration of free calcium ions in the myoplasm starts to decrease. When this concentration decreases, the calcium ions which were binded with troponin are now unbound from the troponin. When the calcium ions are removed from the troponin molecule, the troponin comes back to its relaxed position. As a result, tropomyosine again covers the myosine binding sites on the actin filament and inhibits the cross bridge attachment in this way. When the cross bridges are not formed, the muscle is now in relaxed position. Neuronal remains relaxed until the next neuronal input or nerve impulse arrives.