Read through all these sequential steps of muscle contraction and mention the no. of step which is not in sequence. You have to pick the error .
1.The nerve signal arrives at axon terminal and leads to opening of Calcium voltage gates. Calcium enters the terminal and leads to exocytosis of acetylcholine stored in vesicles inside the axon terminal.
2. The released acetylcholine moves through the cleft and binds with the receptors on the motor end plate. The receptors in response to this binding open sodium and potassium gates. The sodium gates are fast to open and potassium open with slight delay.
3. Opening of sodium gates leads to depolarization of motor end plate and if these stimuli are strong enough lead to generation of action potentials, which means now the muscle is activated to respond by contraction.
4.These action potentials travel along the sarcolemma and where it finds a T-tubule the message is delivered deep inside the muscle cell. On both sides of T-tubules are terminal cisterns, storing calcium and forming a triad.
5. In response to this message, calcium is released from the cisterns and moves in the sarcoplasm towards the myofilaments. Calcium binds with the troponin and troponin then moves the tropomyosin (both troponin and tropomyosin are regulatory proteins.) Movements of tropomyosin leads to exposure of active sites on actin filaments.
6. Both actin and myosin are contractile proteins. With the active site on Actin been exposed the myosin head which is always charged and in cocked position waiting for opportunity, forms crossbridge with the active site on Actin. The crossbridge that are formed generate the power stroke that make the filaments slide past on each other. The actin filament been attached to z discs, pulls the Z disk toward the midline. This leads to shortening of sarcomere and this shortening leads to muscle contraction.
7. ATP is now required to pull the myosin head out so it can move further and form crossbridge with another actin molecule, so muscle contraction is series of these crossbridge being formed and it operates like step ladder pattern or like gliding movements of earthworms in which series of repetitions provide movement.
8. Now for muscle relaxation to occur, first the nerve signals stop to arrive, so no more acetylcholine is released. The acetylcholine already in the cleft is degraded by acetylcholinesterase present in the cleft so it is no more capable of stimulation of muscle cell. Secondly, the Calcium which was released from the cisterns is returned from the sarcoplasm back to cisterns by passive transport requiring ATP. With no more calcium available in the sarcoplasm, troponin moves the tropomyosin and the active sites on the actin filaments are blocked again. Cross bridges cannot be formed now.
