Hint: Muscle contraction is the activation of tension-producing areas inside muscle fibers. Muscular contraction doesn't necessarily mean muscle shortening because muscle tension is often produced without changes in muscle length, like when holding an important book or a dumbbell in the same position.
Complete answer:
While contraction, the skinny filaments drop above the thick filaments. A signal sent by the central nervous system via motor nerve fibre initiates muscular contraction. When Calcium ions combine with troponin then in consequence muscle contraction will initiate. While contraction, the Z-lines come closer together and the sarcomere becomes smaller. The length of the A-band remains constant. I-bands shorten and H-zone narrows. The termination of muscle contraction is obeyed by muscle relaxation, which is a restoration of the muscle fibers to their low tension-producing state.
Additional Information: Muscle contraction takes place in the following steps.
-Nerve impulse goes through a motor nerve to its end sites on muscle fibers.
-A little amount of acetylcholine (neurotransmitter) is secret on the fiber from the nerve end.
-The membranes of muscle fibers open up. The action of acetylcholine.
-The sodium ions go within the membrane by this opening.
-The sodium channels also get opened.
-This initiated action potential inside the membrane.
-The action potential runs along with the muscle fiber.
-Due to the present action potential the sarcoplasm releases the calcium ion
-The calcium ions end in attractive forces between actin and myosin filaments of muscles that cause contractions.
