Question

Which statement is correct for muscle contraction?
(a)Length of H-zone decreases
(b)Length of A- band decreases
(c)Length of two Z-line increases
(d)Length of A-band remains constant

Answer 1

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.
So, the correct answer is ‘Length of A-band remains constant’.

Note: In vertebrates, striated muscle contractions are neurogenic as they require synaptic input from motor neurons to supply muscle contractions. Muscles appear in red color thanks to a pigment called myoglobin. Myoglobin rich muscles are called red fibers. They also include many mitochondria, which they could utilize for energy generation. Muscles that do not contain myoglobin and white in color are known as white fibers. Muscle fatigue occurs because of repeated activation of the muscles resulting in the accumulation of lactic acid.