Question

During muscular contraction which of the following events occur?

(a) 'H' zone disappears
(b) 'A' band widens
(c) 'I' band reduces in width
(d) Myosin hydrolyzes ATP, releasing the ADP and Pi
(e) Z-lines attached to actins are pulled inwards

Choose the correct answer from the options given below.

• A. (b), (d), (e), (a) only
• B. (a), (c), (d), (e) only
• C. (a), (b), (c), (d) only
• D. (b), (c), (d), (e) only

Answer 1

The correct option is B (a), (c), (d), (e) only

Sarcomere is the structural and functional unit of muscle. The shortening of the sarcomere results in muscle contraction.

Sliding filament theory was proposed to explain the mechanism of muscle contraction.

It states that thin actin filaments slide over the thick myosin filaments resulting in the shortening of sarcomere.

The source of energy for the muscle contraction is ATP. The myosin heads bind and hydrolyse ATP releasing ADP and Pi.

During muscle contraction, the I band reduces in width as the overlap between the thick filaments and the thin filaments increases.

Fig: Mechanism of muscle contraction

The H zone also decreases in length because of the same reason. In a fully contracted muscle, the H zone disappears.

As the actin filaments come closer to the M-line, the Z lines attached to actin filaments come closer together, so the
sarcomere also decreases in length.

The A band is the only part of the sarcomere that does not change in length during muscle contraction.