During muscular contraction which of the following events occur?

(a) 'H' zone disappears

(b) 'A' band widens

(c) 'I' band reduces in width

(d) Myosine 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.

The correct answer is (a), (c), (d), (e) only.

To understand the events that occur during muscle contraction, we can refer to the Sliding Filament Theory. According to this theory, muscle contraction occurs when the thin (actin) filaments slide over the thick (myosin) filaments, leading to the shortening of the sarcomere (the functional unit of a muscle fiber). Let us evaluate each event step-by-step:

(a) 'H' zone disappears:
The 'H' zone is the central region of the 'A' band which contains only thick filaments (myosin). During contraction, thin filaments from opposite sides slide inwards towards the center of the sarcomere, overlapping the thick filaments. This causes the 'H' zone to reduce in width and eventually disappear during maximal contraction. Therefore, statement (a) is correct.

(b) 'A' band widens:
The 'A' band corresponds to the full length of the thick filaments. During muscle contraction, the actual lengths of the thick and thin filaments do not change; only their relative positions slide past each other. Consequently, the width of the 'A' band remains constant. Therefore, statement (b) is incorrect.

(c) 'I' band reduces in width:
The 'I' band is the region of the sarcomere containing only thin (actin) filaments. As these thin filaments slide inwards towards the H-zone, the region of non-overlap decreases. This leads to a reduction in the width of the 'I' band. Therefore, statement (c) is correct.

(d) Myosine hydrolyzes ATP, releasing the ADP and Pi:
The myosin head functions as an ATPase. It hydrolyzes ATP into ADP and inorganic phosphate (Pi) to enter an energized state that can bind to the active sites on actin filaments to form a cross-bridge. The release of ADP and Pi triggers the "power stroke," which pulls the actin filaments. Therefore, statement (d) is correct.

(e) Z-lines attached to actins are pulled inwards:
Thin filaments are anchored to the Z-lines. When the thin filaments slide towards the center of the sarcomere, they pull the Z-lines inwards, resulting in the shortening of the sarcomere. Therefore, statement (e) is correct.

Since events (a), (c), (d), and (e) occur during contraction, the correct choice is (a), (c), (d), (e) only.