NEET 2021 Zoology Question 187 Solution

Muscle contraction occurs via the sliding of thin (actin) filaments over thick (myosin) filaments without any change in the physical length of the individual filaments themselves: - (d) ATP Hydrolysis: The globular myosin head acts as an ATPase enzyme, hydrolyzing ATP into $\text{ADP} + \text{P}_i$. This energizes the head into a cocked high-energy state that binds the exposed active sites on actin to form a cross-bridge. - Power Stroke & Z-Line Movement: Release of $\text{ADP}$ and $\text{P}_i$ triggers the power stroke, tilting the myosin head and pulling the attached actin filaments inward toward the centre of the sarcomere ($M$-line). Consequently, the Z-lines attached to actins are pulled inwards ((e) is True). - (c) I-Band Shortening: The isotropic (I) bands (composed of actin filaments only) narrow and reduce in width ((c) is True). - (a) Disappearance of H-Zone: The central H-zone (the region of thick myosin not overlapped by thin actin at rest) progressively narrows and disappears as opposing actin filaments overlap ((a) is True). - (b) Constancy of A-Band: The anisotropic (A) band corresponds to the entire physical length of the myosin filament. Because myosin filaments do not lengthen or expand, the A-band retains its constant length and NEVER widens ((b) is FALSE).

$$\begin{array}{l|l} \textbf{Sarcomere Metric} & \textbf{Status During Contraction} \\ \hline \text{Sarcomere Length (Z to Z)} & \text{Shortens} \\ \text{I-Band (Actin only)} & \text{Reduces in width (Shortens)} \\ \text{H-Zone (Myosin only)} & \text{Narrows and Disappears} \\ \mathbf{A\text{-Band (Total Myosin)}} & \mathbf{Retains\,Length\,(Constant,\,DOES\,NOT\,WIDEN)} \\ \end{array}$$ Therefore, the events that occur are (a), (c), (d), and (e) only.