Contraction of myofibrils in skeletal muscles occurs between :

Question

Accepted Answer

The correct option is 1: Z' lines

The core functional unit of contraction in skeletal muscle is the **sarcomere**, and a sarcomere is precisely defined as the region between two successive **Z-lines** (or Z-discs). According to the universally accepted Sliding Filament Theory, muscle contraction does not involve the shortening of the myofilaments themselves (actin and myosin); instead, it occurs through the thin actin filaments sliding inward along the stationary thick myosin filaments towards the center of the sarcomere (the M-line). Since the actin filaments are directly anchored to the Z-lines, the pulling action of the myosin heads on the actin filaments draws the Z-lines closer together. Therefore, the overall shortening or contraction of the myofibril—and thus the entire muscle fiber—is fundamentally the reduction in the distance *between the two adjacent Z-lines*. This distance is the measure of one sarcomere, making the Z-lines the essential boundaries within which the contractile action takes place.

The incorrect option is 2: T' bands

The term 'T' bands' is not a standard anatomical reference for the bands that define the sarcomere structure. Although there are 'I-bands' and 'A-bands,' the closest structural component that might be confused with this term is the **Transverse tubules (T-tubules)**. T-tubules are internal invaginations of the muscle cell membrane (sarcolemma) that extend deep into the muscle fiber. Their physiological role is to conduct the action potential from the cell surface to the muscle's interior, specifically to the sarcoplasmic reticulum, triggering the release of calcium ions. Since T-tubules are part of the excitation-contraction *coupling* system and not a primary component of the contractile protein organization (like Z-lines or A-bands), they do not define the boundaries of the contracting unit.

The incorrect option is 3: A' bands

The **A-band** represents the length of the thick (myosin) filaments and the region where the thick and thin (actin) filaments overlap. A crucial principle of the Sliding Filament Theory is that the length of the thick myosin filaments, and consequently the length of the A-band, **does not change** during muscle contraction or relaxation. The A-band merely shifts closer to the adjacent A-bands as the sarcomere shortens. Thus, the A-band is an internal feature of the sarcomere that maintains a constant length, rather than representing the overall distance *between* which the shortening occurs. If contraction were to occur *between* the A-bands, it would imply the thick filaments themselves are shrinking, which is factually incorrect.

The incorrect option is 4: H' zones

The **H-zone** is the central, lighter region within the A-band that, in a relaxed muscle, contains only thick (myosin) filaments and is not overlapped by thin (actin) filaments. While it is true that the H-zone **shortens** (narrows or even disappears completely) during maximal muscle contraction, this is a *result* of the inward sliding of the thin filaments. However, the H-zone is not the boundary *between* which the fundamental contractile unit (the sarcomere) is measured. The functional shortening of the entire myofibril unit is defined by the distance the Z-lines are pulled, making the Z-line distance the definitive measurement of contraction.