Q.Explain in detail the mechanism of muscle contraction according to the Sliding Filament Theory.

The Sliding Filament Theory, proposed independently by Hugh Huxley and Jean Hanson, explains that a muscle shortens because the thin actin filaments are actively pulled past the stationary thick myosin filaments — neither filament shortens itself, but their increasing overlap shortens the sarcomere as a whole. This is visible directly in the sarcomere's bands: as contraction proceeds, the I-band and the H-zone both narrow, while the A-band's length (fixed by the length of the thick filaments) stays the same.

The mechanism unfolds in five linked stages.

1. Excitation. A nerve impulse reaching the neuromuscular junction releases acetylcholine, generating an action potential across the sarcolemma. This electrical signal is carried rapidly inward along the T-tubules of the triad system, reaching the sarcoplasmic reticulum deep within the fibre.

2. Cross-bridge formation. The arriving signal triggers the sarcoplasmic reticulum to release its stored Ca2+ into the sarcoplasm. These calcium ions bind to troponin C, causing the troponin-tropomyosin complex on the thin filament to shift and uncover the myosin-binding sites on actin that were previously masked. The myosin head, already primed with energy from an earlier round of ATP hydrolysis (and still holding ADP and inorganic phosphate), binds to this newly exposed site, forming a cross-bridge between the two filaments.

3. Power stroke. The myosin head now releases the bound ADP and phosphate and pivots at its neck region, dragging the attached actin filament a short distance toward the centre of the sarcomere. This bending motion is the actual force-generating step of the whole cycle, and it is what pulls the Z-lines at either end of the sarcomere closer together.

4. Recovery stroke. A fresh molecule of ATP now binds to the myosin head. This binding itself is what causes the head to release its grip on actin. The myosin head then acts as an ATPase, hydrolysing this ATP to ADP and phosphate, and the energy released re-cocks the head back into its original, high-energy orientation — ready to attach to a new binding site further along the actin filament and repeat the whole cycle.

5. Relaxation. When nervous stimulation of the fibre stops, Ca2+ is actively pumped back into the sarcoplasmic reticulum, lowering its concentration in the sarcoplasm. With Ca2+ no longer bound to troponin C, the troponin-tropomyosin complex returns to its resting position, re-masking the myosin-binding sites on actin. No new cross-bridges can form, and the muscle relaxes and lengthens again, either passively or by the pull of an opposing (antagonist) muscle.

Because a single myofibril contains an enormous number of cross-bridges cycling through these five stages continuously and out of step with one another, the net effect is a smooth, sustained shortening of the whole muscle rather than a series of separate jerks, even though each individual myosin head only completes one small power stroke per cycle before releasing and re-cocking.

• Describing the filaments themselves as contracting or shrinking, rather than sliding past each other while staying the same length.
• Leaving out the role of ATP in detaching the myosin head (many students think ATP is only used for the power stroke, when it is actually needed to break the cross-bridge and re-cock the head).
• Omitting relaxation as a distinct, active step (Ca2+ must be actively pumped back into the SR; the muscle does not simply "turn off" on its own).