Q.Explain how tropomyosin and troponin together regulate the availability of the myosin-binding sites on actin in a resting muscle fibre and in a stimulated one.

[!TLDR]

Tropomyosin blocks the myosin-binding site at rest; troponin's calcium-binding subunit removes that block when stimulated.

[!ANSWER]

Resting: tropomyosin covers the site, blocking myosin. Stimulated: Ca2+ binds troponin C, shifting tropomyosin to expose the site.

In a resting muscle fibre, tropomyosin lies along the actin groove in a position that directly covers the sites on actin where a myosin head could otherwise bind, so no cross-bridge can form and the muscle stays relaxed. The troponin complex, made of three subunits, is what controls tropomyosin's position: troponin C is able to bind calcium ions, troponin I helps hold tropomyosin over the site while calcium is absent, and troponin T anchors the whole complex onto tropomyosin. When the muscle fibre is stimulated and sarcoplasmic calcium rises, calcium binds troponin C; this produces a conformational change in the troponin complex that physically drags tropomyosin away from the myosin-binding site, uncovering it and allowing myosin heads to attach and begin the cross-bridge cycle. When stimulation stops and calcium is pumped back into the sarcoplasmic reticulum, troponin releases calcium, tropomyosin slides back to cover the site, and the muscle relaxes.

[!ANSWER]

Resting: tropomyosin covers the site, blocking myosin. Stimulated: Ca2+ binds troponin C, shifting tropomyosin to expose the site.

Trace the causal chain both ways: at rest, tropomyosin blocks the site; on stimulation, calcium binds troponin C, which moves tropomyosin, which unblocks the site.

Saying calcium binds directly to tropomyosin -- calcium actually binds troponin C, and it is only the resulting shape change that then moves tropomyosin.