Biology · Locomotion and Movement · NEET
Calcium binds to a subunit of troponin, and troponin sits on the actin (thin) filament. It does NOT bind to myosin. This is the most common trap. When Ca++ binds troponin, the tropomyosin moves and unmasks (uncovers) the active sites on actin, so the myosin head can attach. Remember the chain: Ca++ to troponin to unmask actin to myosin binds.
The calcium is stored inside the sarcoplasmic reticulum (the smooth ER of the muscle fibre). NCERT clearly calls the sarcoplasmic reticulum the store house of calcium ions. When an action potential spreads through the muscle fibre, Ca++ is released from this store into the sarcoplasm (the muscle cytoplasm).
In a resting muscle, the active sites on actin are covered (masked) by tropomyosin. Calcium binding to troponin shifts tropomyosin away, so the active sites on actin become exposed. This 'unmasking' is the exact word NEET uses. Only after unmasking can the myosin head bind actin using energy from ATP hydrolysis to form a cross-bridge.
No. Calcium is only the trigger or switch that unmasks the active sites. The actual energy for the myosin head to bind and pull the actin comes from ATP hydrolysis (breaking of ATP). Do not confuse the trigger (Ca++) with the energy source (ATP). Both are needed, but they do different jobs.
When the nerve signal stops, calcium is pumped back into the sarcoplasmic reticulum. The Ca++ level in the sarcoplasm drops, so calcium leaves troponin, tropomyosin masks the active sites again, cross-bridges break, and the muscle relaxes. Low calcium equals relaxation; high calcium equals contraction.
Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.
Calcium is important in skeletal muscle contraction because it
Choose the correct statements regarding muscle contraction. A. A motor neuron carries a signal sent by the CNS to the sarcolemma of the muscle fibre. B. The neural signal generates an action potential which causes the release of Ca2+ into sarcoplasm. C. Increase in Ca2+ inactivates the actin for breaking cross bridges. D. Actin binds to the myosin head to form a cross bridge. E. Shortening of sarcomere takes place, by pulling actin filaments towards the centre of 'A' band.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Calcium binds to troponin, unmasks the active sites on actin, and lets myosin form a cross-bridge to start contraction.
From the sarcoplasmic reticulum, which NCERT calls the store house of calcium ions, into the sarcoplasm of the muscle fibre.
Neither directly. Calcium binds a subunit of troponin, which is located on the actin thin filament.
No. Calcium is the trigger that unmasks active sites. The energy comes from ATP hydrolysis by the myosin head.
When calcium drops, tropomyosin re-masks the active sites, cross-bridges break, and the muscle relaxes. Very low blood calcium can also cause tetany.
