Biology · Locomotion and Movement · NEET
The NCERT order is: (1) CNS sends a signal via a motor neuron to the neuromuscular junction. (2) The neuron releases acetylcholine (a neurotransmitter). (3) This generates an action potential in the sarcolemma. (4) The action potential releases Ca2+ from the sarcoplasmic reticulum into the sarcoplasm. (5) Ca2+ binds a subunit of troponin, which unmasks the active sites on actin. (6) The myosin head binds these sites forming a cross-bridge (using ATP energy). (7) The myosin pulls the thin filament toward the centre of the A band, shortening the sarcomere. NEET repeatedly tests this exact order.
Without Ca2+, the active sites on actin stay masked by tropomyosin and troponin, so myosin cannot attach. When Ca2+ rises, it binds a subunit of troponin, moves tropomyosin aside, and exposes the binding sites. This is the single event that lets cross-bridges form. In NEET, the phrase to remember is: 'Ca2+ binds troponin to remove the masking of active sites on actin for myosin.'
Relaxation is the reverse. The stimulation stops, so acetylcholine is broken down and no new action potential forms. Ca2+ is actively pumped back into the sarcoplasmic reticulum. Falling Ca2+ makes troponin-tropomyosin re-mask the active sites on actin, so myosin heads detach. The thin filaments slide back out and the sarcomere returns to its resting length. Relaxation, like contraction, needs ATP (to run the calcium pump).
ATP is used twice. First, the myosin head hydrolyses ATP into ADP + Pi; this energy lets the head bind actin and perform the power stroke (pulling actin inward). Second, ATP is needed to detach the myosin head from actin so it can re-cock, and to power the Ca2+ pump during relaxation. This is why rigor mortis (stiffness after death) happens: no ATP means myosin cannot release from actin.
During contraction the thin filaments slide over the thick filaments: the H zone disappears and the I band shortens, while the A band length stays the same (the A band does NOT widen). The Z lines are pulled inward, so the whole sarcomere shortens. Note carefully: 'A band retains its length' is the classic NEET trap. Full detail is on the band-changes-during-contraction page.
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.
Muscle contraction is initiated by a signal sent by the central nervous system by the release of _____.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
A signal from the central nervous system travels down a motor neuron to the neuromuscular junction, where acetylcholine is released. This is always the first event in the NCERT sequence.
Yes. In relaxation, stimulation stops, Ca2+ is pumped back into the sarcoplasmic reticulum, active sites get re-masked by troponin-tropomyosin, myosin heads detach, and the sarcomere returns to resting length.
Yes. Relaxation still uses ATP, mainly to run the calcium pump that returns Ca2+ to the sarcoplasmic reticulum and to detach myosin heads from actin.
It states that a muscle fibre contracts by the sliding of the thin (actin) filaments over the thick (myosin) filaments, which shortens each sarcomere.
Calcium unmasks the active sites on actin by binding troponin, and the A band length stays constant during contraction while the H zone and I band shrink.
