Biology · Locomotion and Movement · NEET
No. This is the single biggest confusion. Neither the thin (actin) nor the thick (myosin) filaments change their length. They stay the exact same size. Contraction happens because the thin filaments SLIDE inward over the thick filaments, so the two sets overlap more. The extra overlap pulls the Z lines closer, and that is what shortens the sarcomere and the whole muscle.
The A band is the length of the thick (myosin) filaments. Since the thick filaments never change their length, the A band width stays constant during contraction. The parts that shrink are the I band (only thin filament, no overlap) and the H zone (only thick filament, no overlap). As thin filaments slide in, these non-overlap regions get smaller, but the myosin length behind the A band is unchanged. NEET loves this exact point.
During contraction the I band gets reduced (narrower) and the H zone disappears (or shrinks a lot). Both are the gaps that vanish as the thin filaments slide toward the centre. The A band stays the same. Remember: A stays, I and H shrink.
An action potential releases calcium ions (Ca2+) from the sarcoplasmic reticulum into the sarcoplasm. Calcium binds to a subunit of troponin on the thin filament. This shifts tropomyosin and unmasks the active binding sites on actin. Now the myosin heads can attach to actin and form cross bridges. So calcium is the switch that unmasks the sites, it does not directly pull the filament.
ATP is used twice-worth of jobs. The myosin head hydrolyses ATP (to ADP + Pi) to become energised, then it binds actin and performs the power stroke that pulls the thin filament inward, releasing ADP and Pi. A new ATP molecule then binds the myosin head, which breaks the cross bridge so the head can detach and repeat the cycle. No ATP means the cross bridge cannot break, which is why muscles stay stiff after death (rigor mortis).
The sarcomere is the region between two successive Z lines. All the sliding, overlap change, and shortening are measured across one sarcomere. Because contraction is described sarcomere by sarcomere, the sarcomere is the structural and functional unit of contraction. When every sarcomere shortens a little, the whole muscle fibre shortens a lot.
Calcium is important in skeletal muscle contraction because it
During muscular contraction which of the following events occur? (a) H zone disappears (b) A band widens (c) I band reduces in width (d) Myosin hydrolyses ATP, releasing ADP and Pi (e) Z lines attached to actins are pulled inwards.
Name the ion responsible for unmasking of active sites for myosin for cross-bridge activity during muscle contraction.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
The sliding filament theory was proposed independently in 1954 by Hugh Huxley and Jean Hanson, and by Andrew Huxley and Ralph Niedergerke. For NEET you only need the mechanism, not the names, but it is good to know it is called the Huxley sliding filament model.
Contraction of a muscle fibre takes place by the sliding of the thin filaments over the thick filaments, which shortens the sarcomere. This is the exact NCERT definition and is a common short-answer question.
No. The A band equals the length of the thick myosin filaments, which never change length, so the A band width stays constant. Only the I band reduces and the H zone disappears.
When the neural signal stops, calcium ions are pumped back into the sarcoplasmic reticulum. This re-masks the actin binding sites, cross bridges cannot form, the thin filaments slide back out, and the sarcomere returns to its resting length, which is relaxation.
A new ATP molecule must bind the myosin head to detach it from actin. Without ATP the cross bridge cannot break, so the muscle stays contracted. This is the basis of rigor mortis after death.
