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Muscle contraction is a favourite NEET topic because it rewards order. You need to know what happens first, what calcium does, where ATP is used and which bands change length. Students who memorise a paragraph usually lose marks on the band-length question, because it tests a picture, not a sentence. This post gives you the steps in order, a worked example on band lengths, the history behind the sliding filament idea, and a routine to fix it in memory.
What does a sarcomere look like?
NCERT's chapter on locomotion and movement describes each myofibril as alternating dark and light bands. The light I band contains actin, the thin filaments. The dark A band contains myosin, the thick filaments. A Z line bisects each I band, and the thin filaments are attached to it. An M line holds the thick filaments together in the middle of the A band. The stretch between two Z lines is a sarcomere, the functional unit of contraction.
At rest, the ends of the thin filaments overlap the ends of the thick filaments but leave the centre of the A band free. That central part, where there is myosin but no actin, is the H zone.
Each thin filament is two strands of F-actin wound around each other, with tropomyosin running alongside and troponin placed at intervals on it. At rest, a subunit of troponin masks the sites on actin where myosin would bind. Each myosin head is an ATPase with binding sites for ATP and for actin.
What are the steps of contraction?
NCERT sets out the sequence. Learn it as a chain where each step causes the next.
The central nervous system sends a signal along a motor neuron. A motor neuron and the muscle fibres it supplies form a motor unit.
At the neuromuscular junction, the signal releases acetylcholine, which generates an action potential in the sarcolemma.
The action potential spreads through the fibre and causes calcium ions to be released into the sarcoplasm from the sarcoplasmic reticulum, the fibre's calcium store.
Calcium binds to a subunit of troponin, removing the masking of the active sites on actin.
Using energy from ATP hydrolysis, the myosin head binds to actin, forming a cross bridge.
The cross bridge pulls the thin filaments towards the centre of the A band. The Z lines are pulled inwards and the sarcomere shortens.
The myosin head releases ADP and Pi. A new ATP binds and the cross bridge breaks. The ATP is hydrolysed again and the cycle repeats.
When calcium is pumped back into the sarcoplasmic cisternae, the actin sites are masked again, the Z lines return and the muscle relaxes.
What does calcium actually switch on?
Research describes the switch in more detail than NCERT. A review in Physiological Reviews by Gordon, Homsher and Regnier (2000) explains that calcium acts mainly on the thin filament. The position of tropomyosin and troponin decides whether myosin can bind actin. Calcium binding to the troponin C subunit changes how troponin holds tropomyosin, tropomyosin moves over the actin surface, and binding sites that were blocked are exposed. In the exam, use NCERT's wording: calcium binds troponin and unmasks the active sites on actin.
Worked example: which bands change length?
Use round numbers to make the picture concrete. Take thick filaments 1.6 µm long and thin filaments that each reach 1.0 µm from their Z line.
Resting sarcomere, 2.4 µm long
A band = length of the thick filaments = 1.6 µm.
I band = sarcomere − A band = 2.4 − 1.6 = 0.8 µm per sarcomere (0.4 µm on each side).
H zone = sarcomere − 2 × thin filament = 2.4 − 2.0 = 0.4 µm.
Check: H zone + overlap on both sides should equal the A band. Overlap on each side = 1.0 − 0.4 = 0.6 µm, so 0.4 + 1.2 = 1.6 µm. Correct.
Contracted sarcomere, 2.0 µm long
A band = 1.6 µm, unchanged, because the thick filaments do not shorten.
I band = 2.0 − 1.6 = 0.4 µm.
H zone = 2.0 − 2.0 = 0 µm. The thin filaments now meet in the middle.
So the A band stays the same while the I band and H zone shrink, exactly as NCERT states. Neither filament gets shorter; they slide past each other.
Where did the sliding filament idea come from?
In 1954, two papers in Nature reported this pattern. Andrew Huxley and Rolf Niedergerke studied living muscle fibres with interference microscopy, and Hugh Huxley and Jean Hanson studied changes in the cross-striations during contraction and stretch. Hugh Huxley later described, in a 2004 review, how X-ray diffraction work that began in Cambridge led eventually to the first direct evidence that myosin cross bridges tilt while generating force.
Which mistakes cost marks?
"The A band shortens." It does not; only the I band and H zone shorten.
"Actin and myosin filaments contract." They keep their length and slide.
Calcium from the blood. In NCERT's account, calcium comes from the sarcoplasmic reticulum and is pumped back into it.
Mixing troponin and tropomyosin. Calcium binds troponin. Tropomyosin is the long strand that troponin sits on.
Leaving out relaxation. Many answers stop at the power stroke. Relaxation needs calcium removal.
A practice routine for this topic
The NEET (UG) 2026 syllabus lists contractile proteins and muscle contraction under locomotion and movement, along with disorders such as myasthenia gravis and tetany.
Draw two sarcomeres, one relaxed and one contracted, and label the A band, I band, H zone, Z line and M line.
Write the eight steps from memory, then check them against NCERT.
Change the numbers. Redo the worked example for a sarcomere of 2.2 µm.
Link to disorders. For tetany, NCERT gives low calcium in body fluid as the cause; say which step it affects.
When a step is unclear, look at it on its own. The Fermi Concept Library covers every concept in biology, each with its own lesson, so you can study the neuromuscular junction separately before putting the chain together. For how the nerve impulse in steps 1 and 2 is generated, see our guide to neural control and coordination, and for joints and skeletal disorders, our overview of locomotion and movement.
Draw a relaxed and a contracted sarcomere side by side today and mark which lengths stay the same.
