Sliding, not shrinking
"Sliding, not shrinking" is a free, interactive Class 11 Biology simulation for NEET, built on the lesson "Mechanism of muscle contraction" in Locomotion and Movement. Each stroke is one cycle: a myosin head binds actin, pulls, lets go on a fresh ATP and binds again. Add strokes and watch which parts of the sarcomere change. Then try it with calcium low. Ca²⁺ binds troponin, actin sites open, myosin heads form cross bridges using ATP and pull the thin filaments inwards. Z lines move in; I band and H zone shorten; A band stays the same.
From the lesson Mechanism of muscle contraction in Locomotion and Movement. Change the values and watch what happens.
The idea behind it
NCERT §17.2.2
- The sliding filament theory explains contraction: a muscle fibre shortens because the thin filaments slide over the thick filaments.
- The CNS starts contraction by sending a signal along a motor neuron. A motor neuron with the muscle fibres it supplies is a motor unit.
- The junction between a motor neuron and the sarcolemma is the neuromuscular junction, or motor end plate. There the signal releases the neurotransmitter acetylcholine, which sets up an action potential in the sarcolemma.
- The action potential spreads through the fibre and causes calcium ions to be released into the sarcoplasm.
- Ca²⁺ binds a subunit of troponin, which uncovers the active sites for myosin on actin.
- Using energy from ATP hydrolysis, the myosin head binds the exposed sites to form a cross bridge, and pulls the attached actin filaments towards the centre of the A band.
- The Z lines attached to those actins are pulled inwards too, so the sarcomere shortens. During contraction the I bands shorten while the A bands keep their length.
- The myosin head releases ADP and Pi and returns to its relaxed state; a fresh ATP binds and the cross bridge breaks. The ATP is hydrolysed again and the cycle of making and breaking cross bridges repeats, sliding the filaments further.
- This goes on until Ca²⁺ is pumped back into the sarcoplasmic cisternae, the actin sites are covered again, and the Z lines return to their original place: relaxation.
Questions
2 answers
- What does the "Sliding, not shrinking" simulation show?
- Each stroke is one cycle: a myosin head binds actin, pulls, lets go on a fresh ATP and binds again. Add strokes and watch which parts of the sarcomere change. Then try it with calcium low.
- Is this simulation free?
- Yes. It is free and runs in the browser on a phone or a laptop. It belongs to the lesson "Mechanism of muscle contraction" in Locomotion and Movement (Class 11 Biology).
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