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Muscle contraction – the sliding filament model

BiologyHuman & Animal PhysiologyAges 16–17

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Watch a sarcomere contract by the sliding filament model: a nerve impulse reaches the neuromuscular junction, the sarcoplasmic reticulum releases Ca²⁺, Ca²⁺ binds troponin so tropomyosin moves, and myosin heads form cross-bridges with actin, make a power stroke and detach using ATP. Change the Ca²⁺ and ATP supply to see a muscle that cannot contract or goes into rigor, measure the A band, I band and H zone, plot the force–sarcomere-length curve, and compare an antagonistic pair and fast and slow muscle fibres.

Lesson: Muscle contraction – the sliding filament theory, antagonistic muscles and fibre types

What it shows

Skeletal muscle shortens because thin actin filaments slide between thick myosin filaments inside each sarcomere. A nerve impulse at the neuromuscular junction triggers the release of calcium ions from the sarcoplasmic reticulum. Calcium binds troponin, tropomyosin moves aside and myosin heads attach to actin as cross-bridges. Each head makes a power stroke, then ATP binds so it detaches, and ATP hydrolysis re-cocks it. Without ATP the heads stay attached, causing rigor. The filaments never change length, so the A band stays constant while the I band and H zone shorten.

How to use

On the Sarcomere screen, press Send nerve impulse or set the Nerve impulse frequency, and watch the heads and the length trace. Lower Ca²⁺ released per impulse or press Remove ATP and compare. On Length–tension, move Sarcomere length and press Record to build the table. On Muscles & fibres, use Contract biceps and Contract triceps, pick a Fibre type and press Run fatigue test.

Parameters you can change

  • Screen Sarcomere and cross-bridge cycle, Force–sarcomere length curve, Antagonistic muscles, fast and slow fibres
  • Ca²⁺ released per impulse 0–100 %
  • ATP available 0–100 %
  • Nerve impulse frequency 0–50 Hz
  • Sarcomere length (force measurement) 1.2–3.8 µm
  • Fibre type Slow (type I), Fast (type II)

Questions to explore

  1. Why does the A band keep the same length while the I band and H zone get shorter?
  2. Why do muscles become stiff and locked after death, when ATP runs out?
  3. Why does a sarcomere produce the greatest force at about 2.0–2.25 µm?

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