Muscle Contraction: The Sliding Filament Mechanism

Core Concept

Skeletal muscle contracts by the sliding filament mechanism: myosin II heads pull actin thin filaments toward the center of the sarcomere, so the actin and myosin filaments slide past one another and the sarcomere shortens — the filaments themselves do not change length. Contraction is switched on by Ca²⁺, which binds troponin and moves tropomyosin off the myosin-binding sites on actin. Both the power stroke and the detachment of myosin heads require ATP, which is why ATP depletion produces rigor mortis (permanently attached heads).

Key Components

• Sarcomere: the repeating contractile unit between two Z-lines.
• Thin filaments: actin plus the regulatory proteins tropomyosin (blocks myosin-binding sites) and troponin (Ca²⁺ sensor: TnC, TnI, TnT).
• Thick filaments: bipolar bundles of myosin II with protruding heads.
• Z-line (Z-disc): anchors the thin filaments; M-line: anchors thick filaments.
• A-band: length of the thick filament (constant during contraction); I-band: thin-filament-only region (shortens); H-zone: thick-filament-only region (shortens).
• Sarcoplasmic reticulum (SR): stores Ca²⁺; T-tubules conduct the action potential inward.
• DHP receptor / ryanodine receptor: couple membrane depolarization to SR Ca²⁺ release (excitation–contraction coupling).

Mechanism / How It Works

1. Excitation: a motor-neuron action potential depolarizes the muscle membrane and propagates down T-tubules.
2. Ca²⁺ release: the voltage-sensitive DHP receptor triggers the ryanodine receptor to open, releasing Ca²⁺ from the sarcoplasmic reticulum into the cytosol.
3. Activation: Ca²⁺ binds troponin C, causing a conformational change that moves tropomyosin off the myosin-binding sites on actin.
4. Cross-bridge cycle: myosin heads bind actin, undergo the power stroke (released on Pi), pulling thin filaments toward the M-line, and detach when a fresh ATP binds.
5. Shortening: repeated cycles slide thin filaments past thick filaments, shortening the sarcomere (I-band and H-zone shrink; A-band is unchanged).
6. Relaxation: when stimulation stops, Ca²⁺-ATPase pumps (SERCA) return Ca²⁺ to the SR, tropomyosin re-covers the binding sites, and cross-bridges detach.

Energy and Directionality

Contraction is powered by ATP: one ATP per cross-bridge cycle (hydrolysis cocks the head; Pi release drives the stroke), and additional ATP is spent by SERCA to re-sequester Ca²⁺ for relaxation. The direction of force is toward the sarcomere center because thick filaments are bipolar — heads at each end pull the oppositely oriented thin filaments inward. Rigor mortis illustrates both ATP roles: without ATP, heads cannot detach (rigor) and Ca²⁺ cannot be pumped back (so the muscle stiffens after death).

Experimental Evidence / Technique

• Huxley and Niedergerke / Huxley and Hanson (1954): interference microscopy showed the A-band stays constant while the I-band shortens during contraction — direct evidence for the sliding filament model (which replaced the "filament folding" idea).
• X-ray diffraction: revealed cross-bridge movement and the repeating helical arrangement of myosin heads during contraction.
• Glycerinated/skinned fiber experiments: permeabilized fibers contract when given ATP and Ca²⁺, and relax when Ca²⁺ is removed — dissecting the minimal requirements.
• Optical tweezers and in vitro motility: measured single myosin power-stroke size and force.
• Caged ATP / caged Ca²⁺: flash photolysis triggered contraction or relaxation with millisecond resolution, correlating biochemistry with mechanics.

How it works

1. Excitation: a motor-neuron action potential depolarizes the muscle membrane and propagates down T-tubules.
2. Ca²⁺ release: the voltage-sensitive DHP receptor triggers the ryanodine receptor to open, releasing Ca²⁺ from the sarcoplasmic reticulum into the cytosol.
3. Activation: Ca²⁺ binds troponin C, causing a conformational change that moves tropomyosin off the myosin-binding sites on actin.
4. Cross-bridge cycle: myosin heads bind actin, undergo the power stroke (released on Pi), pulling thin filaments toward the M-line, and detach when a fresh ATP binds.
5. Shortening: repeated cycles slide thin filaments past thick filaments, shortening the sarcomere (I-band and H-zone shrink; A-band is unchanged).
6. Relaxation: when stimulation stops, Ca²⁺-ATPase pumps (SERCA) return Ca²⁺ to the SR, tropomyosin re-covers the binding sites, and cross-bridges detach.

Common confusions

• "Filaments shorten during contraction." They slide; the actin and myosin filaments themselves keep a constant length — only the sarcomere shortens.
• "ATP is only needed to contract." ATP is also required for relaxation (detachment and Ca²⁺ reuptake); rigor mortis is caused by lack of ATP.
• "Ca²⁺ binds myosin directly." Ca²⁺ binds troponin C, which moves tropomyosin; myosin's target is actin.
• "The A-band shortens." The A-band (thick filament length) is constant; the I-band and H-zone shorten.
• "Muscle contraction requires one ATP total." It requires one ATP per cross-bridge cycle, continuously, for as long as contraction lasts.

Quick review

• Sliding filament: myosin pulls actin inward; sarcomere shortens, filaments don't.
• Ca²⁺ → troponin → tropomyosin shift → cross-bridge cycling.
• ATP: powers the stroke and detachment; SERCA re-sequesters Ca²⁺.
• Excitation–contraction coupling via DHP and ryanodine receptors.
• Rigor mortis = ATP depletion → locked cross-bridges.