CorpusHuman Anatomy

Sarcomere & Sliding Filaments

From whole muscle to cross-bridge — excitation–contraction coupling (schematic, not to scale)

The top row zooms in step by step from a whole muscle, through a fascicle, a muscle fibre and a myofibril, to a single sarcomere. The middle panels show how a nerve impulse is turned into force. On the left, ACh released at the neuromuscular junction triggers an action potential that runs along the sarcolemma and down the T-tubules, releasing Ca²⁺ from the sarcoplasmic reticulum. On the right, a single myosin head goes through one cross-bridge cycle on an actin filament regulated by troponin and tropomyosin.

The bottom panel animates the sliding filament mechanism in a single 6-second loop with the other panels. The Z discs and thin filaments slide toward the M line, so the I bands and H zone shrink, while the A band, the length of the thick filament, stays constant. Myosin heads row during the contraction phase, and the titin springs compress and recoil. Use the steps to walk through excitation–contraction coupling and the cross-bridge cycle.

Step-by-step

  1. Levels of organisation

    A skeletal muscle (wrapped in epimysium) is made of fascicles (perimysium), each a bundle of muscle fibres (endomysium). Every fibre is a single multinucleate cell packed with myofibrils, which are chains of sarcomeres, the contractile units.

  2. 1 · Nerve impulse reaches the NMJ

    An action potential travels down the α-motor neuron. At the terminal, P/Q-type voltage-gated Ca²⁺ channels open, Ca²⁺ enters, and synaptic vesicles fuse (SNARE proteins) to release acetylcholine into the ≈50 nm synaptic cleft.

  3. 2 · End-plate potential → muscle action potential

    ACh binds nicotinic receptors on the junctional folds, opening cation channels. The resulting end-plate potential always exceeds threshold (safety factor ≈3–5), so voltage-gated Na⁺ channels fire an action potential that spreads along the sarcolemma. Acetylcholinesterase rapidly clears ACh, so each nerve impulse produces exactly one muscle impulse.

  4. 3 · Down the T-tubules

    The action potential dives into the fibre along the T-tubules, two per sarcomere at the A–I junctions. There, voltage-sensing dihydropyridine receptors (DHPR, Caᵥ1.1) change shape and mechanically pull open the ryanodine receptors (RyR1) of the adjacent SR terminal cisternae (the triad).

  5. 4 · Ca²⁺ release from the SR

    Ca²⁺ stored on calsequestrin floods out of the terminal cisternae through RyR1. Cytosolic Ca²⁺ rises about a hundred-fold from ≈0.1 µM within milliseconds and reaches every myofibril almost at once.

  6. 5 · Troponin–tropomyosin switch

    Ca²⁺ binds troponin C. The troponin complex pulls troponin I off actin and rolls tropomyosin deeper into the actin groove, uncovering the myosin-binding sites. Each tropomyosin spans 7 actins, so activation spreads cooperatively along the filament.

  7. 6 · Cross-bridge formation

    The energised myosin head, already cocked and holding ADP·Pᵢ, binds the exposed site on actin to form a cross-bridge.

  8. 7 · Power stroke

    Release of Pᵢ triggers the power stroke. The lever arm swings ≈5–10 nm and pulls the thin filament toward the M line, and ADP is then released. Thousands of asynchronous strokes slide the filaments: the Z discs approach, the I band and H zone narrow, and the A band is unchanged.

  9. 8 · ATP binding → detachment

    A new ATP molecule binds the myosin head and lowers its affinity for actin, so the cross-bridge releases. Without ATP the heads stay locked to actin: this is rigor mortis.

  10. 9 · Hydrolysis re-cocks the head

    Myosin ATPase hydrolyses ATP to ADP·Pᵢ, and the energy returns the head to its high-energy, cocked position. The cycle repeats as long as Ca²⁺ and ATP are present, each head stepping along the filament like a rower.

  11. 10 · Sliding filaments

    Thin filaments slide toward the M line while neither filament shortens. The sarcomere shortens (here ≈2.7 → 2.1 µm), the I bands and H zone shrink, the A band (≈1.6 µm) stays constant, and titin springs in the I band are compressed.

  12. 11 · Relaxation

    When nerve impulses stop, ACh is cleared and RyR1 closes. ATP-driven SERCA pumps return Ca²⁺ to the SR, where calsequestrin binds it. Ca²⁺ leaves troponin, tropomyosin re-covers the binding sites, and cross-bridge cycling stops. The sarcomere lengthens passively, helped by titin recoil and antagonist muscles. Relaxation therefore also needs ATP.

Labelled parts