Zoology · Ch 9 — Locomotion and Movement
Mechanism of muscle contraction
Mechanism of muscle contraction
Mechanism of muscle contraction
In 1954, Andrew F. Huxley and Rolf Niedergerke proposed the sliding filament theory: actin and myosin filaments, each of fixed, unchanging length, slide past one another during contraction rather than shortening themselves. The tension created by a contracting muscle is used to move or resist a load (any weight or force opposing the contraction). Contraction is the active creation of tension; relaxation is the passive release of that tension.
Step 1 — Nerve signal and calcium release. Contraction begins with a nerve impulse from the central nervous system travelling down a motor neuron to the neuromuscular junction (motor end plate), the point where the neuron meets the sarcolemma. There, the neuron releases the neurotransmitter acetylcholine, which opens gated channels in the sarcolemma and triggers an action potential. This electrical signal travels inward along the T-tubules and triggers the release of calcium ions stored in the sarcoplasmic reticulum.
Step 2 — Exposing the binding sites. The released calcium binds to troponin on the thin filament. This causes tropomyosin to shift and uncover the myosin-binding active sites on actin, exposing them to the myosin heads.
Step 3 — Cross-bridge cycling (the sliding). Using energy from ATP hydrolysis, a myosin head rotates to form a 90° angle with the filament axis (the "cocked" position) and binds actin, forming a cross-bridge (the actin-myosin complex is called actomyosin). The head then tilts from 90° to about 45° — the power stroke — pulling the thin filament toward the centre of the sarcomere (the M-line), much like the stroke of an oar. ADP and phosphate are released during this stroke. A fresh ATP molecule then binds the myosin head, weakening its grip on actin and detaching the cross-bridge, which then re-cocks and repeats the cycle on a fresh actin site further along. …
What this figure shows. Own-words cover: a four-stage cycle diagram of one myosin head acting on a thin (actin) filament next to a thick (myosin) filament, with calcium ions, ATP, ADP and inorganic phosphate (Pi) labelled. Stage 1, cocking of the myosin head, as ATP is hydrolysed to ADP+Pi the head returns to a high-energy 'cocked' position. Stage 2, cross-bridge formation, the energised myosin head attaches to an exposed site on the actin filament. Stage 3, the power/working stroke, ADP and Pi are released and the myosin head pivots and bends to its low-energy bent shape, pulling the actin filament toward the M-line. Stage 4, cross-bridge detachment, a new ATP binds to myosin, the myosin-actin link …
What this figure shows. Own-words cover: two stacked diagrams of a single sarcomere bounded by two Z-discs. The upper diagram is a fully relaxed sarcomere showing a wide I-band on either side of the A-band, with the H-zone visible at the centre of the A-band and the M-line at its very centre. The lower diagram is a fully contracted sarcomere of the same muscle fibre, showing the I-bands and H-zone visibly narrowed (nearly disappearing) as the Z-discs are pulled inward toward the M-line, while the A-band width itself stays unchanged, illustrating that filament length does not change during contra …