Muscle contraction and the sliding filament theory, step by step

Sarcomere structure

A sarcomere runs from one Z-line to the next. Thick myosin filaments form the A-band; thin actin filaments form the I-band. The H-zone is the region with only myosin. During contraction the I-band and H-zone shorten, while the A-band stays the same length — evidence that filaments slide rather than shorten.

The sequence of contraction

• An action potential spreads along the sarcolemma and down T-tubules.
• Calcium ions are released from the sarcoplasmic reticulum.
• Calcium binds to troponin, causing tropomyosin to move and expose binding sites on actin.
• Myosin heads bind, forming actomyosin cross-bridges, then pivot in the power stroke, pulling actin towards the centre of the sarcomere and releasing ADP.
• A new ATP binds, the head detaches; ATP hydrolysis (activated by calcium) returns the head to its original position ready to bind again.

Relaxation

When stimulation stops, calcium ions are actively transported back into the sarcoplasmic reticulum using ATP. Tropomyosin blocks the binding sites again, and the filaments slide back.

Slow and fast twitch fibres

• Slow twitch: many mitochondria, rich blood supply, myoglobin stores oxygen — aerobic respiration for endurance.
• Fast twitch: thicker myosin filaments, stored phosphocreatine to regenerate ATP quickly, rely on anaerobic respiration — powerful, short bursts.