Skeletal Muscle Contraction

Key points

• • Each muscle fibre is packed with sarcomeres, the repeating units of the thick filament myosin and the thin filament actin, whose overlap gives muscle its striped look.
• • In the sliding filament theory the filaments themselves do not shorten; instead the myosin heads pull the actin filaments past them, so the sarcomere shortens.
• • A nerve impulse at the neuromuscular junction spreads along the fibre and down the T-tubules, making the sarcoplasmic reticulum release its stored calcium.
• • Calcium binds troponin and moves tropomyosin off the actin, uncovering the binding sites so that myosin heads can attach and form cross-bridges.
• • Each cross-bridge cycle uses one ATP: ATP binding detaches the myosin head and its hydrolysis re-cocks it, so the muscle keeps contracting while both calcium and ATP are present.
• • When the signal stops, calcium is pumped back into the sarcoplasmic reticulum, the binding sites are re-covered, and the muscle relaxes.