Structure of Skeletal Muscle Fibre: Sarcomere Explained

A skeletal muscle fibre is a long, cylindrical muscle cell specialised for contraction. Inside it, repeating units called sarcomeres contain actin and myosin filaments whose sliding produces force and shortening.

Knowing this structure matters because it explains muscle strength, fatigue, injury and diseases. Training, nutrition and rehabilitation all depend on how fibres and sarcomeres are built and repaired.

Key Concepts

• Muscle fibre: A multinucleated cell, also called a myofibre, enclosed by the sarcolemma membrane.
• Myofibrils: Long rods inside the fibre made of repeating sarcomeres.
• Sarcomere: The functional unit between two Z-lines, about 2 micrometres long at rest, containing A-band, I-band and H-zone.
• Actin (thin filament): Protein filament with myosin-binding sites, associated with troponin and tropomyosin.
• Myosin (thick filament): Protein with heads that bind actin and pull using ATP.
• Sarcoplasmic reticulum: Specialised endoplasmic reticulum storing calcium ions needed to start contraction.
• T-tubules: Membrane folds that carry the nerve signal deep into the fibre.

How It Works

1. The fibre is wrapped by endomysium; bundles of fibres called fascicles are wrapped by perimysium; the whole muscle is wrapped by epimysium. These layers join to form tendon.
2. Under a microscope, alternating dark A-bands and light I-bands give skeletal muscle its striated appearance.
3. The sarcomere is defined from one Z-line to the next. Thin filaments anchor at Z-lines and extend inward; thick filaments sit in the centre.
4. During contraction, myosin heads pull actin filaments toward the centre. Sarcomeres shorten, but the filaments themselves do not shorten.
5. Calcium released from the sarcoplasmic reticulum exposes binding sites on actin, and ATP powers repeated pulling cycles.

Worked Example

If one sarcomere shortens from 2.2 micrometres to 1.9 micrometres, shortening = 0.3 micrometres. A myofibril with 10,000 sarcomeres in series shortens by 0.3 × 10,000 = 3,000 micrometres = 3 millimetres. This shows how tiny changes in each sarcomere add up to visible muscle shortening because thousands are arranged end to end.

Common Misconceptions

• Actin and myosin filaments shrink during contraction: Wrong. They slide past each other; their lengths stay nearly constant.
• Each muscle fibre has one nucleus: Wrong. Skeletal fibres are multinucleated, formed by fusion of many cells.
• Bigger muscles always mean more fibres: Usually wrong in adults. Training mainly increases fibre size (hypertrophy), not fibre number greatly.

Real-World Applications

Muscle biopsy examines fibre structure in muscular dystrophy. Strength training and protein intake support myofibril growth. Stretching and warm-up reduce strain injury risk by preparing muscle and tendon for load.

Overall, the skeletal muscle fibre is a highly ordered machine: membranes deliver signals, calcium switches it on, and sarcomeres convert ATP energy into pulling force.