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Histology · Muscle tissue

Skeletal muscle: sarcomere and contraction !

  • Tissue

Skeletal muscle is the striated, voluntary muscle that moves the skeleton; its repeating unit, the sarcomere, shortens to make force.

Updated 2026-11-01

See it in 3D: Biceps (long head) · Middle deltoid · Rectus femoris

What it is

A skeletal muscle fibre is a single huge cell, formed in the embryo by the fusion of many myoblasts, so it has hundreds of nuclei at its edge. Inside, myofibrils are lined up end to end in sarcomeres, the contractile units, which give the cross-striations. A nerve impulse at the neuromuscular junction releases calcium inside the fibre and the thin filaments slide past the thick ones.

Key facts

  • During contraction the I band and H zone shorten; the A band stays the same length.
  • Without ATP myosin cannot detach from actin: this is rigor mortis.
  • Titin anchors thick filaments to the Z line and gives passive elasticity; nebulin sets thin-filament length; dystrophin links the cytoskeleton to the sarcolemma.
  • Satellite cells under the basal lamina are the stem cells for repair.

Structure

  • Longitudinal section: long unbranched cylinders with alternating dark (A) and light (I) bands and many flattened nuclei just under the sarcolemma.
  • Cross section: polygonal fibres with peripheral nuclei, packed into fascicles.
  • Wrappings: epimysium around the whole muscle, perimysium around each fascicle, endomysium around each fibre.
  • Sarcomere: from Z line to Z line; the A band (thick myosin filaments, constant length), the I band (thin actin filaments only), the H zone (myosin only) and the M line in the middle.
  • T tubules dip in at each A–I junction and meet two terminal cisterns of sarcoplasmic reticulum, forming a triad.
  • Fibre types: type I (slow, red, oxidative, many mitochondria and myoglobin) and type II (fast, white, glycolytic).

Step by step

  1. Acetylcholine released at the neuromuscular junction depolarizes the sarcolemma.
  2. The action potential runs down the T tubules.
  3. Voltage sensors (dihydropyridine receptors) in the T tubule open ryanodine receptors in the sarcoplasmic reticulum, releasing calcium.
  4. Calcium binds troponin C; tropomyosin shifts and uncovers the myosin-binding sites on actin.
  5. Myosin heads bind actin, release ADP and phosphate and pull (power stroke).
  6. ATP binds myosin, which lets go of actin; splitting ATP re-cocks the head for the next cycle.
  7. Calcium is pumped back into the sarcoplasmic reticulum (SERCA) and the muscle relaxes.

Clinical notes

  • Duchenne muscular dystrophy: absent dystrophin; fibres degenerate and are replaced by fat and fibrous tissue.
  • Myasthenia gravis: antibodies against acetylcholine receptors at the neuromuscular junction cause fatigable weakness.
  • Malignant hyperthermia: a ryanodine receptor defect lets calcium flood out under some anaesthetics.
  • Rhabdomyolysis: injured fibres release myoglobin and creatine kinase into the blood.

Test yourself

Think of the answer, then tap the question to see it.

Which band of the sarcomere keeps the same length during contraction?

The A band, because it corresponds to the length of the thick myosin filaments; the I band and H zone shorten.

Where are the nuclei of a skeletal muscle fibre, and why are there so many?

Many nuclei sit at the periphery under the sarcolemma, because each fibre forms by fusion of many myoblasts.

What structure forms a triad in skeletal muscle?

One T tubule flanked by two terminal cisterns of the sarcoplasmic reticulum, located at the junction of the A and I bands.

Why do muscles stiffen after death?

ATP runs out, and without ATP the myosin heads cannot detach from actin, so cross-bridges stay locked (rigor mortis).

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Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

Sources: 10.2 Skeletal Muscle (OpenStax, 2022); 10.3 Muscle Fiber Contraction and Relaxation (OpenStax, 2022); Muscle Tissue (National Cancer Institute, SEER Training Modules, 2024).

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