Structure of a skeletal muscle

A skeletal muscle is made up of many thin cells called muscle fibres. Going from large to small:

Muscle → muscle bundles (fascicles) → muscle fibres (cells) → myofibrils → myofilaments (actin and myosin).

• The muscle fibres are grouped in bundles, each surrounded by a membrane called the perimysium.
• Each muscle fibre is a long multinucleated cell (many nuclei). It contains:
  • cytoplasm called sarcoplasm;
  • a surrounding cell membrane, the sarcolemma;
  • myofibrils, 1000 to 2000 per fibre, running parallel to the long axis of the fibre.
• Each myofibril shows alternating light and dark bands (cross-striations) — which is why skeletal and cardiac muscle is called striated; smooth muscle does not have this pattern and is called unstriated.
  • Light band (I-band): formed of thin protein filaments, actin. It is bisected by a dark line, the Z-line.
  • Dark band (A-band): formed of thick protein filaments, myosin (the thin filaments overlap the ends of the thick ones). It is bisected by a lighter region, the H-zone, where only myosin is present.
  • Sarcomere: the distance between two successive Z-lines. It is the structural and functional unit of contraction of the muscle.

Diagram in words: Z — [half I-band] — A-band (with H-zone in the middle) — [half I-band] — Z.

Worked example 7 (sarcomere arithmetic). A relaxed sarcomere is 2.6 µm long and a muscle fibre contracts so that each sarcomere is 1.8 µm. A myofibril has 10,000 sarcomeres in series. Relaxed length = 10,000 × 2.6 µm = 26,000 µm = 2.6 cm; contracted length = 10,000 × 1.8 µm = 1.8 cm; shortening = 0.8 cm, i.e. about 31%. (The numbers are for practice; the point is that the whole muscle shortens because every sarcomere shortens.)