Structure of Skeletal Muscle

This lesson covers: 

  1. The structure of skeletal muscle fibres
  2. The components within muscle fibres that allow contraction

The structure of skeletal muscle fibres

Skeletal muscles comprise numerous bundles of long, cylindrical muscle fibres.


Individual cells are fused to form muscle fibres during development to avoid weakness at the junctions between cells and to increase overall muscle strength.

Diagram showing the structure of a skeletal muscle fibre including sarcolemma, sarcoplasm, T tubule, sarcoplasmic reticulum, myofibril, nuclei, and mitochondrion.

Key components of a muscle fibre include:

  • Sarcolemma - The cell-surface membrane of a muscle fibre.
  • Sarcoplasm - The cytoplasm of a muscle fibre.
  • Transverse tubules (T tubules) - Extensions of the sarcolemma that transmit electrical signals, to ensure that the entire muscle receives the impulse to contract simultaneously.
  • Sarcoplasmic reticulum - A specialised endoplasmic reticulum that is responsible for storing and releasing calcium ions.
  • Myofibrils - Subcellular structures designed for contraction.
  • Multiple nuclei - Skeletal muscle fibres have many nuclei because several cells merge to form one muscle fibre.
  • Mitochondria - These release energy in the form of ATP for muscle contraction.

The structure of myofibrils

Myofibrils, the core units of muscle fibres, contain organised bundles of protein filaments.


These filaments slide past each other to enable muscle contraction.

The structure of a sarcomere

Myofibrils are made up of repeating units called sarcomeres.


Main filaments in a sarcomere:

  • Myosin filaments - Thick filaments composed of long rod-shapes with bulbous heads that project to the side.
  • Actin filaments - Thin filaments composed of two strands twisted around each other.


Under a microscope, myofibrils exhibit a distinctive banding pattern, which corresponds to the arrangement of myosin and actin within each sarcomere.

Diagram showing the structure of a sarcomere including A band, I band, M-line, Z-line, H-zone, myosin, and actin filaments.

Key sections of the sarcomere:

  • A band (dark band) - Area with both myosin and overlapping actin filaments (darker in colour).
  • I band (light band) - Area containing only light actin filaments (lighter in colour).
  • Z-lines - Mark the boundaries of each sarcomere unit (central line within the I band).
  • M-line - The central line of a sarcomere.
  • H-zone - Area with only myosin filaments (central lighter region in the A band).


The sliding of actin and myosin filaments within sarcomeres is fundamental to the process of muscle contraction. During muscle contraction, sarcomeres shorten, causing a change in the pattern of light and dark bands.