Skeletal muscle fibre

Myocytus striatus skeletalis

Also known as: muscle fiber, muscle cell, myofibre, rhabdomyocyte, sarcolemma

A long, cylindrical, multinucleated syncytium formed by the fusion of myoblasts. It is packed with myofibrils and specialised membranes for rapid, synchronous contraction.

Overview

Diameter≈10–100 µm

Myofibrils per fibrehundreds to thousands

Myofibril share of volume≈80%

A skeletal muscle fibre is a single giant cell, ≈10–100 µm in diameter and from millimetres up to ≈30 cm long. Its plasma membrane, the sarcolemma, is covered by a basal lamina and folds inwards as T-tubules at every A–I junction. Hundreds of flattened nuclei sit just beneath the sarcolemma. Each nucleus controls a fixed volume of cytoplasm, called the myonuclear domain. Most of the sarcoplasm (≈80%) is filled with myofibrils. Between them lie mitochondria, glycogen granules, lipid droplets and the oxygen-binding pigment myoglobin, and around each myofibril runs the sarcoplasmic reticulum.

The dystrophin–glycoprotein complex links the cytoskeleton to the extracellular matrix. Dystrophin binds actin near the sarcolemma, then dystroglycan, then laminin-211 in the basal lamina. This link transmits force sideways and protects the membrane during contraction, which is why defects cause muscular dystrophies. Satellite cells (Pax7⁺) are quiescent stem cells between the sarcolemma and the basal lamina. When muscle is injured or trained, they activate, proliferate and fuse with the fibre, adding nuclei for repair and hypertrophy.

Parts & subdivisions

Sarcolemma

The excitable plasma membrane. It carries the action potential and contains voltage-gated Na⁺ channels (Naᵥ1.4) and the dystrophin complex.

Sarcoplasm

The cytoplasm, rich in glycogen, myoglobin and mitochondria (amounts differ between fibre types).

Myofibrils

Contractile organelles, ≈1–2 µm wide, made of sarcomeres in series.

Sarcoplasmic reticulum

A smooth ER network that stores and releases Ca²⁺.

T-tubules

Invaginations of the sarcolemma that carry the action potential deep into the fibre.

Peripheral nuclei

Many nuclei under the sarcolemma. Central nuclei indicate regeneration or myopathy.

Satellite cells

Muscle stem cells beneath the basal lamina that allow regeneration and hypertrophy.

Anatomy

Histology

Peripheral nuclei, cross-striations, and myofibrils seen as dots in cross-section. Fibre types can be told apart with ATPase or NADH histochemistry, which gives a "checkerboard" pattern in normal muscle and "fibre-type grouping" after denervation and reinnervation.

Development

Myoblasts fuse into myotubes with central nuclei. As myofibrils fill the cell, the nuclei move to the periphery. The number of fibres is set at around birth.

Clinical significance

Dystrophinopathies

Absent (Duchenne) or truncated (Becker) dystrophin makes the sarcolemma fragile. CK leaks into the blood and repeated cycles of necrosis and regeneration end in fibro-fatty replacement.

Inflammatory myopathies

In polymyositis and inclusion body myositis, CD8⁺ T cells invade the endomysium. Dermatomyositis is a complement-mediated microangiopathy with perifascicular atrophy. All cause proximal weakness with a raised CK.