One giant cell formed by the fusion of hundreds of embryonic precursors, so it holds many nuclei
Muscle fibre — a syncytium formed from myoblasts.
Anatomy · Cells & tissues
Muscle fibre
The contractile cell of skeletal muscle
Muscle tissue · a single multinucleated cell
A long, cylindrical, striped cell that is the building block of a skeletal muscle. It forms in the embryo when many small myoblasts fuse, so each fibre has hundreds of nuclei pressed against its outer membrane, the sarcolemma. The cell is filled with hundreds to thousands of rod-like myofibrils, each a chain of sarcomeres made of overlapping actin and myosin filaments, which slide past one another to shorten the cell. Fibres are usually 50–100 micrometres wide but can be many centimetres long.
How to spot them
- ▸Very long, unbranched cylinder with regular cross-striations along its length
- ▸Many oval nuclei lying just beneath the surface membrane at the edge of the cell
- ▸In cross-section, polygonal profiles with peripheral nuclei, each surrounded by a fine sheath of connective tissue
- ▸Inside, bundles of fine parallel rods, the myofibrils, with alternating dark A bands and light I bands
- ▸Fibres in a fascicle are gathered by endomysium, and the fascicles by perimysium
What to know
Its stripes come from repeating units about 2 micrometres long between Z discs, in which thin actin slides between thick myosin
Muscle fibre — the sarcomere and the sliding filament mechanism.
Tiny tubular inpushings of the outer membrane carry the electrical impulse deep into the cell
Muscle fibre — the T-tubules, which with the sarcoplasmic reticulum form triads.
An internal store of calcium wraps every myofibril, and its release triggers shortening
Muscle fibre — the sarcoplasmic reticulum.
Reserve stem cells lie beside it under its coat and repair it after injury
Muscle fibre — satellite cells.
It comes in slow, fatigue-resistant and fast, quickly tiring varieties, told apart by their myoglobin, mitochondria and speed
Muscle fibre — type I (slow oxidative) and type II (fast) fibres.
Its outer membrane is named from the Greek words for 'flesh' and 'husk'
Muscle fibre — sarcolemma from Greek sarx, flesh, and lemma, husk.
Key ideas
- When a motor neuron fires, acetylcholine at the neuromuscular junction triggers an action potential along the sarcolemma and down the T-tubules, releasing calcium from the sarcoplasmic reticulum.
- Calcium exposes binding sites on actin so that myosin heads can bind, pivot and pull the thin filaments inward, using ATP, shortening each sarcomere.
Quick-fire facts
- •A single fibre may contain hundreds of nuclei because of its origin in the fusion of myoblasts
- •Fibres are about 50–100 micrometres in diameter, and in long muscles such as the sartorius may run for tens of centimetres
- •Each contains hundreds to thousands of myofibrils, each with thousands of sarcomeres in series
- •A motor unit is one motor neuron and all the fibres it supplies; the number ranges from a handful in eye muscles to hundreds or thousands in the calf
- •Type I fibres are slow, fatigue-resistant and rich in myoglobin and mitochondria; type IIx fibres are fast and tire quickly
- •Fibres are red because of myoglobin, which stores oxygen — the same reason meat from stamina muscles is darker
- •Growth after exercise comes mainly from enlargement of existing fibres, not from making new ones
- •In Duchenne muscular dystrophy the protein dystrophin, which anchors the sarcolemma, is missing
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