Structure of a skeletal muscle fibre

Respiration, Muscles and the Internal Environment · Muscles and movement · note 2 of 4

Spec 7.10(i)

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Structure of a skeletal muscle fibreSpec 7.10(i)

In short

A mammalian skeletal muscle fibre is a long, multinucleate cell with a cell surface membrane called the sarcolemma and cytoplasm called sarcoplasm. It is packed with myofibrils, each made of repeating sarcomeres containing thick myosin and thin actin filaments, and it has many mitochondria, sarcoplasmic reticulum and T-tubules.

A skeletal muscle fibre is a single, long, cylindrical cell with many nuclei. Its cell surface membrane is the sarcolemma and its cytoplasm is the sarcoplasm. The sarcolemma folds inwards to form T-tubules (transverse tubules), which carry action potentials deep into the fibre.

Myofibrils: long, thin, cylindrical bundles of protein filaments that run along the length of the fibre and cause contraction.
Sarcoplasmic reticulum: a network of membranes around each myofibril that stores calcium ions (Ca²⁺).
Many mitochondria: lying between the myofibrils, to supply the ATP needed for contraction.
Many nuclei, lying just under the sarcolemma.

The sarcomere

Each myofibril is divided along its length into repeating units called sarcomeres, from one Z line to the next. Two kinds of filament lie in the sarcomere: thick filaments of myosin and thin filaments of actin (with the proteins troponin and tropomyosin). Their arrangement gives the striped (striated) appearance under the microscope.

Bands of a sarcomere
Region	Appearance	What is present
A band	Dark	The full length of the myosin filaments (including where actin overlaps them)
I band	Light	Actin filaments only (no myosin)
H zone	Lighter region in the middle of the A band	Myosin only (no actin overlap)
Z line	Thin dark line	Boundary of the sarcomere; actin is attached here
M line	Centre of the sarcomere	Myosin filaments held together

Three levels of skeletal muscle structure: a muscle fibre with sarcolemma, nuclei, mitochondria, sarcoplasmic reticulum, T-tubule and myofibrils; one myofibril with dark and light bands; and one sarcomere with A band, I band, H zone, Z lines, M line, actin and myosin. — From fibre to sarcomere: each myofibril is a chain of sarcomeres.

Exam tip:

In a contracted muscle the A band stays the same length, but the I band and H zone get shorter. Examiners often test this.

Written and checked against the Edexcel International A Level Biology (YBI11) specification · Updated October 2026

Frequently asked questions

How do antagonistic muscles work?

Antagonistic muscles are pairs that have opposite effects at a joint, such as the biceps (flexor) and triceps (extensor). Muscles can only pull. When one contracts and bends or straightens the joint, the other relaxes and is stretched, and then they swap roles to move the joint back.

What is the sliding filament theory?

The sliding filament theory explains muscle contraction. Calcium ions move tropomyosin away from the actin binding sites, so myosin heads bind and pull the actin filaments inwards using energy from ATP. The filaments slide past each other, the sarcomere shortens and the muscle contracts, although the filaments themselves stay the same length.

What are the differences between fast and slow twitch muscle fibres?

Slow twitch fibres contract slowly, resist fatigue, respire aerobically and have many mitochondria, much myoglobin and many capillaries. Fast twitch fibres contract quickly with more force, tire quickly, rely on anaerobic glycolysis and have more glycogen but fewer mitochondria, so they suit sprinting.

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