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7.1.2Topic 7 · Run for your Life

Muscle Fibres — Edexcel A Level Biology A (SNAB) 9BI0 Revision Notes & Topical Questions

5 min read 8 parts Lesson 4 of 25 1 quiz

📚 Edexcel A Level Biology A (SNAB) 9BI0 Hub / Topical Revision
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Lesson summary

7.1.2 Muscle Fibres — Edexcel A Level Biology A (SNAB) 9BI0 notes: Describe the ultrastructure of a skeletal muscle fibre and compare slow and fast twitch fibres.A skeletal muscle is a bundle of long, cylindrical cells called muscle fibres.

  • multinucleate
  • myofibrils
  • Sarcolemma
  • Sarcoplasm
  • Mitochondria
  • Thin filaments

Section 1 of 8From muscle to myofibril

A skeletal muscle is a bundle of long, cylindrical cells called muscle fibres. Each fibre is formed by the fusion of many embryonic cells, so it is multinucleate and can be several centimetres long. Inside, each fibre contains hundreds of parallel myofibrils, thread-like structures made of protein filaments, and these are the units that actually shorten.

Specialised names are used for the fibre's parts:

1Sarcolemma
Sarcolemma the cell surface membrane. It folds inwards as T-tubules that carry the action potential deep into the fibre.
2Sarcoplasm
Sarcoplasm the cytoplasm, rich in glycogen granules and the oxygen-binding pigment myoglobin.
3Sarcoplasmic reticulum (SR)
Sarcoplasmic reticulum (SR) a modified smooth endoplasmic reticulum wrapped around every myofibril that stores Ca2+ ions.
4Mitochondria
Mitochondria numerous, packed between myofibrils to supply ATP.

Section 2 of 8Structure of a sarcomere

Under the electron microscope each myofibril shows a repeating pattern of light and dark bands. The repeating unit between two Z lines is the sarcomere, the functional unit of contraction. Two types of protein filament lie in the sarcomere:

Structure of a sarcomere
Thin filaments
Thin filaments are made mainly of actin, with the regulatory proteins tropomyosin and troponin wound along them. They are anchored to the Z lines.
Thick filaments
Thick filaments are made of myosin. Each myosin molecule has a long tail and a globular head with an ATP-binding site and an actin-binding site. Thick filaments are held at the centre by the M line.

The bands seen are named as follows.

Structure of a sarcomere
A band
The A band is the dark region equal to the length of the thick filaments (it includes any overlap with actin).
I band
The I band is the lighter region containing only thin filaments.
H zone
The H zone is the lighter central part of the A band where there is no overlap with actin.
When the muscle contracts the I band and H zone get narrower, while the A band keeps the same length, because the filaments slide rather than shorten.

Section 3 of 8Slow and fast twitch fibres

Skeletal muscle contains two main fibre types, and most muscles contain a mixture. The proportion is influenced by genes and by training.

FeatureSlow twitch (type I)Fast twitch (type II)
Contraction speedSlow, sustainedFast, powerful
FatigueResistant; used for enduranceFatigue quickly; used for sprints
Main respirationAerobicAnaerobic (glycolysis)
MyoglobinHigh, so fibres look redLow, so fibres look pale
Mitochondria and capillariesManyFewer
Glycogen and phosphocreatineLower storesLarge stores
Examples of usePosture, marathon runningSprinting, weight lifting

Slow twitch fibres are well suited to long aerobic activity because the dense capillary network, myoglobin and mitochondria give a continuous oxygen supply and ATP production. Fast twitch fibres rely on rapid anaerobic ATP production and on phosphocreatine, so they generate strong force but build up lactate and tire quickly.

Section 4 of 8Reading micrographs

Exam questions often show an electron micrograph of a sarcomere and ask you to identify the bands or calculate a length.

Reading micrographs
Remember: the distance from one Z line to the next is one sarcomere; measure with a ruler, convert using the scale bar or magnification (actual size = image size ÷ magnification), and compare relaxed and contracted sarcomeres.
In a contracted sarcomere the Z lines move closer together, the H zone may disappear and the I band shrinks.
Because thick filaments keep their length, the A band is the reliable fixed reference when comparing two micrographs.

Section 5 of 8Why the structure fits the function

The architecture of a fibre explains its speed and strength.

1
The T-tubules ensure the signal reaches every myofibril at almost the same instant, so the whole fibre contracts together rather than just its outer surface.
2
The sarcoplasmic reticulum lies next to the T-tubules, so Ca2+ is released close to the filaments and diffuses only a very short distance.
3
Having many nuclei allows a large amount of protein synthesis along a long cell, and the many mitochondria close to the myofibrils supply ATP exactly where the myosin heads use it.
4
Glycogen granules provide a local fuel store for glycolysis.

When muscle fibres are trained, the proportions do not change dramatically, but fibres can adapt: endurance training increases capillary density, myoglobin and mitochondrial number in slow twitch fibres, while resistance training increases the number of myofibrils per fibre, making the muscle larger and stronger.

Section 6 of 8Exam-style questions

  1. A myofibril shows a sarcomere 2.4 µm long when relaxed and 1.8 µm when contracted. Calculate the percentage decrease in length. [2]2 marks

  2. Explain why a marathon runner's leg muscles are likely to contain a high proportion of slow twitch fibres. [3]3 marks

  3. Describe the appearance of the A band, I band and H zone when a sarcomere contracts. [3]3 marks

Section 7 of 8Key terms

TermMeaning
Muscle fibreA long multinucleate muscle cell containing many myofibrils
MyofibrilBundle of actin and myosin filaments arranged in sarcomeres
SarcomereRepeating contractile unit between two Z lines
Sarcoplasmic reticulumModified endoplasmic reticulum storing calcium ions
T-tubuleInward fold of the sarcolemma carrying the action potential into the fibre
MyoglobinOxygen-storing pigment in muscle fibres
Slow twitch fibreFibre with many mitochondria and high myoglobin, suited to endurance
Fast twitch fibreFibre that contracts quickly and powerfully but fatigues rapidly

Section 8 of 8Exam technique

1
Saying the filaments themselves shortenthey slide, so the A band stays constant.
2
Confusing the sarcomere with the whole myofibril: the sarcomere is the segment between two Z lines.
3
Saying fast twitch fibres use mainly aerobic respiration: they use anaerobic respiration and phosphocreatine.
4
Forgetting that muscle fibres are multinucleate cells.

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