7.2.2Unit 6: Practical Skills in Biology II
The Muscle Fibre — IAL Edexcel Linear Revision Notes & Topical Questions
5 min read 4 parts Lesson 24 of 48 Unit quiz
Section 1 of 4Structure of a Muscle Fibre
A fibre is multinucleate and formed by fusion of cells, so it is described as cell-like rather than a normal cell.
Sarcolemma
- The cell surface membrane of the fibre, which folds inwards to form transverse tubules called T-tubules.
T-tubules
- Tube-like extensions that pass close to the sarcoplasmic reticulum and spread the electrical impulse quickly through the fibre.
Sarcoplasm
- The cytoplasm of the fibre, which holds the myofibrils, many mitochondria and many nuclei.
Sarcoplasmic reticulum
- Specialised endoplasmic reticulum that pumps in and stores calcium ions needed for contraction.
Mitochondria
- Supply the ATP for contraction by aerobic respiration, and are abundant in fibres used for endurance.
Myofibrils
- Long bundles of actin and myosin filaments that slide past each other when the fibre contracts.
Section 2 of 4Banding Pattern of a Myofibril
Banding Pattern of a Myofibril
Sarcomere
- The repeating contractile unit, running from one Z disc to the next.
A band
- The dark band covering the whole length of the thick myosin filaments;
- its length stays the same during contraction.
I band
- The light band containing only thin actin filaments, which becomes narrower as the muscle contracts.
H zone
- The central region containing only myosin, which also becomes narrower or disappears on contraction.
Z disc and M line
- Z discs anchor thin filaments at the ends of each sarcomere, and the M line lies in the middle holding thick filaments.
Section 3 of 4Fast and Slow Twitch Fibres
Fast twitch
- Contract rapidly and rely mainly on anaerobic respiration, so they fatigue quickly because lactate builds up.
Slow twitch
- Contract more slowly, rely on aerobic respiration and resist fatigue, which makes them suited to endurance.
Blood supply
- Slow twitch fibres have a denser capillary network, giving a short diffusion distance for oxygen and glucose.
Myoglobin
- This red oxygen-storing pigment is plentiful in slow twitch fibres, which therefore look dark red, while fast twitch fibres look paler.
Examples
- Eyelid muscles contain many fast twitch fibres, while human back muscles contain many slow twitch fibres.
| Feature | Fast twitch | Slow twitch |
|---|---|---|
| Contraction speed | Fast | Slower |
| Main respiration | Anaerobic | Aerobic |
| Fatigue | Quickly | Slowly |
| Mitochondria and myoglobin | Fewer | Many |
| Typical use | Short bursts | Sustained activity |
Section 4 of 4Exam Checklist & Tips
7.2.2 – Key Points for the Exam
- Sarcomere The unit of a myofibril between two Z discs, which shortens during contraction.
- T-tubule An inward extension of the sarcolemma that carries impulses deep into the fibre.
- Myoglobin An oxygen-storing pigment in muscle.
- Sarcoplasmic reticulum The calcium-storing endoplasmic reticulum of muscle.
- Myofibril A bundle of actin and myosin filaments within a fibre.
Common Mistakes
- Stating that a muscle fibre is a single-nucleus cell; it contains many nuclei.
- Saying the A band shortens during contraction; only the I band and H zone narrow.
- Mixing up the sarcolemma and the sarcoplasm, which are the membrane and cytoplasm.
- Suggesting that fast twitch fibres use mainly aerobic respiration.
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