Muscle fibres
A muscle fibre is a single skeletal muscle cell. It is long and thin, up to 100 μm across and, in the longest muscles, up to 30 cm long, and it has many nuclei because it forms when hundreds of embryonic cells fuse together. Each fibre is packed with myofibrils, the strands of actin and myosin that shorten when the muscle contracts. Muscles contain a mix of slow twitch fibres, built for endurance, and fast twitch fibres, built for short bursts of power.
From muscle to muscle fibre
Skeletal muscle is organised in layers, each wrapped in connective tissue:
| Level | What it is | Wrapped in |
|---|---|---|
| Muscle | The whole organ, such as the biceps | Epimysium |
| Fascicle | A bundle of muscle fibres | Perimysium |
| Muscle fibre | One muscle cell | Endomysium |
| Myofibril | A strand inside the fibre, about 1.2 μm across, running its whole length | |
| Sarcomere | One contracting unit of a myofibril, about 2 μm long |
A single fibre holds hundreds to thousands of myofibrils, lined up side by side, and each myofibril is a chain of sarcomeres joined end to end. That is why skeletal muscle looks striped (striated) under a microscope: the bands of the sarcomeres line up across the whole fibre.
Structure of a muscle fibre
Muscle fibres have their own names for the usual parts of a cell, plus some structures found only in muscle:
| Part | What it is | What it does |
|---|---|---|
| Sarcolemma | The cell surface membrane | Carries the action potential along the fibre |
| T-tubules (transverse tubules) | Folds of the sarcolemma reaching deep into the fibre | Carry the action potential into the interior of the cell |
| Sarcoplasm | The cytoplasm | Holds the myofibrils, mitochondria and glycogen |
| Sarcoplasmic reticulum | Specialised smooth endoplasmic reticulum around each myofibril | Stores, releases and takes back calcium ions |
| Many nuclei | Left from the cells that fused to make the fibre | Many copies of the genes, so large amounts of protein can be made |
| Myofibrils | Bundles of actin and myosin filaments | Shorten to make the fibre contract |
| Mitochondria | Organelles of aerobic respiration | Make ATP for contraction |
How the structure triggers contraction
- A motor neurone releases the neurotransmitter acetylcholine at the neuromuscular junction.
- An action potential spreads over the sarcolemma, then the T-tubules take it into the middle of the fibre.
- The sarcoplasmic reticulum releases calcium ions.
- Calcium ions cause tropomyosin to move off the binding sites on actin, so myosin heads can bind and pull the actin filaments.
- When stimulation stops, calcium ions are taken back into the sarcoplasmic reticulum and the fibre relaxes.
The cross-bridge cycle itself, and how the sarcomere bands change, are in the sliding filament theory guide.
Slow twitch and fast twitch muscle fibres
AQA asks for the structure, location and general properties of slow and fast skeletal muscle fibres. Their differences all come from how they make ATP:
| Slow twitch | Fast twitch | |
|---|---|---|
| Speed of contraction | Relatively slow | Fast |
| Main way of making ATP | Aerobic respiration | Anaerobic glycolysis |
| Mitochondria | Many | Few |
| Myoglobin (stores oxygen) | Lots, so the fibres are red | Little, so the fibres are white |
| Blood capillaries | Extensive supply | Fewer |
| Glycogen | Less | Lots |
| Diameter and force | Small diameter, low force | Large diameter, high force |
| Fatigue | Can work for long periods without tiring | Tire quickly |
| Where they do most work | Muscles that keep your posture and steady your joints | Quick, powerful movements |
Most muscles contain both types, and the mix reflects the main job of the muscle. University anatomy books split fast fibres further into fast oxidative fibres, which contract quickly but still respire mainly aerobically and are used for movements such as walking, and fast glycolytic fibres, the white, quick-tiring fibres in the table above.
How muscle fibres get their ATP
Contraction and relaxation both need ATP, but a fibre only holds a little at a time. It tops it up in three ways, which take over from each other as exercise goes on:
| Source | How it works | How long it lasts at full effort |
|---|---|---|
| Phosphocreatine (creatine phosphate) | Gives its phosphate to ADP to make ATP, catalysed by creatine kinase. Very fast, and needs no oxygen | About 15 seconds |
| Anaerobic glycolysis | Breaks glucose down without oxygen; fast but makes little ATP per glucose | About 1 minute |
| Aerobic respiration | Uses oxygen; slower, but makes far more ATP per glucose | As long as oxygen keeps arriving |
The figures are from OpenStax Anatomy and Physiology. Phosphocreatine is the first reserve used in a burst of intense effort. AQA names ATP and phosphocreatine together as the energy sources for contraction.
Skeletal, cardiac and smooth muscle
Muscle fibre usually means a skeletal muscle cell, but there are three types of muscle tissue:
| Skeletal | Cardiac | Smooth | |
|---|---|---|---|
| Striated (striped) | Yes | Yes | No |
| Nuclei per cell | Many | One or two | One |
| Found in | Muscles attached to the skeleton | The heart | Walls of arteries, gut and other organs |
| Control | Depends on signals from the nervous system | Myogenic: starts each beat itself; nerves and hormones adjust it | Nerves, hormones and local signals |
Skeletal muscles act in antagonistic pairs against the skeleton, which cannot be compressed: when one muscle of the pair contracts, the other relaxes, so between them they move a joint both ways, as the biceps and triceps do at the elbow.
Worked examples
Example 1: the real width of a fibre
In a photomicrograph taken at × 400, a muscle fibre is 32 mm wide. Find its real width in micrometres.
- Convert the image size: 32 mm = 32,000 μm.
- Real size = image size ÷ magnification = 32,000 ÷ 400 = 80 μm.
- That fits the range for skeletal fibres, which are up to about 100 μm across. The magnification calculator does the unit conversion for you.
Example 2: sarcomeres in a myofibril
A muscle fibre is 4.0 cm long. Each sarcomere in it is about 2 μm long. Estimate how many sarcomeres there are end to end in one myofibril.
- Convert to the same unit: 4.0 cm = 40,000 μm.
- Number of sarcomeres = 40,000 ÷ 2 = 20,000.
- Because they are in series, each sarcomere only has to shorten a little for the whole fibre to shorten a lot.
Common mistakes
- Calling a muscle fibre a myofibril. The fibre is the whole cell; myofibrils are the strands inside it.
- Saying slow twitch fibres have more glycogen. Fast twitch fibres store more, because glycolysis uses glucose quickly.
- Saying fast twitch fibres have no mitochondria. They have fewer, not none.
- Mixing up myoglobin and haemoglobin. Myoglobin stores oxygen inside muscle fibres; haemoglobin carries it in red blood cells.
- Forgetting the T-tubules. They are how the action potential reaches the sarcoplasmic reticulum deep in the fibre.
Practise with a short timed paper
Skeletal muscle is in the second year of AQA A-level Biology (3.6.3). The daily A-level Biology: Year 2 paper is a short timed paper of 8 questions, with a new one every day.
Common questions
What is a muscle fibre?
What is the difference between slow and fast twitch muscle fibres?
Why do muscle fibres have many nuclei?
What is the sarcoplasmic reticulum?
Why are slow twitch fibres red?
What are the three types of muscle?
Revision cards
Say the answer first, then flip the card to check it.
Where to go next
- Solve it
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Biology · Nerves and homeostasisSliding filament theory: muscle contraction step by stepThe sliding filament theory for A-level Biology: sarcomere bands, the roles of calcium, tropomyosin and ATP, the cross-bridge cycle and muscle fibre types.