Muscle Contraction

This lesson is mapped to AQA 7402 Section 3.6.2 — muscle contraction, structure of skeletal muscle, sliding-filament theory (refer to the official AQA specification document for exact wording). Skeletal muscle is the effector of the somatic nervous system — the structure that converts the all-or-nothing electrical events covered in lessons 0–2 into the mechanical work of movement, posture, breathing and locomotion. It is one of the most metabolically active tissues in the body (a sprinting muscle can increase its ATP turnover several hundred-fold within seconds of activation) and one of the most elegant examples of nanometre-scale molecular machinery in biology.

The sliding-filament theory is associated with the independent 1954 publications of Andrew Huxley with Niedergerke and of Hugh Huxley with Hanson (paraphrased here as their convergent demonstration from light- and electron-microscopy that the A band does not shorten during contraction). Their model — that contraction occurs by relative motion of two interdigitated filament arrays rather than by any change in individual filament length — replaced earlier ideas in which the proteins themselves were thought to coil up. The mechanism has since been extended at molecular resolution by structural biology, but the original A-band-constancy observation is still the load-bearing piece of evidence and is examined directly in AQA mark schemes.

Key Definition: The sliding-filament theory states that muscle contraction occurs when actin (thin) filaments slide over myosin (thick) filaments, shortening the sarcomere, without the filaments themselves changing length. The mechanical work is performed by ATP-driven conformational changes of the myosin head.

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Structure of Skeletal Muscle

Skeletal muscle is organised in a strict hierarchical structure, each level wrapped in a connective-tissue sheath that ultimately fuses into the tendon transmitting force to bone:

1. Muscle — the whole organ, attached to bones by tendons (made of dense collagen).
2. Muscle fascicles — bundles of muscle fibres wrapped in perimysium connective tissue.
3. Muscle fibres (muscle cells) — individual multinucleated cells, typically 10–100 µm in diameter and up to several centimetres long.
4. Myofibrils — cylindrical organelles running the length of the muscle fibre, composed of repeating units called sarcomeres. A single muscle fibre contains hundreds to thousands of parallel myofibrils.
5. Sarcomere — the functional unit of contraction, ~2.2 µm long at rest.

Key Features of Muscle Fibres

• Multinucleate: Muscle fibres form during development by the fusion of many embryonic cells (myoblasts), so each fibre contains many nuclei located at the periphery of the cell. This is necessary because a single nucleus could not support transcription for a cell that is hundreds of times longer than a typical somatic cell.
• Sarcoplasm: The cytoplasm of the muscle fibre, rich in glycogen granules (immediate energy store, hydrolysed to glucose-1-phosphate by glycogen phosphorylase during exercise) and myoglobin (an oxygen-binding protein similar to haemoglobin but with a single subunit and higher O₂ affinity that stores oxygen and releases it at the very low partial pressures encountered during sustained exercise).
• Sarcoplasmic reticulum (SR): A specialised form of smooth endoplasmic reticulum that stores and releases calcium ions (Ca²⁺). The SR wraps around each myofibril and accumulates Ca²⁺ via the SERCA pump (ATP-driven Ca²⁺-ATPase) — the SR Ca²⁺ concentration is ~1 mM, around 10,000× the resting cytosolic concentration.
• T-tubules (transverse tubules): Invaginations of the sarcolemma (cell membrane) that penetrate deep into the muscle fibre at every Z line, allowing action potentials to rapidly reach the interior of the cell. T-tubules form triads with adjacent SR cisternae — this is the electrical-to-chemical coupling point.
• Mitochondria: Present in very large numbers between myofibrils, providing ATP by aerobic respiration for muscle contraction. Slow-twitch fibres have many more mitochondria than fast-twitch fibres.

graph TD
    A["Muscle"] --> B["Fascicle"]
    B --> C["Muscle fibre<br/>multinucleate cell"]
    C --> D["Myofibril<br/>repeating sarcomeres"]
    D --> E["Sarcomere<br/>~2.2 µm"]
    E --> F["Thick filaments<br/>myosin"]
    E --> G["Thin filaments<br/>actin + tropomyosin + troponin"]

    style C fill:#27ae60,color:#fff
    style E fill:#3498db,color:#fff

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The Sarcomere

The sarcomere is the repeating unit of a myofibril, bounded by Z lines (also called Z discs). Each sarcomere is approximately 2.2 µm long at rest, and shortens by up to ~30% during full contraction.

Bands and Lines

Structure	Description
Z line	The boundary of the sarcomere; anchor point for actin filaments
I band	Light band; contains actin only (no overlap with myosin). Bisected by the Z line. Shortens during contraction.
A band	Dark band; the full length of the myosin filaments. Includes regions of overlap with actin. The A band does not change length during contraction — this is the load-bearing evidence for the sliding-filament theory.
H zone	The central, lighter region of the A band; contains myosin only (no overlap with actin). Shortens during contraction.
M line	The centre of the sarcomere and the H zone; anchor point for myosin filaments.

Changes During Contraction

During contraction, the actin filaments slide inwards over the myosin filaments. The diagnostic observations are:

• I band gets shorter (more overlap between actin and myosin).
• H zone gets shorter or disappears (actin filaments extend further into the centre, into the previously myosin-only region).
• A band stays the same length (myosin filaments do not change length — the key Huxley observation).
• Sarcomere gets shorter (Z lines move closer together).
• The filaments themselves do not shorten — only the overlap changes.

Exam Tip: Examiners regularly test the band changes. The pattern is: I band ↓, H zone ↓, A band unchanged, sarcomere ↓. Practise sketching the relaxed and contracted sarcomere side by side.

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The Proteins of the Sarcomere

Myosin (Thick Filaments)

• Each myosin molecule has a tail (two intertwined alpha-helical "coiled-coil" rods) and two globular heads that project laterally.
• The heads contain ATPase activity (they can hydrolyse ATP) and an actin-binding site.
• The heads can bind to actin, forming cross-bridges that span the gap between the thick and thin filaments.
• Many myosin molecules are bundled together with their heads protruding outwards at regular intervals (~14 nm apart) to produce the thick filament. The thick filament is bipolar — the heads at each end point toward the nearer Z line, so the two ends pull in opposite directions and the sarcomere shortens symmetrically.

Actin (Thin Filaments)

• Actin is a globular protein (G-actin) that polymerises into long chains (F-actin). Two F-actin chains twist around each other to form the thin filament backbone.
• Each actin monomer has a myosin binding site.

Tropomyosin

• A fibrous protein that winds around the actin filament in the groove between the two F-actin chains, covering seven consecutive actin monomers.
• At rest, tropomyosin blocks the myosin binding sites on actin, preventing cross-bridge formation. This is the molecular safety catch that prevents continuous contraction.

Troponin

• A globular protein complex of three subunits attached to tropomyosin at regular intervals along the actin filament.
• Troponin has a binding site for calcium ions (Ca²⁺) on its TnC subunit.
• When Ca²⁺ binds to troponin, it causes a conformational change that moves tropomyosin away from the myosin binding sites on actin, exposing them for cross-bridge formation.

Titin (beyond-spec extension)

• An enormous (~3.7 MDa) elastic protein spanning each half-sarcomere from Z line to M line.
• Acts as a molecular spring, generating passive elastic tension when a relaxed muscle is stretched and helping to restore sarcomere length on relaxation. Not required for AQA but useful for A* synthesis: it explains why a stretched muscle springs back even without an active contraction.

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The Mechanism of Muscle Contraction (Cross-Bridge Cycle)

The following steps describe the sliding-filament mechanism. The cycle is what produces force; the cycle repeats many times per contraction and many cross-bridges cycle asynchronously across the sarcomere.

1. Stimulation

• A nerve impulse arrives at the neuromuscular junction (a specialised cholinergic synapse between a motor neurone and a muscle fibre — see lesson 2).
• Acetylcholine is released, opens nicotinic ligand-gated Na⁺ channels in the sarcolemma, depolarising it.
• The action potential travels along the sarcolemma and down the T-tubules.
• The T-tubules stimulate the sarcoplasmic reticulum to release Ca²⁺ ions into the sarcoplasm via voltage-coupled Ca²⁺-release channels (ryanodine receptors).

2. Exposure of Binding Sites

• Ca²⁺ ions bind to troponin on the actin filament.
• This causes troponin to change shape, pulling tropomyosin away from the myosin binding sites on actin.
• The binding sites are now exposed.

3. Cross-Bridge Formation

• The myosin head, which is in an energised (cocked) state and has ADP + Pᵢ bound to it, binds to the exposed site on actin, forming a cross-bridge.
• Pᵢ is released first, followed by ADP, on cross-bridge formation.

4. The Power Stroke

• The myosin head changes angle (pivots ~45°), pulling the actin filament towards the M line (centre of the sarcomere).
• This is the power stroke — the step that produces force and movement (~5 pN per cross-bridge; ~10 nm of displacement per stroke).
• The sarcomere shortens as the actin filaments slide over the myosin.

5. Detachment

• A new molecule of ATP binds to the myosin head.
• This causes the myosin head to detach from actin, breaking the cross-bridge.
• Without ATP, the cross-bridge cannot be broken — this is the molecular basis of rigor mortis: after death, ATP supply ceases, all attached cross-bridges remain bound, and the muscle becomes stiff. Rigor mortis resolves only when muscle proteins begin to degrade.

6. Re-Energising (Re-Cocking)

• The ATP bound to the myosin head is hydrolysed by myosin ATPase to ADP + Pᵢ.
• The energy released re-cocks the myosin head into its high-energy position (the "lever arm" swings back), ready to bind to the next actin binding site further along the filament.

7. Repeat

• The cycle repeats as long as Ca²⁺ and ATP are available, with each cycle pulling the actin a little further. At maximal activation, each myosin head completes ~5 cycles per second; with ~300 heads per thick filament and thousands of thick filaments per fibre, this produces the smooth macroscopic contraction.
• Many cross-bridges form and release asynchronously — at any instant most heads are attached, producing sustained force.

Relaxation

• When nerve impulses stop, Ca²⁺ is actively pumped back into the sarcoplasmic reticulum by SERCA (using ATP).
• Without Ca²⁺, troponin returns to its original shape, tropomyosin covers the binding sites, and cross-bridges can no longer form.
• The muscle returns to its resting length (with the help of antagonistic muscles, gravity, or titin-mediated elastic recoil).

Exam Tip: ATP is used in three stages of muscle contraction: (1) hydrolysis by myosin ATPase to energise the myosin head (re-cocking); (2) binding to myosin to cause detachment from actin (breaking the cross-bridge); and (3) active transport of Ca²⁺ back into the sarcoplasmic reticulum by SERCA during relaxation. A common A* discriminator is naming all three uses, not just one.

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ATP Supply During Muscle Contraction

Skeletal muscle can call on three ATP-supplying systems in series:

1. Stored ATP — only enough for ~2 seconds of maximal contraction.
2. Creatine phosphate (phosphocreatine, PCr) — ADP + PCr → ATP + creatine. Creatine kinase catalyses this rapid phosphate transfer; PCr is the muscle's fast-access energy buffer, good for ~5–10 s of maximal effort. (Beyond the strict AQA 7402 spec but examined synoptically in sport-physiology contexts.)
3. Anaerobic respiration (glycolysis → lactate) — supplies ATP for ~60–120 s of high-intensity work; produces lactate which lowers cytosolic pH and contributes to muscle fatigue.
4. Aerobic respiration — the only sustainable supply for prolonged contraction; requires continuous O₂ delivery via the circulatory system and is favoured in slow-twitch fibres.

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Fast and Slow Twitch Muscle Fibres

Skeletal muscle contains two main types of fibre, each adapted to a different mechanical and metabolic profile:

Feature	Slow Twitch (Type I)	Fast Twitch (Type II)
Speed of contraction	Slow (~100 ms time-to-peak)	Fast (~30 ms)
Fatigue resistance	High (fatigue-resistant)	Low (fatigue quickly)
Main energy source	Aerobic respiration	Anaerobic respiration (glycolysis)
Mitochondria	Many	Few
Myoglobin content	High (appear red)	Low (appear pale / white)
Glycogen stores	Lower	Higher
Capillary density	High	Lower
Myosin ATPase activity	Low	High
Example use	Endurance (postural muscles, marathon running)	Short bursts of power (sprinting, jumping, weightlifting)
Example muscle	Soleus	Gastrocnemius (mixed); ocular muscles (almost pure type II)

Training can shift the proportion of fibre types modestly, but the broad endowment is genetic; elite sprinters and marathon runners differ measurably in fibre composition.

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A-Level Deep Dive

Spec mapping

This content sits in AQA 7402 Section 3.6.2 — skeletal muscle, sliding-filament theory, fibre types (refer to the official AQA specification document for exact wording). Examined on Paper 2 and synoptically on Paper 3, particularly in essay questions linking respiration with movement.

Synoptic links

This lesson connects to: