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Spec Mapping — OCR H420 Module 5.1.5 — Plant and animal responses, content statements covering the structure of skeletal muscle (sarcomere, myofibril, fibre), the sliding filament theory of muscle contraction, the role of Ca²⁺, ATP, troponin, tropomyosin, the neuromuscular junction, and the energy sources for muscle contraction including phosphocreatine and aerobic / anaerobic respiration (refer to the official OCR H420 specification document for exact wording). This lesson closes the loop from neuronal communication (motor neurone fires) through synaptic transmission (NMJ) to mechanical work (sarcomere shortening).
Every voluntary movement you make depends on skeletal muscle, and every contraction of skeletal muscle depends on a microscopic dance of protein filaments sliding past one another. The sliding filament theory, proposed by Andrew Huxley & Rolf Niedergerke and independently by Hugh Huxley & Jean Hanson in 1954 (paraphrase), is one of the most elegant and thoroughly proven theories in physiology. (Curiously, the two senior authors share a surname but are unrelated.) The theory was the first to explain muscle contraction in terms of filament sliding rather than filament shortening; subsequent decades of electron microscopy, biochemistry, and biophysics have provided unprecedented detail on how myosin motor proteins generate force at the molecular level.
The molecular details have a clear scientific lineage. Hugh Huxley (paraphrase) used X-ray diffraction and electron microscopy to characterise the interdigitating thick and thin filaments. Wilson Albert Engelhardt and Militsa Lyubimova (paraphrase, 1939) demonstrated that myosin is an ATPase, linking the protein directly to energy metabolism. Ebashi and Endo (paraphrase, 1968) discovered troponin and the role of Ca²⁺ in regulating contraction. Roger Cooke and Edwin Pate (paraphrase, 1980s) characterised the chemistry of the cross-bridge cycle, including the order of ATP binding, hydrolysis, and product release. Modern single-molecule biophysics has measured the step size of a single myosin head (~10 nm) and the force it generates (~3 pN). The textbook account at A-Level reflects this accumulated body of evidence.
Key Definitions:
- Sarcomere — the functional unit of a skeletal muscle fibre, from one Z line to the next.
- Myofibril — a longitudinal bundle of sarcomeres within a muscle fibre.
- Actin — the thin filament protein of the sarcomere.
- Myosin — the thick filament protein of the sarcomere, with ATPase activity.
- Tropomyosin and troponin — regulatory proteins associated with actin that control muscle contraction.
- Sliding filament theory — the explanation that contraction occurs when actin filaments slide over myosin filaments, shortening the sarcomere.
Skeletal muscle has a beautifully hierarchical structure. Understanding each level of organisation makes the contraction mechanism much easier to grasp.
Each level is a container for the level below. The striations you can see in a muscle cell under a light microscope are produced by the highly ordered alignment of sarcomeres across all the myofibrils in the fibre.
The sarcomere is the unit that does the work. OCR expects you to know its regions, and to be able to identify them on a diagram.
flowchart LR
Z1[Z line] -.- I1[I band] -.- A[A band] -.- I2[I band] -.- Z2[Z line]
A -.- H[H zone in middle]
H -.- M[M line in centre]
| Region | What it contains | Changes on contraction |
|---|---|---|
| Z line | Anchors actin filaments | Moves closer together |
| I band | Only actin (light, isotropic) | Shortens |
| A band | The whole length of myosin (dark, anisotropic) | Stays the same |
| H zone | Only myosin, no overlap with actin | Shortens |
| M line | Anchors myosin filaments in the middle | Unchanged (stays central) |
The key observation: during contraction, the A band does not change length, but the I band and H zone both shorten, and the Z lines move closer together. This immediately tells you that the myosin filaments cannot be getting shorter — they are simply being overlapped by more actin. This is the central evidence for sliding filament theory.
Actin is the thin filament. Two strands of globular (G-) actin monomers polymerise into a fibrous (F-) actin helix. Each actin monomer has a myosin-binding site. Wound around the actin helix is:
Myosin is the thick filament. Each myosin molecule has a globular head (with ATPase activity and an actin-binding site) and a long tail. About 300 myosin molecules associate tail-to-tail to form a thick filament, with the heads projecting outwards to form cross-bridges that can bind actin.
Myosin heads are the motor units of the muscle. Each one acts like a tiny lever: when it binds actin, it swings through an angle of ~45°, dragging the actin filament past the myosin.
OCR does not require detail on titin, nebulin, desmin, etc., but in case you see them in a diagram: titin is a giant elastic protein connecting the ends of myosin to the Z line, keeping the sarcomere organised.
Muscle contraction is a cycle of interactions between actin and myosin. Each cycle shortens the sarcomere by a tiny amount (~10 nm). Many cycles, in many sarcomeres, produce a visible contraction. OCR expects you to describe the cycle in detail.
An action potential arrives at the neuromuscular junction and is transmitted across the synapse by acetylcholine. This produces an action potential on the sarcolemma, which spreads along transverse (T) tubules deep into the fibre.
The T-tubule action potential triggers the sarcoplasmic reticulum to release stored Ca²⁺ into the sarcoplasm. Intracellular Ca²⁺ rises sharply.
Ca²⁺ binds to troponin C, causing a conformational change. Troponin pulls tropomyosin away from the myosin-binding sites on actin.
Myosin heads can now bind to the exposed sites on actin. Each myosin head attaches, forming a cross-bridge — like a hand grabbing a rope.
The myosin head pivots, swinging through about 45°. Because the other end of the myosin head is fixed to the thick filament, this movement drags the actin filament past the myosin. The Z line is pulled closer to the centre of the sarcomere. As this happens, ADP and Pi are released from the myosin head.
A new ATP molecule binds to the myosin head. This causes the myosin to release actin. (Without ATP, the cross-bridge cannot release — this is the basis of rigor mortis, when dead muscle stays contracted until the proteins degrade.)
The myosin head hydrolyses the bound ATP to ADP + Pi, using the energy to swing back into the "cocked" position, ready for the next stroke.
As long as Ca²⁺ remains elevated and ATP is available, the cycle repeats. Each individual stroke moves the actin by ~10 nm, but over many cycles a sarcomere can shorten dramatically.
When the action potentials stop, the sarcoplasmic reticulum actively pumps Ca²⁺ back in (using ATP). Cytosolic Ca²⁺ falls; troponin releases Ca²⁺; tropomyosin moves back to cover the binding sites; cross-bridge formation stops; the muscle relaxes (passive lengthening).
flowchart TB
AP[Action potential reaches neuromuscular junction] --> ACH[ACh released, depolarises sarcolemma]
ACH --> TT[Depolarisation spreads down T-tubules]
TT --> CA[Sarcoplasmic reticulum releases Ca2+]
CA --> TR[Ca2+ binds troponin]
TR --> TM[Tropomyosin moves; binding sites exposed]
TM --> XB[Myosin heads bind actin]
XB --> PS[Power stroke: actin pulled over myosin]
PS --> AD[ADP + Pi released]
AD --> AT[New ATP binds; myosin detaches]
AT --> HY[ATP hydrolysed; myosin re-cocks]
HY --> XB
CA --> PUMP[Eventually Ca2+ pumped back into SR]
PUMP --> REL[Muscle relaxes]
Muscle contraction needs ATP for several reasons:
This is why rigorous exercise is so metabolically expensive and why muscles contain vast numbers of mitochondria and large stores of glycogen.
Muscles can draw ATP from several sources during activity:
| Energy source | Time scale | How much ATP |
|---|---|---|
| Stored ATP | 1–2 seconds | Tiny |
| Phosphocreatine | 5–10 seconds | Instant but limited |
| Anaerobic glycolysis | 30 seconds – 2 minutes | Lactate produced |
| Aerobic respiration | Minutes onwards | Sustained, large amount |
Phosphocreatine (also called creatine phosphate) is a high-energy phosphate store that can rapidly regenerate ATP from ADP. Sprint athletes rely heavily on it, and creatine supplementation aims to boost it.
The neuromuscular junction is a specialised chemical synapse between a motor neurone and a muscle fibre. OCR does not require detailed knowledge beyond noting that it is structurally and functionally similar to the cholinergic synapse of lesson 5, but with the postsynaptic membrane being the motor end plate — a specialised region of the sarcolemma — rather than another neurone. The neurotransmitter is acetylcholine, released in the same way, and it opens nicotinic ACh receptors on the muscle fibre.
OCR does not test detailed knowledge of muscle fibre types, but it is useful context for appreciating how muscle adapts to different functional demands:
| Feature | Slow-twitch (Type I) | Fast-twitch (Type II) |
|---|---|---|
| Contraction speed | Slow | Fast |
| Fatigue resistance | High | Low |
| Colour | Red (myoglobin, mitochondria) | White or pink |
| Main energy system | Aerobic | Anaerobic / mixed |
| Example use | Posture, marathon running | Sprinting, jumping |
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