Skeletal Muscle and Sliding Filament Theory

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 Hugh Huxley and Andrew Huxley in 1954, is one of the most elegant and thoroughly proven theories in physiology. This lesson covers the structure of skeletal muscle and the molecular mechanism of contraction, in line with OCR A-Level Biology A specification 5.1.5(h)–(j).

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.

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Levels of Organisation

Skeletal muscle has a beautifully hierarchical structure. Understanding each level of organisation makes the contraction mechanism much easier to grasp.

1. Whole muscle (e.g. biceps) — a bundle of muscle fibres wrapped in epimysium.
2. Muscle fibre — a single multinucleate cell, up to 30 cm long, formed by fusion of many myoblasts during development. Surrounded by the sarcolemma.
3. Myofibril — a cylindrical organelle within the fibre, ~1 μm across, running the full length of the cell. Each fibre contains hundreds of myofibrils packed in parallel.
4. Sarcomere — a repeating unit along the length of the myofibril, about 2.5 μm long when relaxed.
5. Filaments — actin (thin) and myosin (thick) proteins arranged within each sarcomere.

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.

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The Sarcomere — Anatomy of the Functional Unit

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.

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The Proteins in Detail

Actin

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:

• Tropomyosin — a long rod-shaped protein that, when the muscle is at rest, covers the myosin-binding sites on actin, preventing contraction.
• Troponin — a globular complex of three subunits (TnT, TnI, TnC) bound to tropomyosin. TnC has a binding site for calcium ions. When calcium binds, troponin changes shape, pulling tropomyosin aside and uncovering the myosin-binding sites.

Myosin

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.

Other Structural Proteins

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.

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The Sliding Filament Theory — The Cross-Bridge Cycle

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.

1. Resting State

• Calcium is sequestered in the sarcoplasmic reticulum (smooth ER of muscle cells).
• Tropomyosin covers the myosin-binding sites on actin.
• Myosin heads are "cocked" and loaded with ADP + Pi, but cannot bind.
• The muscle is relaxed.

2. Action Potential Arrives

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.

3. Calcium Release

The T-tubule action potential triggers the sarcoplasmic reticulum to release stored Ca²⁺ into the sarcoplasm. Intracellular Ca²⁺ rises sharply.

4. Tropomyosin Moves

Ca²⁺ binds to troponin C, causing a conformational change. Troponin pulls tropomyosin away from the myosin-binding sites on actin.

5. Cross-Bridges Form

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.

6. Power Stroke