Muscle Contraction

Skeletal muscle is the effector that produces voluntary movement. The mechanism of contraction — the sliding filament theory — is a core topic at A-Level, requiring detailed knowledge of the structure of muscle fibres, the sarcomere, and the roles of specific proteins. Understanding muscle contraction also links to topics on the nervous system, ATP, and cellular respiration.

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.

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Structure of Skeletal Muscle

Skeletal muscle is organised in a hierarchical structure:

1. Muscle — the whole organ, attached to bones by tendons.
2. Muscle fascicles — bundles of muscle fibres wrapped in 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.
5. Sarcomere — the functional unit of contraction.

Key Features of Muscle Fibres

• Multinucleate: Muscle fibres form by the fusion of many embryonic cells (myoblasts), so each fibre contains many nuclei located at the periphery of the cell.
• Sarcoplasm: The cytoplasm of the muscle fibre, rich in glycogen granules (energy store) and myoglobin (an oxygen-binding protein similar to haemoglobin that stores O₂).
• Sarcoplasmic reticulum (SR): A specialised form of smooth endoplasmic reticulum that stores and releases calcium ions (Ca²⁺). The SR wraps around each myofibril.
• T-tubules (transverse tubules): Invaginations of the sarcolemma (cell membrane) that penetrate deep into the muscle fibre, allowing action potentials to rapidly reach the interior of the cell and the sarcoplasmic reticulum.
• Mitochondria: Present in very large numbers between myofibrils, providing ATP by aerobic respiration for muscle contraction.

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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.

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.
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.
H zone	The central, lighter region of the A band; contains myosin only (no overlap with actin).
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:

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

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The Proteins of the Sarcomere

Myosin (Thick Filaments)

• Each myosin molecule has a tail and two globular heads.
• The heads contain ATPase activity (they can hydrolyse ATP).
• The heads can bind to actin, forming cross-bridges.
• Many myosin molecules are bundled together with their heads protruding outwards at regular intervals.

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.
• 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.
• At rest, tropomyosin blocks the myosin binding sites on actin, preventing cross-bridge formation.

Troponin