Muscle Contraction: The Sliding Filament Theory
Types of Muscle
Mammals have three types of muscle tissue:
| Type | Location | Control | Striations | Nuclei |
|---|---|---|---|---|
| Skeletal (striated) | Attached to bones | Voluntary (somatic nervous system) | Yes | Multinucleate |
| Cardiac | Heart wall | Involuntary; myogenic | Yes (with intercalated discs) | Usually one per cell |
| Smooth | Walls of hollow organs (gut, blood vessels, airways) | Involuntary (autonomic) | No | One per cell |
A-Level Biology focuses primarily on skeletal muscle.
Structure of Skeletal Muscle
From Whole Muscle to Myofibril
A skeletal muscle is composed of bundles of muscle fibres (cells). Each fibre is a single, very long cell formed by the fusion of many embryonic cells, which is why fibres are multinucleate (multiple nuclei, typically located at the periphery beneath the cell membrane).
Each muscle fibre contains many myofibrils — cylindrical structures running the length of the fibre, each approximately 1-2 μm in diameter. Myofibrils are composed of repeating units called sarcomeres, which are the functional units of contraction.
Key membrane structures:
- Sarcolemma — the cell surface membrane of a muscle fibre
- Sarcoplasmic reticulum (SR) — a specialised smooth endoplasmic reticulum that stores calcium ions (Ca²⁺)
- T-tubules (transverse tubules) — infoldings of the sarcolemma that penetrate deep into the fibre, allowing action potentials to reach the interior rapidly
The Sarcomere
A sarcomere extends from one Z-line (Z-disc) to the next. It contains two types of protein filament:
- Thick filaments — composed of myosin. Each myosin molecule has a globular head that can bind to actin and has ATPase activity, and a tail that interacts with other myosin molecules.
- Thin filaments — composed primarily of actin (a double helix of G-actin monomers), with associated regulatory proteins tropomyosin and troponin.
Banding Pattern
The arrangement of thick and thin filaments produces the characteristic striped (striated) appearance:
| Band/Zone | Composition | Appearance | During contraction |
|---|---|---|---|
| A-band (anisotropic) | Region where thick filaments are present (may overlap with thin) | Dark | Stays the same length |
| I-band (isotropic) | Region of thin filaments only (no overlap with thick) | Light | Gets shorter |
| H-zone | Central region of A-band with thick filaments only (no overlap with thin) | Lighter region within A-band | Gets shorter |
| Z-line | Anchor point for thin filaments; marks the boundary of each sarcomere | Thin dark line | Z-lines move closer together |
| M-line | Centre of the sarcomere; holds thick filaments together | Thin line in middle of H-zone | Stays in centre |
The Sliding Filament Theory
The sliding filament theory (Huxley and Hanson, 1954) explains how muscles contract. The key principle is that the thick and thin filaments slide past each other — they do NOT shorten themselves.
Step-by-Step Mechanism
1. Nerve Impulse Arrives
An action potential travels along a motor neurone to the neuromuscular junction. Acetylcholine (ACh) is released and binds to receptors on the sarcolemma, generating an action potential in the muscle fibre membrane.
2. Calcium Release
The action potential travels along the sarcolemma and down the T-tubules into the interior of the fibre. This stimulates the sarcoplasmic reticulum (SR) to release stored Ca²⁺ ions into the sarcoplasm (cytoplasm of the muscle fibre).
3. Exposure of Binding Sites
At rest, the myosin binding sites on actin are blocked by tropomyosin (a filamentous protein wound around the actin helix). Troponin holds tropomyosin in this blocking position.
When Ca²⁺ is released, it binds to troponin, causing a conformational change that moves tropomyosin away from the myosin binding sites on actin. The binding sites are now exposed.
4. Cross-Bridge Formation
The myosin heads, which are in a high-energy configuration (cocked position, with ADP + Pᵢ attached from previous ATP hydrolysis), bind to the exposed sites on actin, forming cross-bridges (actin-myosin cross-bridges).
5. Power Stroke
The myosin head undergoes a conformational change — it pivots, pulling the thin filament toward the centre of the sarcomere. This is the power stroke. ADP and Pᵢ are released from the myosin head during this movement. Each power stroke slides the filaments approximately 10 nm.
6. Detachment
A new molecule of ATP binds to the myosin head, causing it to detach from actin. (This is why muscles become rigid after death — rigor mortis — because without ATP, the cross-bridges cannot detach.)
7. Recovery Stroke
The myosin head hydrolyses the bound ATP (using its ATPase activity) → ADP + Pᵢ. The energy from hydrolysis returns the myosin head to its cocked (high-energy) position, ready to bind to the next actin binding site along the thin filament.
8. Repeat
As long as Ca²⁺ and ATP are available, the cycle repeats rapidly (each myosin head cycles ~5 times per second). The cumulative effect of thousands of cross-bridges cycling asynchronously produces smooth, sustained contraction.
The Cross-Bridge Cycle Summary
1. Ca²⁺ binds troponin → tropomyosin moves → binding sites exposed
2. Myosin head binds actin → cross-bridge forms
3. Power stroke → thin filament slides → ADP + Pᵢ released
4. ATP binds myosin → cross-bridge detaches
5. ATP hydrolysed → myosin head re-cocked
6. Cycle repeats
Relaxation
When nervous stimulation stops:
- Ca²⁺ is actively pumped back into the sarcoplasmic reticulum by Ca²⁺-ATPase pumps (requiring ATP)
- Ca²⁺ concentration in the sarcoplasm drops
- Ca²⁺ dissociates from troponin
- Tropomyosin slides back to block the myosin binding sites
- No more cross-bridges can form
- The muscle relaxes and returns to its resting length (pulled by antagonistic muscles or elastic recoil)
Energy for Contraction
Muscle contraction requires ATP for:
1. The power stroke — energy from ATP hydrolysis cocks the myosin head
2. Detachment of cross-bridges — ATP binding causes release from actin
3. Ca²⁺ reuptake — active transport back into the SR
ATP is supplied by:
- Creatine phosphate (phosphocreatine) — provides rapid but short-lived ATP regeneration (first ~10 seconds of intense exercise). Creatine phosphate + ADP → creatine + ATP (catalysed by creatine kinase)
- Aerobic respiration — the main source during sustained, moderate exercise
- Anaerobic respiration (glycolysis + lactate fermentation) — provides ATP when oxygen supply is insufficient during intense exercise, but produces lactate and is less efficient
Exam Tips
- AQA frequently asks you to describe the sequence of events from nerve impulse to contraction — learn the cross-bridge cycle in order
- Remember which bands/zones change during contraction: I-band and H-zone get SHORTER; A-band stays the SAME
- Always state that filaments SLIDE past each other — they do not shorten
- The role of ATP is commonly tested — it is needed for BOTH the power stroke (hydrolysis cocks the head) AND detachment (binding causes release)
- Link rigor mortis to the absence of ATP — cross-bridges cannot detach, so muscles remain stiff