7.9 Muscle Contraction - Sliding Filament Model

The sliding filament theory of contraction states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree. This model explains how muscles shorten and generate force.

Definition

Sliding Filament Model: The sliding filament theory of contraction states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree.

Main Idea

• During contraction, thin myofilaments slide past thick ones.
• They overlap to a greater degree.
• In a relaxed muscle fibre, thick and thin myofilaments overlap only at the ends of the A band.
• When muscle fibres are stimulated by the nervous system, the myosin heads attach to myosin binding sites on actin in the thin myofilaments.
• Sliding begins.
• These links are called cross bridges.
• Cross bridges are formed and broken several times during a contraction.
• They act like tiny ratchets to generate tension and propel the thin myofilaments toward the centre of the sarcomere.
• As this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens.
• The I bands shorten.
• The distance between successive Z discs is reduced.
• The H zone disappears.
• The contiguous A bands move closer together but do not change in length.

Core Concept

• Sliding filament model = thin filaments slide past thick filaments
• Cross bridges = links between myosin heads and actin
• Cross bridges = formed and broken several times
• Cross bridges = act like tiny ratchets
• **I bands** = shorten
• Z discs = move closer together
• **H zone** = disappears
• A bands = move closer together but do not change in length
• Muscle cell = shortens

Detailed Explanation

Muscle Contraction - Sliding Filament Model

• The sliding filament theory of contraction states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree.
• In a relaxed muscle fibre, the thick and thin myofilaments overlap only at the ends of the A band.
• But when muscle fibres are stimulated by the nervous system, the myosin heads are attached on to myosin binding sites on actin in the thin myofilaments, and the sliding begins.
• These links are called cross bridges which are formed and broken several times during a contraction, acting like tiny ratchets to generate tension and propel the thin myofilaments toward the centre of the sarcomere.
• As this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens.
• The I bands shorten.
• The distance between successive Z discs is reduced.
• The H zone disappears.
• The contiguous A bands move closer together but do not change in length.

Control of Cross Bridges

• Muscle contraction is initiated by nerve impulse arriving at the neuromuscular junction.
• The nerve impulse is carried through the sarcolemma to the T tubule then to the sarcoplasmic reticulum (SR).
• The calcium gates of the SR open releasing calcium into the cytosol.
• When muscle is at rest the tropomyosin is disposed in such a way that it covers the sites on the actin chain where the heads of myosin become attach.
• When calcium ions bind with the troponin molecules they cause them to move slightly.
• This has the effect of displacing the tropomyosin and exposing the binding sites for the myosin head.
• Once the myosin head has become attached to the actin filament, ATP is hydrolysed and the crossed bridges are broken down.
• The formation and breakdown of cross bridges occur again and again during the sliding of the filament.

Summary Table of Sliding Filament Model

Summary of the Process

Intermediate Level

Sliding Filament Model

Control of Cross Bridges

Role of Calcium and ATP

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Sliding filament theory explains muscle contraction.
• 2. Thin myofilaments slide past thick ones.
• 3. They overlap more during contraction.
• 4. In relaxed muscle, overlap is only at ends of A band.
• 5. Myosin heads attach to actin.
• 6. Links are called cross bridges.
• 7. Cross bridges formed and broken several times.
• 8. Cross bridges act like tiny ratchets.
• 9. They generate tension.
• 10. They propel thin myofilaments toward centre of sarcomere.
• 11. I bands shorten.
• 12. Z discs move closer.
• 13. H zone disappears.
• 14. A bands move closer but do not change in length.
• 15. Muscle cell shortens.
• 16. Calcium is released from sarcoplasmic reticulum.
• 17. Calcium binds to troponin.
• 18. Tropomyosin is displaced.
• 19. Binding sites for myosin are exposed.
• 20. ATP is hydrolysed to break cross bridges.

Remember

• - Sliding filament model = thin filaments slide past thick filaments
• - Cross bridges = links between myosin heads and actin
• - Cross bridges = formed and broken several times
• - Cross bridges = act like tiny ratchets
• - **I bands** = shorten
• - Z discs = move closer together
• - **H zone** = disappears
• - A bands = move closer together but do not change in length
• - Muscle cell = shortens
• - Calcium = binds to troponin
• - ATP = hydrolysed to break cross bridges

Common Mistakes

• 1. Thinking A bands shorten — A bands move closer but do not change in length.
• 2. Confusing the changes in bands — I bands shorten; H zone disappears; A bands remain constant.
• 3. Forgetting the role of calcium — Binds to troponin; displaces tropomyosin.
• 4. Mixing up troponin and tropomyosin — Troponin binds calcium; tropomyosin covers binding sites.
• 5. Forgetting the role of ATP — Hydrolysed to break cross bridges.
• 6. Thinking cross bridges form once — Cross bridges form and break several times.
• 7. Mixing up the sliding direction — Thin myofilaments slide toward centre of sarcomere.
• 8. Believing the muscle fibre lengthens — Muscle fibre shortens.
• 9. Forgetting the role of the T tubule — Carries nerve impulse to sarcoplasmic reticulum.
• 10. Mixing up the steps — Nerve impulse, calcium release, cross bridge formation, sliding.

Quick Revision

The sliding filament theory of contraction states that during contraction the thin myofilaments slide past the thick ones so that they overlap to a greater degree. In a relaxed muscle fibre, the thick and thin myofilaments overlap only at the ends of the A band. But when muscle fibres are stimulated by the nervous system, the myosin heads are attached on to myosin binding sites on actin in the thin myofilaments, and the sliding begins. These links are called cross bridges which are formed and broken several times during a contraction, acting like tiny ratchets to generate tension and propel the thin myofilaments toward the centre of the sarcomere. As this event occurs simultaneously in sarcomeres throughout the cell, the muscle cell shortens. The I bands shorten, the distance between successive Z discs is reduced, the H zone disappears, and the contiguous A bands move closer together but do not change in length. Muscle contraction is initiated by nerve impulse arriving at the neuromuscular junction. The nerve impulse is carried through the sarcolemma to the T tubule then to the sarcoplasmic reticulum (SR). The calcium gates of the SR open releasing calcium into the cytosol. When muscle is at rest the tropomyosin is disposed in such a way that it covers the sites on the actin chain where the heads of myosin become attach. When calcium ions bind with the troponin molecules they cause them to move slightly. This has the effect of displacing the tropomyosin and exposing the binding sites for the myosin head. Once the myosin head has become attached to the actin filament, ATP is hydrolysed and the crossed bridges are broken down. The formation and breakdown of cross bridges occur again and again during the sliding of the filament.

60-Second Revision

• Sliding filament model = thin filaments slide past thick filaments
• Cross bridges = links between myosin heads and actin
• Cross bridges = formed and broken several times
• Cross bridges = act like tiny ratchets
• I bands = shorten
• Z discs = move closer together
• H zone = disappears
• A bands = move closer together but do not change in length
• Muscle cell = shortens
• Calcium = binds to troponin
• ATP = hydrolysed to break cross bridges
• Warning: A bands do not change in length during contraction

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• 7. Support and Movement

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• 7.10 ANTAGONISTIC ARRANGEMENT OF SKELETAL MUSCLES

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