Edexcel A Level (IAL) Biology -7.11 The Process of Skeletal Muscle Contraction- Study Notes- New Syllabus

Sliding Filament Theory of Skeletal Muscle Contraction

🌟 Introduction

Muscle contraction happens because myosin and actin filaments slide over each other, shortening the sarcomere. This sliding is tightly controlled by regulatory proteins and triggered by calcium ions.

🧩 Key Players and Their Roles

Actin (Thin filament)

• Long, twisted protein chain.
• Has binding sites for myosin heads.

Myosin (Thick filament)

• Has myosin heads that pull actin.
• Each head has:
  • ATP binding site
  • Actin binding site

Tropomyosin

• Long protein lying along actin.
• Blocks actin’s binding sites when the muscle is relaxed.

Troponin

• Attached to tropomyosin.
• Has a binding site for Ca²⁺ ions.
• Acts like a “switch” for contraction.

Calcium ions (Ca²⁺)

• Released from the sarcoplasmic reticulum (SR).
• Bind to troponin to start contraction.

ATP

• Powers myosin head movement.
• Needed for both cross bridge formation and detachment.

ATPase

• Enzyme on myosin head.
• Breaks ATP to release energy for the power stroke.

⚡ Full Process of Muscle Contraction (Sliding Filament Theory)

1. Nerve impulse arrives

• Travels down the motor neurone.
• Causes Ca²⁺ release from the sarcoplasmic reticulum into the sarcoplasm.

2. Calcium activates the actin sites

• Ca²⁺ binds to troponin.
• Troponin changes shape.
• Pulls tropomyosin away from the actin binding sites.
• Myosin heads can now attach.

Key line: Calcium removes the “tropomyosin block”.

3. Cross-bridge formation

• Myosin head (energised by ATP breakdown) attaches to actin.
• This connection is called a cross bridge.

4. Power stroke

• Myosin head bends, pulling actin toward the centre of the sarcomere.
• This uses energy from ATP hydrolysis by ATPase.
• Result: Sarcomere shortens and the muscle contracts.

5. Detachment

• A new ATP molecule binds to the myosin head.
• This breaks the cross bridge.
• Myosin releases actin.

6. Resetting (Re-cocking)

• ATP is hydrolysed by ATPase.
• Energy resets the myosin head to its upright position ready to bind again.

7. Cycle repeats

• As long as Ca²⁺ is present and ATP is available, cross bridges keep forming and actin keeps sliding.

🧊 Muscle Relaxation

• When the nerve impulse stops, Ca²⁺ is pumped back into the SR.
• Troponin returns to its original shape.
• Tropomyosin covers actin binding sites again.
• Myosin cannot attach, so sliding stops.
• Muscle fibre relaxes.

📌 Summary Table