Sliding Filament Theory Lecture Notes

Overview

• The sliding filament theory explains how muscles contract by the sliding action of actin and myosin filaments.
• It describes how myosin heads pull actin filaments closer, reducing the sarcoma (sarcomere) distance and causing muscle contraction.

Key Components

• Myofibrils: Comprised of actin (thin) and myosin (thick) filaments.
• Sarcoma (Sarcomere): The distance between two Z-lines in a myofibril; shortens during contraction.
• Myosin Heads: Attach to actin filaments and perform "power strokes" to pull them closer.

Sliding Filament Theory Process

1. Initial State

   • Myosin heads and actin filaments are not initially touching.
   • Calcium ions bind to troponin on actin filaments.

2. Attachment

   • Tropomyosin shifts due to calcium ion binding, exposing actin.
   • Myosin heads attach to actin forming cross-links.

3. Power Stroke

   • Myosin heads tilt backward, pulling actin filaments along, shortening the sarcoma.
   • This process is referred to as the "power stroke."

4. Detachment and Reattachment

   • Myosin heads release actin, hydrolyzing ATP into ADP and phosphate.
   • ATP binding resets and reenergizes the myosin heads for subsequent contractions.

Detailed Mechanism

• Troponin and Tropomyosin: Regulate the exposure of actin for myosin binding.
• Calcium Ions: Trigger the initial attachment of myosin to actin.
• ATP: Essential for detachment of myosin heads and reattachment for continuous contraction.

Steps in Detail

1. Calcium Binding

   • Calcium ions from the sarcoplasmic reticulum bind to troponin on actin.
   • Causes tropomyosin to change shape and expose actin sites.

2. Cross-Link Formation

   • Myosin heads attach to exposed actin filaments.

3. Power Stroke Execution

   • Myosin heads pull actin filaments inward, decreasing sarcoma distance.

4. ATP Hydrolysis

   • ATP is hydrolyzed, causing myosin heads to release actin.

5. ATP Rebinding

   • New ATP binds to myosin heads, allowing reattachment to actin.
   • Process repeats until maximum contraction or depletion of ATP.

Simplified Explanation for Exams

• At a minimum, remember:
  • Calcium ions bind to troponin.
  • Myosin heads bind and pull actin filaments.
  • Sarcoma distance decreases, leading to muscle contraction.

Conclusion

• The sliding filament theory describes the mechanism of muscle contraction through myosin and actin interactions.
• Understanding the detailed processes is crucial for full marks in exams.
• Remember the role of calcium, ATP, and structural proteins (troponin, tropomyosin) in the contraction cycle.