Lecture Notes on Muscle Contraction and the Sliding Filament Model

Introduction

• Muscle movement involves complex chemical processes.
• Sliding Filament Model: Describes the mechanism where muscle filaments slide across each other to create contraction.
• Muscle contraction begins with a signal from the brain and ends with the sliding filament action.

Excitation-Contraction Coupling

• Signal Origin: Begins in the brain, travels down the spinal cord, and through nerves to muscles.
• Neuromuscular Junction: Synapse between a neuron and muscle cell where the signal triggers muscle contraction.
  • Axon Terminal: Contains vesicles with neurotransmitters.
  • Muscle Fiber: Has receptors that interact with neurotransmitters.
  • Organelles: Transverse Tubule and Sarcoplasmic Reticulum
    • Transverse Tubule (T-tubule): Conducts signals into muscle cells.
    • Sarcoplasmic Reticulum (SR): Stores and releases calcium ions.

Process of Muscle Contraction

1. Action Potential: A signal from the brain reaches the axon terminal causing neurotransmitter release.
2. Synaptic Transmission: Neurotransmitters bind to muscle fiber receptors, causing sodium influx and depolarization.
3. Signal Propagation: Signal travels along the sarcolemma to T-tubules, reaching the SR.
4. Calcium Release: SR releases calcium ions into myofibrils causing interaction with myofilaments.
5. Myofilaments Interaction: In the presence of calcium, actin and myosin filaments engage in contraction, shortening the sarcomere.

Sliding Filament Model: Detailed Mechanics

• Components:
  • Myosin Filament (Thick Filament): Contains heads that bind to actin.
  • Actin Filament (Thin Filament): Has binding sites for myosin heads.
  • Regulatory Proteins:
    • Tropomyosin: Covers binding sites on actin.
    • Troponin: Binds with calcium and moves tropomyosin to expose binding sites.

Muscle Contraction Stages

1. Calcium Binding: Calcium binds to troponin causing tropomyosin to uncover binding sites.
2. Cross-Bridge Formation: Myosin heads bind to actin forming cross-bridges.
3. Power Stroke: Myosin heads pivot, pulling actin filaments toward the center of the sarcomere.
4. Release and Reset: ATP binds to myosin heads, breaking cross-bridges and resetting heads for another cycle.

Continuous Contraction

• As long as calcium and ATP are present, the cycle of grab, pull, release, and reset continues.
• Muscle contraction persists until calcium is sequestered back into SR.

Relaxation

• Calcium Reuptake: Calcium ions are pumped back into the SR, stopping contraction.
• Tropomyosin Blockage: Tropomyosin re-covers binding sites, leading to muscle relaxation.

Summary

• Muscle contraction is a complex process involving electrical signals, calcium ion regulation, and mechanical interactions between actin and myosin.
• Understanding each step of the sliding filament model is crucial for grasping how muscles contract and relax.