Muscle Cell Contraction and Calcium Role

Introduction

• Action potential: Initiation of muscle cell stimulation.
• Calcium release: Key to muscle contraction.
• Muscle cell architecture: Specialized for efficient calcium distribution.

Role of T-tubules

• T-tubules structure: Deep tubular invaginations in the plasma membrane.
• Function: Facilitate rapid spread of action potential into the cell.

Action Potential and Calcium Release

• Depolarization wave: Action potential spreads from synapse through the plasma membrane.
• Calcium release channel: Triggered by voltage-sensitive proteins.
• Sarcoplasmic reticulum: Major calcium store in muscle cells.
  • Releases burst of calcium ions into the cytosol.

Muscle Contraction Mechanism

• Contractile bundle (Myofibril): Location of contraction.
• Sarcomere: Basic unit of a contractile bundle.
• Protein filaments:
  • Actin (thin) and Myosin (thick): Interaction causes contraction.
  • Tropomyosin: Covers myosin binding sites on actin in absence of calcium.

Role of Troponin and Tropomyosin

• Troponin: Calcium-sensitive complex attached to tropomyosin.
• Calcium Binding: Troponin changes conformation, moving tropomyosin off.
• Exposure of Myosin Binding Sites: Allows muscle contraction by myosin motors crawling along actin.

Contraction Cycle and Relaxation

• Calcium Pump: Returns calcium to the sarcoplasmic reticulum post-contraction.
• Relaxation: Absence of calcium, myosin releases actin, and filaments return to original positions.