Muscle Cell – Actin, Myosin & Molecular Motors

Muscle Structure Overview

  • Skeletal muscles → bundles (fascicles) of elongated, multinucleated muscle fibers
  • Muscle fiber components:
    • Sarcolemma = plasma membrane
    • Sarcoplasm = cytoplasm
    • Sarcoplasmic reticulum (SR) = specialized Ca2+\text{Ca}^{2+} store
    • Myofibrils run full fiber length; anchored to sarcolemma

Sarcomere Anatomy

  • Functional contractile unit (Z-disk → Z-disk)
  • Bands
    • II band = light (thin filaments only)
    • AA band = dark (overlap of thick + thin; thick entire length)
  • Filaments
    • Thick = myosin
    • Thin = actin (with regulatory proteins)

Major Muscle Proteins

Myosin
  • Two globular heads + long tail; heads form actin & ATP binding sites
  • Proteolysis: S1 (head) fragment released by trypsin/papain
  • Hundreds aggregate tail-to-tail → bipolar thick filament (~325 nm325\,\text{nm})
Actin
  • FF-actin = helical polymer of GG-actin dimers (double helix)
  • Each monomer exposes myosin-binding active site

Sliding Filament Model

  • Cross-bridge cycling pulls thin filaments toward sarcomere center → Z-disks move closer → fiber shortens

Excitation–Contraction Coupling

  1. Motor neuron action potential → ACh\text{ACh} release at neuromuscular junction
  2. Sarcolemma depolarization propagates via T-tubules
  3. SR releases Ca2+\text{Ca}^{2+} into cytosol
  4. Ca2+\text{Ca}^{2+} binds troponin → tropomyosin shifts, exposing actin sites
  5. Cross-bridge cycle (see below) generates tension
  6. Ca2+\text{Ca}^{2+} re-sequestered by ATP-driven SR pumps → relaxation

Cross-Bridge Cycle (per head)

  • Ca2+\text{Ca}^{2+} exposes actin site → myosin (ADP·Pi) binds (cross-bridge)
  • Pi release → power stroke; ADP released
  • New ATP\text{ATP} binds → detachment
  • ATP hydrolysis → head re-cocks (ADP·Pi)

Regulation of Contraction & Calcium Role

  • Continuous ATP-driven SR pump maintains low cytosolic Ca2+\text{Ca}^{2+} at rest
  • Contraction: SR releases Ca2+\text{Ca}^{2+}
  • Relaxation: pump resequesters Ca2+\text{Ca}^{2+} (ATP dependent)

ATP Role and Rigor Mortis

  • ATP required for: SR Ca2+\text{Ca}^{2+} pump, myosin detachment, head re-cocking
  • Post-mortem: pH↓ → ATP production stops → SR leaks Ca2+\text{Ca}^{2+} → tropomyosin off actin → myosin binds but cannot detach → rigor mortis