Muscle Relaxation and Related Physiology

Introduction

• This lecture focuses on the relaxation phase of muscle contraction.
• Review of muscle contraction and the transition to relaxation.

Action Potential and Ion Channel Dynamics

• Voltage-Gated Channels
  • Sodium channels open, causing peak contraction at +30mV.
  • Potassium channels open at peak potential, allowing efflux of K+.
• Repolarization Phase
  • Efflux of K+ leads to repolarization.
  • Inactivation gates of potassium channels slowly close, completing at resting membrane potential (-90mV).
• Calcium Dynamics
  • Calcium binds to troponin during contraction, changing tropomyosin configuration.
  • Upon relaxation, calcium returns to the sarcoplasmic reticulum via calcium ATPase and sodium-calcium exchangers.

Sarcomere and Muscle Relaxation

• Tropomyosin blocks actin-binding sites as calcium detaches, preventing myosin binding.
• Sarcomere returns to resting state: H-zone reappears, I-band returns to normal.

Clinical Correlations

Autoimmune Disorders

• Myasthenia Gravis
  • Autoimmune disorder where antibodies block nicotinic receptors, preventing acetylcholine binding.
  • Leads to muscle weakness, paralysis.
• Lambert-Eaton Syndrome
  • Antibodies block calcium channels, preventing acetylcholine release.

Toxins and Drugs

• Tetanus and Botulinum Toxins
  • Tetanus: Prevents GABA release, causing spastic paralysis.
  • Botulinum: Prevents acetylcholine release, used in Botox.
• Snake Venoms
  • Dendrotoxin: Blocks K+ channels, leading to convulsions.
  • Bungarotoxin: Blocks nicotinic receptors, causing paralysis.
• Succinylcholine
  • Mimics acetylcholine, causes temporary muscle relaxation.

Pharmacological Interventions

• Acetylcholinesterase Inhibitors
  • Drugs like neostigmine increase acetylcholine levels by inhibiting breakdown.
  • Used to manage myasthenia gravis symptoms.

Conclusion

• Understanding action potentials and muscle physiology is critical in both clinical and pharmacological contexts.
• Important to recognize the impact of disorders and toxins on muscular function.