Understanding Skeletal Muscle Contraction

Aug 30, 2024

Skeletal Muscle Contraction

Overview

  • Focus on skeletal muscle contraction.
  • Review of action potential transmission and calcium release.
  • Transition to muscle contraction mechanics.

Action Potential Transmission

  1. Motor Neuron Activity
    • Somatic motor neurons fire action potentials.
    • Movement of action potentials down the axon.
  2. Ion Movement
    • Voltage-gated sodium channels open, allowing sodium influx.
    • Sodium influx causes depolarization (more positive cell interior).
  3. Synaptic Vesicle Fusion
    • Calcium influx links synaptic proteins (SNAP-25, synaptobrevin, synaptotagmin, syntaxin).
    • Fusion of vesicles with the synaptic membrane releases acetylcholine.

Acetylcholine and Neuromuscular Junction

  1. Acetylcholine (ACh) Release
    • ACh synthesized from acetyl CoA (from mitochondria) and dietary choline.
    • Released into the synaptic cleft.
  2. ACh Receptors
    • Binds to nicotinic receptors on muscle cell membrane.
    • Sodium influx dominates over potassium efflux, creating an end plate potential (EPP).

Calcium Release and Muscle Contraction

  1. T-Tubule and SR Interaction
    • T-Tubule depolarization affects sarcoplasmic reticulum (SR).
    • Dihydropyridine receptors pull on ryanodine receptors, releasing calcium.
  2. Calcium's Role
    • Calcium binds to troponin, shifting tropomyosin to expose myosin-binding sites on actin.
  3. Sliding Filament Theory
    • ATP hydrolysis "cocks" myosin heads.
    • Myosin binds to actin, performs a power stroke as phosphate is released.
    • Myosin heads detach upon ATP binding, restarting the cycle.

Sarcomere Changes During Contraction

  • H Zone: Disappears as actin slides over myosin.
  • Z-Disc: Moves closer together.
  • A Band: Stays the same.
  • I Band: Decreases or disappears.

Importance of Titin

  • Elastic protein maintaining sarcomere integrity.
  • Prevents overextension during contraction.

Next Steps

  • Future video will address muscle relaxation.
  • Discuss relaxation mechanisms: potassium channels, calcium channels, sodium-calcium exchangers, and SR proteins.
  • Clinical correlations to be explored.

Summary

  • Reviewed action potential and acetylcholine role in muscle contraction.
  • Detailed sliding filament theory.
  • Prepared for discussion on muscle relaxation and associated clinical aspects.