Understanding Acetylcholine and Muscle Contraction

Sep 13, 2024

Lecture Notes: Acetylcholine and Muscle Contraction

Overview

  • Previous video: Neuromuscular junction and neurotransmission.
  • Focus: How acetylcholine stimulates muscle cells to develop end plate potential and action potential.

Synaptic Cleft and Acetylcholine

  • Synaptic Cleft: Space between neurons and muscle cell membrane.
  • Diffusion Process: Acetylcholine diffuses from high to low concentration across the synaptic cleft.
  • Post-Junctional Folds: Increases surface area to receive stimulus.

Ligand-Gated Ion Channels

  • Channels: Abundant at the post-junctional folds.
  • Nicotinic Receptor: A specific ligand-gated ion channel.
    • Pentameric Protein: Composed of 5 subunits (2 alpha, 1 beta, 1 gamma, 1 delta).

Resting Membrane Potential

  • Definition: Voltage difference across the muscle cell membrane at rest.
  • Muscle Cells: ~-90 mV, more negative than neurons (~-70 mV).
  • Maintenance:
    • Sodium-Potassium Pump: 3 Na+ out, 2 K+ in, contributing to electronegativity.
    • Passive Potassium Channels: K+ leaks out, enhancing negative charge.

Action Potential Formation

  • End Plate Potential (EPP): Initial change in membrane potential.
  • Threshold Potential: Skeletal muscle threshold ~-55 mV.
  • Depolarization: Na+ influx leads to positive charge buildup.

Voltage-Gated Sodium Channels

  • Activation Gate: Opens at threshold (~-55 mV), allowing Na+ influx.
  • Inactivation Gate: Closes at peak potential (~+30 mV) stopping Na+ influx.

T-Tubules and Sarcoplasmic Reticulum

  • Sarcolemma: Muscle cell membrane, continuous with T-tubules.
  • Transverse Tubules (T-Tubules): Invaginations of the sarcolemma.
  • Sarcoplasmic Reticulum: Stores calcium; releases it upon stimulation.
  • Triad Structure: Composed of T-tubule and two sarcoplasmic reticulum sacs.

Calcium Release and Contraction

  • Dihydropyridine Receptor: Voltage-sensitive receptor on T-tubules.
  • Ryanodine Receptor: Coupled to dihydropyridine receptor; releases Ca²⁺ from sarcoplasmic reticulum.
  • Calcium Function: Binds to troponin, initiating the sliding filament theory.

Repolarization

  • Potassium Efflux: Potassium channels open, K+ exits, cell becomes negative again.
  • Restoration: Returns to resting potential (~-90 mV).

Key Terms and Concepts

  • Sarcolemma: Muscle cell plasma membrane.
  • Endomycium: Connective tissue surrounding muscle cell.
  • Excitation-Contraction Coupling: Linking electrical signals to muscle contraction.

Next Steps

  • Upcoming video (Part 3): Sliding filament theory and contraction coupling.

These notes capture the key concepts and processes discussed in the lecture about neuromuscular junctions and muscle cell stimulation by acetylcholine.