M.10.4 Understanding Neuromuscular Junction Events

Apr 2, 2025

Lecture Notes: Neuromuscular Junction Events

Introduction

  • Focus: Conversion of electrical signals from motor neuron to muscle fiber.
  • Neuromuscular Junction (NMJ): Comprises axon terminus, synapse (gap), and motor end plate.
  • Key Components:
    • Motor Neuron
    • Axon
    • Motor End Plate: Part of the muscle plasma membrane.
    • Synapse: Gap between motor neuron and muscle fiber.

Action Potential Propagation

  • Voltage-Gated Channels:
    • Sodium (Na+) and Potassium (K+): Facilitate rapid depolarization and repolarization.
    • Calcium (Ca2+): Triggers exocytosis of neurotransmitters.
  • Resting Membrane Potential:
    • Maintained by leaky channels and the sodium-potassium pump.

Neurotransmitter Release

  • Acetylcholine (ACh):
    • Exclusive neurotransmitter in motor neurons.
    • Released into the synapse following calcium influx.

Ion Permeability and Membrane Potential

  • Chemically Gated Channels:
    • Located at the motor end plate.
    • Allow simultaneous sodium influx and potassium eflux.
    • Sodium influx is greater due to:
      • Electrochemical gradient.
      • Smaller size compared to potassium.
    • Result: Membrane potential becomes more positive (depolarization).

Graded Potentials and End Plate Potential

  • End Plate Potential (EPP):
    • Converted chemical signal (ACh) into an electrical signal (EPP) in muscle cell.
    • Distinguished from graded potentials seen in neurophysiology.
    • Typically reaches super-threshold levels, often leading to an action potential.

Key Differences

  • Chemically Gated Channel: Allows both sodium and potassium permeability.
  • EPP Characteristics:
    • No summation of graded potentials due to single motor neuron.
    • Almost always results in an action potential.

Conclusion

  • Events at the neuromuscular junction involve conversion of electrical signals in motor neurons into chemical signals and then back to electrical signals in muscle cells.
  • Next video will explore conversion of EPP into an action potential leading to muscle contraction.