Action Potential Basics

Snapshot

• A 24-year-old patient presents with rapid onset dizziness, nausea, and weakness at a restaurant in Japan. Prior to this presentation, it was disclosed to the paramedics that he had been eating a Japanese delicacy. (Tetrodotoxin poisoining from consumption of pufferfish)

Overview

Action Potential

• Action potential 
  • resting membrane potential (-70 mV)
    • determined by permeability to potassium (K+), sodium (Na+), and chloride (Cl-)
    • roughly -70 mV, with the greatest permeability being to K+
  • threshold (approximately -55 mV)
    • neuron must receive enough stimulus to reach threshold
      • depolarization is all or nothing
        • if threshold is met the neuron depolarizes
        • if it is not met nothing happens
  • depolarization (+50 mV) 
    • once threshold is met, voltage-gated Na+ channels open
      • Na+ rushes into the cell (there is a high electrochemical gradient for Na+ to enter)
        • this creates a short term positive feedback loop where the increasing voltage opens more voltage-gated Na+ channels
      • membrane potential becomes more positive and reaches a value of roughly +50 mV
        • as the membrane reaches its peak voltage, the voltage-gated Na+ channels begin to inactivate spontaneously and rapidly, and voltage gated K+ channels begin to open
      • faster inactivation of sodium channels will lead to lower amplitude of depolarization
    • pathophysiology correlate
      • tetrodotoxin (pufferfish consumption) binds fast voltage-gated Na+ channels in nerve tissue, which does not allow for depolarization or action potential formation 
        • can result in nausea, weakness, dizziness or can be fatal
        • treatment is supportive
  • hyperpolarization
    • at the peak membrane voltage (+50 mV), voltage-gated Na+ channels close and delayed outward-rectifier K+ channels open
      • Na+ no longer enters the cell and K+ leaves the cell at a rate greater than baseline
      • the cell hyperpolarizes at a voltage more negative than its baseline (-80 to -90 mV) due to the increased outward K+ flow
  • baseline
    • eventually the K+ channels return to their baseline state and the membrane potential reaches -70 mV again until another stimulus surpasses threshhold
• NOTE: the ion gradients are continually reestablished by the activity of the Na+/K+ ATPase which pumps 3 Na+ out of the cell for every 2 K+ transported in