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Updated: Jan 3 2022

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
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