Updated: 1/3/2022

Action Potential Basics

Review Topic
  • 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)
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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