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A-Level Biology OCR A

Sodium Channels Voltage Gated Axon Ion Potential Resting Na

An action potential fires when stimulus opens voltage-gated Na+ channels, causing Na+ influx and depolarization. This triggers more Na+ channels to open (positive feedback) until +40mV. Then, Na+ channels close, K+ channels open, causing repolarization and hyperpolarization. Finally, the resting potential is restored by the Na+/K+ pump.

An action potential fires when stimulus opens voltage-gated Na+ channels, causing Na+ influx and depolarization. This triggers more Na+ channels to open (positive feedback) until +40mV. Then, Na+ channels close, K+ channels open, causing repolarization and hyperpolarization. Finally, the resting potential is restored by the Na+/K+ pump.

Front Describe how an action potential is fired.
Back Action potential:
  • Neurone is initially at resting potential - K+ channels open, voltage-gated Na+ closed
  • Energy of stimulus triggers sodium voltage-gated ion channels to open, making the membrane more permeable to sodium ions
  • Na+ diffuse into the axon down their electrochemical gradient, making the inside of neurone less negative - depolarisation
  • This change in charge causes more sodium ion channels to open, allowing more sodium ions to diffuse into the axon - positive feedback
  • When PD is ~ +40mV, voltage-gated sodium ion channels close & voltage-gated potassiom ion channels open - sodium no longer enters axon but potassium ions diffuse out of axon down electrochemical gradient - repolarisation
  • This reduces charge, making the inside of axon more negative than in its resting state - hyperpolarisation
  • Voltage-gated potassium ions now close & SOPI pump working causes Na+ out, K+ in returning to resting potential

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