When is K permeability highest in AP?

[LuckiestOne]

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One of the qbank questions say that the K permeability is the highest during the rapid falling phase, but I think the permeability probably should be higher at the hyperpolarization phase because hyperpolarization phase is closer to the K equilibrium potential.

I was wondering if anyone had a good explanation why K permeability is higher in the rapid repolarizing phase than hyperpolarization phase, if that is the case.

 

[LuckiestOne]

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[tk0102]

One of the qbank questions say that the K permeability is the highest during the rapid falling phase, but I think the permeability probably should be higher at the hyperpolarization phase because hyperpolarization phase is closer to the K equilibrium potential.

I was wondering if anyone had a good explanation why K permeability is higher in the rapid repolarizing phase than hyperpolarization phase, if that is the case.

This is indeed the case. Repolarization of the action potential occurs when the slow Na+ inactivation gates close and voltage-gated K+ channels open in response to depolarization. As the membrane potential returns to its resting state, the voltage-gated K+ channels close, reducing the K+ conductance. I think what you might be missing in your conceptual picture is the role of these voltage-gated channels. Remember, it's these channels that TEA (tetraethylammonium) blocks.