1
$\begingroup$

In my textbook, it is stated that after the closure of potassium voltage-gated channels and during hyperpolarization, potassium leakage channels allow potassium influx passively and this returns the cell to its normal polarised state. I don't understand why will potassium ions enter the cell against its electrochemical gradient ?

My textbook quote :

during Hyperpolarizatio: The Leakage K+ Channels tend to drive the membrane potential from hyperpolarized state to the resting state as they drive K+ ions inward

Improve this question
$\endgroup$
5
  • $\begingroup$ @BryanKrause and then comes another question if the stable membrane potential is for example -90 MV and potassium gated channels open why would potassium ions disrupt their original equilibrium (increase the inside negative charge) and move outside the cell more than before. $\endgroup$
    – mohamed
    Dec 21, 2019 at 17:45
  • $\begingroup$ Which textbook is it? I think that phrase is completely wrong. $\endgroup$
    – BPinto
    Dec 21, 2019 at 19:33
  • $\begingroup$ @BPinto I think the phrase is correct and if it's wrong then what drives the membrane from hyperpolarization to normal resting membrane potential $\endgroup$
    – mohamed
    Dec 21, 2019 at 19:36
  • $\begingroup$ When you`re in the hyperpolarized phase, the resting potential is closer to the K+ reversal potential. When the voltage dependent K+ channels close the membrane permeability to potassium decreases and thus the resting potential shifts to more depolarized. @BrianKrrause answer should make it clear now $\endgroup$
    – BPinto
    Dec 21, 2019 at 19:45
  • $\begingroup$ @BPinto thank you this answer made every clear $\endgroup$
    – mohamed
    Dec 22, 2019 at 8:13

1 Answer 1

Reset to default
2
$\begingroup$

I don't get to say this on very many occasions with questions like this...

Your textbook is wrong.

A typical potassium reversal potential in a cell is ~-90 mV. Hyperpolarization through voltage-gated potassium channels can never go more negative than that reversal potential.

An experimenter with access to the cell through a patch clamp electrode could possibly set the voltage more negative than potassium reversal, in which case indeed, potassium would flow against it's concentration gradient into the cell due to the electrical potential. It is not possible to reach those very negative potentials using potassium channels alone.

For a typical cell, the resting membrane potential is more like -70 mV. This resting potential is due to the "leak conductance", which does include potassium but also includes other ions. The ratio of sodium to potassium permeability is typically around 1:20; potassium dominates but you can't just ignore the other ions. A more accurate replacement statement for your textbook would be:

The leakage conductances tend to drive the membrane potential from hyperpolarized state to the resting state as the net positive flow of ions is inward.

Even at rest, you will always find a slow potassium leak out of the cell. The reason that the cell is at equilibrium at rest is because this leak out of the cell is perfectly balanced by net positive charge flowing in to the cell; this is mostly sodium but can be other ions like calcium as well (note also that that chloride leaving the cell is also a net positive charge in).

Some of the sodium leak current is through specific sodium leak channels, but co-transporters that use the sodium concentration gradient also contribute.

Ren, D. (2011). Sodium leak channels in neuronal excitability and rhythmic behaviors. Neuron, 72(6), 899-911.

Improve this answer
$\endgroup$
3
  • $\begingroup$ @Byran Krause what I understood from your answer is that due to the specific limited number of K+ channels (and not only the electrochemical gradient) the efflux of K+ ions is limited to a certain number and after increasing the conductance of membrane to K+ (causing hyperpolarization) ,the membrane conductance to K+ returns to normal slowing down its efflux .So, what causes the return from hyperpolarization to polarization is the normal flow of other cations with their electrochemical gradient (Na+) and the slowed down efflux of potassium not as was written by the influx of K+. Right? $\endgroup$
    – mohamed
    Dec 21, 2019 at 20:00
  • $\begingroup$ @mohamed First think of each ion's reversal potential and the membrane voltage. Every ion will travel according to its own reversal potential. If potassium's reversal potential is -90mV, it will always be flowing out of the cell if the membrane is more positive than -90mV, and flowing in if the cell is more negative than -90mV. The rate of flow depends on the difference from -90mV. If the reversal potential for sodium is +60mV, it will always be flowing into the cell if the cell is more negative than +60mV. The membrane potential you actually observe, then, depends on a ratio of conductances. $\endgroup$
    – Bryan Krause ♦
    Dec 21, 2019 at 20:04
  • $\begingroup$ and that's why the book is wrong K+ never enters the cells and the cell's membrane potential shifts towards sodium's potential stoping hyperpolarization ,but then the number of sodium inside the cell increases relative to before the action potential, so here comes the Na+-K+ in returning the original number of potassium ions inside the cell and expelling and excess sodium. $\endgroup$
    – mohamed
    Dec 21, 2019 at 20:29

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .