Action Potentials Confusion in Kaplan

[nonzerosum]

Hi

so I was reading up on this in Kaplan and it says that the

neuron membrane is impermeable to Na+ yet they have a diagram with Na+ moving into the cell lol. I've googled with no luck. Can anyone explain this?

thanks

 

[rhythmspirit]

Hi

so I was reading up on this in Kaplan and it says that the

neuron membrane is impermeable to Na+ yet they have a diagram with Na+ moving into the cell lol. I've googled with no luck. Can anyone explain this?

thanks

maybe they mean it's impermeable when the cell is at rest or during repolarization? The only time the membrane allows Na+ to flow passively is during depolrization, i think. Other times, Na+ has to be actively pumped (or some do leak).

 

[diastole]

Na+ is impermeable to the membranes because it has a charge on it and won't move through the non-polar part of the phospholipid bilayer. The only way it gets in or out of the cell is through gates in the cell wall. There is a voltage gated Na+ channel that opens when the threshold membrane potential is met and then all this Na+ rushes in the cell and raises the membrane potential. Then the Na+ gate closes, the K+ gate opens and K+ rushes out of the cell. The cell goes back to a negative potential and overshoots the resting potential and you get hyperpolarization. Then the Na+/K+ pump returns the cell back to normal by pumping Na+ out and K+ in.

So basically the cell membrane is impermeable to Na+ or any ion but they do enter or exit the cell through gates in the membrane.

 

[nonzerosum]

thanks!

so does this mean that diffusion of Na does not occur at all or that only passive diffusion thru a protein mediate channel occur?

also so when biologists use the word impermeable it always only refers to the membrane and not the channels?

 

[mug3n]

edit: misunderstood q

 

[rhythmspirit]

thanks!

so does this mean that diffusion of Na does not occur at all or that only passive diffusion thru a protein mediate channel occur?

also so when biologists use the word impermeable it always only refers to the membrane and not the channels?

I guess you can call it facilitated diffusion. Diffusion of Na+ only occurs during depolarization, when the channels open and Na+ rush into the cell. Without the protein channels, Na+ is impermeable to pass through the membrane.