attention zmedic and jrm818

[phillybadboy]

question about leak channels and action potential. sodium leaks into cells potassium leaks out of cells. potassium is pumped in, sodium is pumped out. but does sodium leak out of cells through leak channels and does potassium leak into cells through leak channels? thanks

 

[systemet]

This gets very complicated, very quickly, but put simply:

* Individual sodium and potassium ions can (and will) move in any direction across the leak channels.

However, most sodium ions will tend to move into the cell through these channels, as sodium is more concentrated in the extracellular space. So, if we add up the movements of all the individual ions, there's a tendency towards movement into the cell.

For potassium, there's a tendency towards net movement out of the cell through these channels as there is a greater concentration of potassium inside of the cell.

*However, until resting conditions, the permeability of the cell membrane to both sodium and potassium is very low.


There just aren't that many leak channels. So the total amount of movement is relatively small. And ion movement at these channels is compensated for by other processes, including sodium-potassium exchange. This keeps the membrane potential at a steady value.

---------------------------

Where all this starts getting interesting is when the membrane potential begins to change, and voltage-gated channels start opening more often, and the permeability to sodium (or potassium) becomes greater. Then we start dealing with larger sodium, potassium, and calcium fluxes, and greater changes in the membrane potential.

 

[phillybadboy]

so the same leak channels tht allows sodium in can let sodium out also or does it have to leak out through another leak channel? the same leak channel that lets potassium out can also let potassium leak in or does it hav to leak in throuh another leak channel?

 

[systemet]

This gets quite complicated, quite quickly. I'm don't know enough about this area to give you a really detailed answer, but here's my best shot.

* A "leak current" is any background current present at rest. Usually we use it to describe currents that vary relatively little during the cardiac action potential, although some of them do show a reduced conductance as the membrane potential rises.

* There's not a single "leak potassium channel", for example, that is responsible for all background K+ currents. There are many, many different channels, each with their own properties. Most of the potassium channels are selective for potassium, however, under some circumstances they may also allow sodium to pass.

* At equilibrium the membrane is much more permeable to potassium than sodium. Membrane potential lies much closer to the voltage we would predict if sodium was the only ion present.

This page tries to explain the resting membrane potential in a simplified form:

http://www2.yvcc.edu/Biology/109Modules/Modules/RMP/RMP.htm

[There's some stuff you can click on, to manipulate the experimental conditions and see how membrane potential changes.]

 

[phillybadboy]

i know that poatssium channels are selective to potassium and sodium channels are selective to sodium, and that there are many of them. and i think they can leak in and out, what i want to know is can that exact same channel that let potassium out let potassiun in? and can that same exact channel that let sodium in let sodium out? thanks sorry to go over and over

 

[systemet]

i know that poatssium channels are selective to potassium and sodium channels are selective to sodium, and that there are many of them. and i think they can leak in and out, what i want to know is can that exact same channel that let potassium out let potassiun in? and can that same exact channel that let sodium in let sodium out? thanks sorry to go over and over

For non-gated channels, i.e. channels that don't close in response to voltage or ligand-binding, there's always going to be bidirectional movement of ions. The net movement may be inward or outward when one adds all the movements of all the individual ions together, but individual ions are always continuously moving in both directions.

[It should be noted that many of these channels still rectify, i.e. they show greater conductances at higher or lower membrane potentials, even if they don't exhibit classical gating].

Even when we start considering gating properties, channels open and close in a probablistic manner. If we consider voltage-gated sodium channels, individual channels will open and close, even below what we consider to be the threshold activation voltage. They just won't do it very often. Even when the membrane voltage rises, at a given point in time some channels will be open and others closed.

Sometimes it's just easier to consider net fluxes (movements) of ions, and consider the whole population of channels to be "open" or "closed" for simplicities sake.

 

[phillybadboy]

thanks