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why do both Cl and K have negative resting membrane potentials even though they both cause hyperpolarization via influx into the cell?
Membrane potential of Cl and K
- Thread starter Chaoticsheath
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They don't both influx into the cell. Potassium EFFLUXES via various channels. Since this is a positive charge leaving the cell interior, the effect is to hyperpolarize the inner membrane leaflet (which is the membrane surface we talk about in these cases by convention). Chloride INFLUXES, and since it is a negative charge entering the cell interior, the effect is to hyperpolarize the inner membrane leaflet as well.
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ahhhh thanks!
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In all honesty, I suggest you pick up your favorite physiology text and refresh yourself on membrane ion permeabilities, resting membrane potential, depolarization/hyperpolarization, and the relative concentrations of ions in the extracellular and intracellular space, respectively. Potassium does not flow into the cell; it flows out. It causes hyperpolarization due to positive charges leaving the inside of the cell, just as sodium causes depolarization due to positive charges entering the cell.
I'll give a short quip about RMP: the resting membrane potential (typically around -70mV) is the charge on the cell membrane in the resting state. It is influence by the relative membrane permeability to different ions. If a membrane is permeable to potassium, the RMP will approach the equilibrium potential for potassium. The equilibrium potential is defined as the charge necessary to balance out the tendency of an ion to move down its concentration gradient. For example, if the intracellular space has a high concentration of potassium (as it usually does), and the cell membrane is relatively permeable to potassium, potassium will flow down its concentration gradient (in this case, from inside the cell to outside the cell). As the potassium ions, which are positively charged, flow out of the cell, they create a negative charge on the inside of the cell. Obviously, as this negative charge increases it will begin to inhibit the flow of potassium out of the cell (stated another way: it will balance out the tendency of potassium to flow out of the cell down its concentration gradient). When this negative charge inside the cell balances the movement of potassium down its concentration gradient, the system is said to be in equilibrium, and thus the equilibrium potential for potassium can be defined. This is normally about -80mV. Why is the RMP for a cell -70mV and the equilibrium potential for potassium -80mV? Because a cell is permeable to more than one ion, and as a result, the membrane potential will be in between equilibrium potentials to which the membrane is permeable, but will always be closer to the equilibrium potential of the ion to which the membrane is most permeable.
If I were you, I would refresh yourself on this. A physiology text will probably give you a better explanation than someone here will. I suggest Costanzo physiology.
Hopefully this didn't confuse you too much and helped a little.
I'll give a short quip about RMP: the resting membrane potential (typically around -70mV) is the charge on the cell membrane in the resting state. It is influence by the relative membrane permeability to different ions. If a membrane is permeable to potassium, the RMP will approach the equilibrium potential for potassium. The equilibrium potential is defined as the charge necessary to balance out the tendency of an ion to move down its concentration gradient. For example, if the intracellular space has a high concentration of potassium (as it usually does), and the cell membrane is relatively permeable to potassium, potassium will flow down its concentration gradient (in this case, from inside the cell to outside the cell). As the potassium ions, which are positively charged, flow out of the cell, they create a negative charge on the inside of the cell. Obviously, as this negative charge increases it will begin to inhibit the flow of potassium out of the cell (stated another way: it will balance out the tendency of potassium to flow out of the cell down its concentration gradient). When this negative charge inside the cell balances the movement of potassium down its concentration gradient, the system is said to be in equilibrium, and thus the equilibrium potential for potassium can be defined. This is normally about -80mV. Why is the RMP for a cell -70mV and the equilibrium potential for potassium -80mV? Because a cell is permeable to more than one ion, and as a result, the membrane potential will be in between equilibrium potentials to which the membrane is permeable, but will always be closer to the equilibrium potential of the ion to which the membrane is most permeable.
If I were you, I would refresh yourself on this. A physiology text will probably give you a better explanation than someone here will. I suggest Costanzo physiology.
Hopefully this didn't confuse you too much and helped a little.
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thx for the explanation. I just had a temporary mix up as far as directionality is concerned. Of course K flows out because several processes, including the cardiac cycle repolarization depend on this. But then I started reading BRS to refresh membrane potential stuff and I got fixated on the Na/K pump and how K flows inward through that pump, forgetting that the Na/K pump is ATP powered and that the natural tendency due to EC gradient is for K to efflux. Thanks for the summary though. Not a topic that we ever touched on in medical school (maybe it wasn't interesting enough for anyone to teach) so I'm pulling on crap I knew for the MCAT.
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Uworld has a few questions on the topic, so its fair game for Step 1, and I personally think its pretty important to understand.
Agreed, you'll be surprised how wild they can get with these questions. If you just need a quick refresher I actually found BRS physio's first chapter to be very helpful for getting me back on track with these questions. There's about 8-10 pages of membrane potential stuff and it's all great. Frankly, I have no idea why Costanzo doesn't get as much love as goljan and sattar on here
