muhammad0112
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I know that the axon has a constant resting potential of -70mv.

During resting potential 3 Na+ ions constantly leave the axon while 2 K+ ions constantly enter the axon. This gives the axon a net negative potential difference. K+ ions can exit the axon afterwards and Na+ ions cannot come back into the axon. This makes the potential difference even less. But doesn't that mean that the potential difference in the axon can go down to negative infinity. I get why the inside of the axon is more negative than the outside, but I don't get what keeps it at constant -70mv?
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Jpw1097
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(Original post by muhammad0112)
I know that the axon has a constant resting potential of -70mv.

During resting potential 3 Na+ ions constantly leave the axon while 2 K+ ions constantly enter the axon. This gives the axon a net negative potential difference. K+ ions can exit the axon afterwards and Na+ ions cannot come back into the axon. This makes the potential difference even less. But doesn't that mean that the potential difference in the axon can go down to negative infinity. I get why the inside of the axon is more negative than the outside, but I don't get what keeps it at constant -70mv?
The sodium-potassium pulp actually has very little effect on the resting membrane potential. The resting membrane potential is more to do with the relative permeability of the cell membrane to Na+ and K+ ions. The membrane is around 40x permeable to K+ ions compared to Na+ ions. Since the membrane is relatively impermeable to Na+ ions and the Na+ concentration is much higher outside the cell, the voltage inside the cell is much lower than outside the cell, hence the negative resting membrane potential.

The sodium-potassium pump is only important for maintaining the concentration gradients of Na+ and K+ ions - it is NOT responsible for the resting membrane potential.
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muhammad0112
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(Original post by Jpw1097)
The sodium-potassium pulp actually has very little effect on the resting membrane potential. The resting membrane potential is more to do with the relative permeability of the cell membrane to Na+ and K+ ions. The membrane is around 40x permeable to K+ ions compared to Na+ ions. Since the membrane is relatively impermeable to Na+ ions and the Na+ concentration is much higher outside the cell, the voltage inside the cell is much lower than outside the cell, hence the negative resting membrane potential.

The sodium-potassium pump is only important for maintaining the concentration gradients of Na+ and K+ ions - it is NOT responsible for the resting membrane potential.
I've done some more research and correct me if I'm wrong, but the axon is very permeable to potassium but without the sodium potassium pump, the electrical gradient will equal to the chemical gradient. (the reason why there's an electrical gradient is because the K+ ions would want to come back in the axon as the outside is more positive and the inside is negative). Therefore, the sodium potassium pump increases the chemical gradient of the potassium (as 2 potassium would enter in the axon). And the chemical and electrical gradient would balance out when the membrane is -70mv.

But can you explain why the sodium potassium pump doesn't make a difference to the resting potential?I don't understand it. If the Na/K pump went off once, wouldn't the net pd be -1 in the axon? - If it went off 10 times, that would be 30 sodium ions out and 20 K+ ions in - Thats a net pd of -10mv?. Obviously I understand that 1 Na+ ion isn't equal to 1mv, but wouldn't it be the same idea?
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Jpw1097
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(Original post by muhammad0112)
I've done some more research and correct me if I'm wrong, but the axon is very permeable to potassium but without the sodium potassium pump, the electrical gradient will equal to the chemical gradient. (the reason why there's an electrical gradient is because the K+ ions would want to come back in the axon as the outside is more positive and the inside is negative). Therefore, the sodium potassium pump increases the chemical gradient of the potassium (as 2 potassium would enter in the axon). And the chemical and electrical gradient would balance out when the membrane is -70mv.

But can you explain why the sodium potassium pump doesn't make a difference to the resting potential?I don't understand it. If the Na/K pump went off once, wouldn't the net pd be -1 in the axon? - If it went off 10 times, that would be 30 sodium ions out and 20 K+ ions in - Thats a net pd of -10mv?. Obviously I understand that 1 Na+ ion isn't equal to 1mv, but wouldn't it be the same idea?
As you say, 1 Na+ ions does not equal 1 mV. A huge number of Na+ ions are needed to exit the axon to create a 1 mV change in membrane potential, in the grand scheme of things, the sodium-potassium pumps do not move enough ions to make a difference - though they are important in maintaining high extracellular Na+ concentrations/high intracellular K+ concentrations.

At the resting membrane potential (say -70 mV in an axon), the 3 Na+ ions that get pumped out by the Na/K pump simply move back into the cell along their electrochemical gradient (as they are in equilibrium). Likewise, the K+ ions that get pumped into the axon simply diffuse out of the axon along their electrochemical gradient.

The Na/K pump is important in maintaining the concentration gradients for Na+ and K+, but it has little to no effect on the resting membrane potential, that is all to do with the differential permeability of the membrane to different ions, as reflected in the Nernst equation.
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muhammad0112
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(Original post by Jpw1097)
As you say, 1 Na+ ions does not equal 1 mV. A huge number of Na+ ions are needed to exit the axon to create a 1 mV change in membrane potential, in the grand scheme of things, the sodium-potassium pumps do not move enough ions to make a difference - though they are important in maintaining high extracellular Na+ concentrations/high intracellular K+ concentrations.

At the resting membrane potential (say -70 mV in an axon), the 3 Na+ ions that get pumped out by the Na/K pump simply move back into the cell along their electrochemical gradient (as they are in equilibrium). Likewise, the K+ ions that get pumped into the axon simply diffuse out of the axon along their electrochemical gradient.

The Na/K pump is important in maintaining the concentration gradients for Na+ and K+, but it has little to no effect on the resting membrane potential, that is all to do with the differential permeability of the membrane to different ions, as reflected in the Nernst equation.
Okay last question, The potassasium diffuses out, potassium then gets pumped in. Increasing the chemical gradient. Even if there is an electrical gradient the other way, more potassium would enter the axon. What keeps it capped at -70mv. If you don't know the answer, I'd rather you reply with "idk" instead of not reply. Also, I appreciate the help
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Jpw1097
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(Original post by muhammad0112)
Okay last question, The potassasium diffuses out, potassium then gets pumped in. Increasing the chemical gradient. Even if there is an electrical gradient the other way, more potassium would enter the axon. What keeps it capped at -70mv. If you don't know the answer, I'd rather you reply with "idk" instead of not reply. Also, I appreciate the help
Okay, let’s try to figure out why the membrane potential is -70mV.

Firstly you have a high concentration of organic anions (negatively charged protein) in the axon. This is balanced by K+ ions. The K+ concentration [K+] is higher inside the axon compared to outside, therefore the chemical gradient drives K+ ions out of the cell. As K+ ions move out of the cell, this creates a negative membrane potential, which causes K+ ions to move back into the axon (electrical gradient). As more K+ ions leave the axon, the electrical gradient gets larger (as more K+ ions leave the axon) and the chemical gradient remains relatively unchanged (the number of ions moving across the membrane has a negligible impact on the chemical gradient for short periods of time). The number of K+ ions leaving the axon gets progressively less and less as the electrical gradient gets larger until the electrical gradient = chemical gradient. At this point, there is no net movement of K+ ions - this is the equilibrium potential for K+ ~ -80 mV.

Now let’s consider Na+. Na+ has a much higher concentration outside the cell, therefore the chemical gradient drives Na+ ions into the cell. As Na+ ions enter the cell, this creates a positive membrane potential (electrical gradient), and this causes the number of Na+ ions entering the cell to slow down as the electrical gradient gets larger. When the electrical and chemical gradients are equal, there is no net movement of Na+ ions - the membrane potential when this occurs is around + 60 mV for Na+ (equilibrium potential).

So if the equilibrium potential for Na+ is ~ +60mV and for K* is ~ -80mV, why isn’t the resting membrane potential somewhere in the middle. This is because the membrane is far more permeable to K+ ions compared to Na+ ions, therefore the resting membrane potential is far closer to the equilibrium potential for K+ (-80mV) compared to Na+ (+60mV).

At the resting membrane potential, the movement of Na+ into the cell and the movement of K+ out of the cell are equal. Even though there is a large electrical gradient driving Na+ into the cell (there is a large difference between the equilibrium potential for Na+ and the resting membrane potential), the cell is relatively impermeable to Na+ ions. While the electrical gradient is small for K+ ions (K+ equilibrium potential is not far from the resting membrane potential), the membrane is highly permeable to K+ ions.

As you can see, at no point have I discussed the Na/K pump, because it operates on such a small level, it has very little effect on the resting membrane potential. Far more ions are moving across the membrane through leak channels compared to the N/K pump. The Na/K pump is only important in maintaining the chemical gradients, but not for establishing them.
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