Question RE Potentials & Depolarization

That doesn't sound right- maybe others can chime in. In it's simplest sense (electrically, that is), an increase in cations extracellularly will, in its immediacy, lead to a more negative membrane potential, and vice versa.

Take, for example, a question brought up on the Step I forum on the etiology of hypocalcemic tetany: fewer calcium ions extracellularly lead to a "less-negative" membrane potential, which brings the membrane potential closer to depolarization treshold, facilitating depolarization.

 

Yuck. I'm so sorry to see that these topics resurface in medical school. What you've noted seems correct. It's a ratio/concentration thing. This is what my intuition tells me:

High cation concentration outside the cell causes the cations to want to move from high to low concentration -thus net positive flux into cell. When the cation [] is lower outside, then they want to move in that direction, leaving the cell more negatively charged. I get confused here, too because we're talking negative/postive in terms of cations. It's relative I guess.

This is just my take; correct if needed.

 

You are correct in saying an increase in cation presence outside of the cell would cause a natural movement of positive ions into the cell. I am really confused by the original post though. The actual process of depolarization is the action of decreasing the difference in charge across the cell membrane. This is acheived by an influx of positively charged Sodium into the nerve cell. Repolarization occurs when Potassium leaves the interior of the cell, thus causing it become negative again. However, before resting potential occurs, there is a hyperpolarization due to TOO MUCH potassium leaving...thus the inside is slightly more negative than it was initially. A return to resting membrane potential (negative charge within the cell) is acheived by the sodium potassium pump. That is the entire process that occurs during action potential propogation (a four step process).

I hope this makes sense and didn't confuse.

 

Oh I just re-read the original statement and I think I gave you a long response that doesn't even answer your question. Spectrum hit the nail on the head. Remember everything naturally wants to flow DOWN hill in concentration gradient (like a car sitting on top of a hill). By increasing the concentration of an ion, its like increasing the steepness of a hill...the car is more apt to roll down a hill that is on a 45 degree angle as opposed to a 5 degree slope. Make sense?

 

Your book is correct. Saying cations is a bit vague, but there are 3 major cations to consider, K+, Na+, and Ca2+. Assume the cell is permeable to all 3 cations (not normally true -- most cells are very permeable to K+ and not normally permeable to Na+ and Ca2+.) The K+ concentration is greater inside the cell than outside, so the inside of the membrane is negative with respect to the outside (looking only at the K+ gradient.) If you increase the extracellular K+ concentration, you lower the K+ gradient, making the membrane potential LESS NEGATIVE (depolarizing.) Both Na+ and Ca2+ concentrations are higher outside the cell than inside the cell, so the inside of the membrane is positive with respect to the outside (looking only at either the Na+ and Ca2+ gradients.) Therefore, increasing the extracellular Na+ or Ca2+ makes the membrane potential MORE POSITIVE (also depolarizing.)

Although I made a lot of assumptions up there, that are not really necessary. Regardless of which of the extracellular cation concentrations you increase, it results in a depolarization of the membrane.

Hope that helps.