Plasma Membrane Voltages

[taj]

Assalamualaikum,

In FHD-21 © Royal Free & University College Medical School University College London, there are some questions that I find rather perplexing:
-why do most cells have a negative plasma resting voltage?
-what will happpen to the resting voltage of a typical cell when the potassium concentration in the extracellular fluid rises (as happens, for instance during hemolysis and during kidney failure)?

Thanks for your time!

Wassalamualaikum.

[Sajjad]

Yeah this was quite confusing.

Forget what you think you know about membrane voltages, in particular with reference to the Na and K pump.

Basically there are many anionic proteins in the cell which bind to and sequester potassium inside the cell. This leaves a huge concentration gradient between the cell and the extracellular space. Therefore potassium has a natural tendancy to want to move outside the cell (the Na/K pump prevents this and maintains the concentration gradient). The relative charge of the cell at which potassium would be "happy", as it were, would be at roughly -70mv this is because at this voltage potassium would have left the cell down its concentration gradient and struck the correct balance for potassium to want to move back in the cell because its too negative. So you have two forces at work: the concentration gradient which favours potassium exit and the electrochemical gradient which favours potassium entry, it so happens that the correct balance would be struck at -70mv.

Hence if you increase the extracellular potassium concentration you decrease the concentration gradient between the cell and the extracellular space, therefore the balance would be achieved when far fewer potassium leaves the cell; leaving the membrane potential less negative. This has the effect of depolarising the cell and explains potassium cardiac toxicity. Potassium can be very toxic.

The picture isn't really this simple, we must also consider the sodium channels, the concentration gradient here is steep from the extracellular space to the cell and is also favoured to enter due to the relative negative charge established as described above by potassium. Therefore sodium will have struck the correct balance at around +40mv or so (not strictly true but we won't complicate matters). But since the cell is more permeable to potassium than sodium, it leans more towards the side of favouring potassium and so a net relative negative charge prevails.

The Na/K pump just therefore maintains the concentration gradients - it doesn't produce them as taught in A levels (at least when I was doing them).

Hope this helps and sorry for the long explanation, please ask more questions if this wasn't clear.

Wslm,

Saj

[taj]

Thanks a lot! I'll try not to make a habit of this, everybody here seems very busy

Jazakallah khairun

Wassalamualaikum

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Insha'Allah