TBR test 1 biological sciences

[2010premed]

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Decreases in the magnitude of the action potential can be achieved by:

• decreasing the chloride ion influx
• increasing the sodium ion influx
• decreasing the extracellular sodium ion concentration
• increasing the extracellular potassium ion concentration

Answer: C
I understand that chloride ion concentration doesn’t play a big role in the action potential, and that increasing sodium ion influx causes depolarization of the cell, making the action potential more positive, but why does decreasing the extracellular sodium ion concentration decrease the magnitude of the AP? In the explanation it says “by decreasing the concentration of NA+ outside the cell, you decrease its resting membrane potential, so the magnitude of cell depolarization will also decrease.” And then it says “increasing K+ concentration only makes depolarization more difficult to trigger, due to a resting membrane potential that is more negative and extends the refractory phase of the cell.” How does both a decrease in extracellular sodium and an increase in extracellular potassium cause a decrease in membrane potential, if they are both positive ions??

 

[dextor2003]

that explanation confuses me. i thought of it picturing the action potential graph in my head. the depolarization phase occurs because of the influx of the Na+ ions. the more sodium that comes inside, the more positive the inside of the cell becomes (the greater the depolarization). so if you decrease the amount of Na+ on the outside, less can come in, so you cant get the inside as positive as you normally would (ie depolarization would be less)

hope that helps

 

[2010premed]

Yes it does, thanks!

 

[brood910]

I actually got this question right by reasoning in the same way as dextor did.
But, I am kinda confused by "Increasing extracellular K+ concentration only makes depolarization more difficult to trigger, due to a resting membrane potential that is more negative".

How is it that resting membrane potential becomes more negative? if you increase extracelullar k concentration, then k+ will not flow out of the cell much. Doesnt this make the inside of the cell more positive?

Also, when they say PE increased, does this mean PE is more positive or negative?
Also, if what they said is true, is it easier to fire action potential when PE is more positive (less negative)?

 

[inasensegone]

I actually got this question right by reasoning in the same way as dextor did.
But, I am kinda confused by "Increasing extracellular K+ concentration only makes depolarization more difficult to trigger, due to a resting membrane potential that is more negative".

How is it that resting membrane potential becomes more negative? if you increase extracelullar k concentration, then k+ will not flow out of the cell much. Doesnt this make the inside of the cell more positive?

Also, when they say PE increased, does this mean PE is more positive or negative?
Also, if what they said is true, is it easier to fire action potential when PE is more positive (less negative)?

I would have reasoned this answer the way Dextor did as well, as that seems to the most intuitive.

Having said that, if you increase extracellular K+ concentration, this would make the resting membrane potential more negative. The whole reason the outside of the cell is more negative in the first place, is because there are more Na+ ions outside of the cell than there are K+ ions inside of the cell. It's not that the inside of the cell is actually negative, it is just negative relative to the outside (hence the proper name: Resting membrane potential difference). So if you increase the amount of positive charge outside of the cell, this would make the inside more negative because it would increase the difference in concentration of positive charges (i.e. cations Na+ and K+).

The same logic can be applied to the Na+ ions. If you have less (decrease as the answer states) Na+ (positive charge) on the outside, the resting membrane potential difference is reduced. This would decrease the magnitude of the action potential because there isn't as much of a difference in charge.

Edit:
In regards to your question about increasing the extracellular K+ concentration and thus making it more difficult for K+ to flow out, Brood, this does not explain why it would make the inside of the cell more positive. It is true, that it would make it more difficult for the efflux of K+ ions out of the cell, but the efflux doesn't occur on a large scale until after an action potential is initiated anyway. Increasing positive charge outside of the cell makes it more difficult to initiate an action potential, as they explained.

What is PE? Potential Energy? (last part of your post)