Hypocalcemia / neural membrane potential

[LuminousTruth]

Which of the following observations would best explain the convulsions induced by hypocalcemia?

1) Hypocalcemia increases parathyroid hormone secretion
2) Decreased plasma calcium levels increase neuronal membrane permeability to sodium.

The answer was 2. If the question had been rephrased as "Which of the following observations, if true, ...", I would have chose 2 instantly.

I know that more sodium will result in depolarization and as a result, more action potentials but the reason I got this wrong was I thought the neuronal membrane permeability does not change, since sodium can not simply just diffuse in. They are facilitated only by voltage gated sodium channels and Na/K ATPase.

Is my reasoning incorrect and that membrane permeability can [/B ]actually change for sodium?

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[uoguelph]

Well, if you changed the question to "if true" both would be correct answers. Regarding your question as serum calcium levels fall, neuronal membranes become increasingly more permeable to sodium, enhancing excitation because an action potential is easily initiated.. In terms of sodium, you have to take into account that there are leakage channels. You are thinking about the sodium-potassium pump. In axon terminals we have voltage gated calcium channels.

 

[shouldvestudied]

Why do decreased extracellular levels of Ca2+ increase membrane permeability to Na+?

(I am going to feel so stupid after I read the answer)

 

[Hanguk]

Well, if you changed the question to "if true" both would be correct answers. Regarding your question as serum calcium levels fall, neuronal membranes become increasingly more permeable to sodium, enhancing excitation because an action potential is easily initiated.. In terms of sodium, you have to take into account that there are leakage channels. You are thinking about the sodium-potassium pump. In axon terminals we have voltage gated calcium channels.

He's right. Where is this question from? Because voltage-gated Ca2+ channels are also subjected to leakage with Na+, which to be honest seems like a fact that's really specific to memorize for the MCAT.

So are you saying if Ca2+ levels decrease, Na+ will feel the effects of a concentration gradient even more due to low calcium levels, increasing leakage?

 

[uoguelph]

Low calcium levels would reduce the amount that the threshold must change to initiate a full depolarization of the axon while leaving the resting potential unchanged. Therefore a progressive depolarization would occur. The resting potential stays the same but as calcium levels drop the amount of partial depolarization needed to allow for continuous sodium influx is reduced. This would cause sodium channels to open prematurely and only a very small membrane potential increase would be required to open sodium channels making the nerve fiber EXTREMELY excitable. In addition, leakage channels would allow for the passive influx of sodium.

I wouldn't think that the MCAT would require this much detail to answer this question but it's quite interesting! 😛

 

[Hanguk]

Low calcium levels would reduce the amount that the threshold must change to initiate a full depolarization of the axon while leaving the resting potential unchanged. Therefore a progressive depolarization would occur. The resting potential stays the same but as calcium levels drop the amount of partial depolarization needed to allow for continuous sodium influx is reduced. This would cause sodium channels to open prematurely and only a very small membrane potential increase would be required to open sodium channels making the nerve fiber EXTREMELY excitable. In addition, leakage channels would allow for the passive influx of sodium.

I wouldn't think that the MCAT would require this much detail to answer this question but it's quite interesting! 😛

Thanks! Good stuff 😀

On second thought, this question may be testing your deductive techniques. You know that convulsions would occur when the nervous system is activated to cause such dramatic responses, so you can rule out A because raising calcium levels in the blood through calcitonin from the parathyroid has no effect on nerve signaling. As a result that just leaves B, assuming C and D were easily incorrect too.

 

[MedPR]

Which of the following observations would best explain the convulsions induced by hypocalcemia?

1) Hypocalcemia increases parathyroid hormone secretion
2) Decreased plasma calcium levels increase neuronal membrane permeability to sodium.

The answer was 2. If the question had been rephrased as "Which of the following observations, if true, ...", I would have chose 2 instantly.

I know that more sodium will result in depolarization and as a result, more action potentials but the reason I got this wrong was I thought the neuronal membrane permeability does not change, since sodium can not simply just diffuse in. They are facilitated only by voltage gated sodium channels and Na/K ATPase.

Is my reasoning incorrect and that membrane permeability can [/B ]actually change for sodium?

This is a typical best answer MCAT question. You have to pay attention to the question stem.

He's right. Where is this question from? Because voltage-gated Ca2+ channels are also subjected to leakage with Na+, which to be honest seems like a fact that's really specific to memorize for the MCAT.

So are you saying if Ca2+ levels decrease, Na+ will feel the effects of a concentration gradient even more due to low calcium levels, increasing leakage?

You don't have to know that fact, though I think it's a pretty basic fact, to answer this correctly.

Thanks! Good stuff 😀

On second thought, this question may be testing your deductive techniques. You know that convulsions would occur when the nervous system is activated to cause such dramatic responses, so you can rule out A because raising calcium levels in the blood through calcitonin from the parathyroid has no effect on nerve signaling. As a result that just leaves B, assuming C and D were easily incorrect too.

Almost, but not quite. Parathyroid secretes PTH, and the thyroid secretes calcitonin. In addition, calcitonin decreases serum calcium. Your reasoning is correct otherwise.

 

[Mornhavon]

.

 

[MedPR]

Question:

By "Decreased plasma calcium levels increase neuronal membrane permeability to sodium" does this mean decreased calcium levels INSIDE the cytosol or is it extracellular calcium?

If you have more sodium outside the cell (Na/K) pump, the gradient makes sodium want to get inside...so to increase the gradient you would have to make the inside of the cell even more negative by removing calcium...which, logically means it is inside the cell...right?

Plasma calcium = calcium in the blood = extracellular.

 

[brood910]

Why does the low concentration of [CA] in extracelullar space cause sodium to flow into the cell even more?
As Mornhavon said above, doesnt electrochemical gradient decrease as extracelullar space will become more negative and become more similar to the intercellular space?

 

[inasensegone]

Why does the low concentration of [CA] in extracelullar space cause sodium to flow into the cell even more?
As Mornhavon said above, doesnt electrochemical gradient decrease as extracelullar space will become more negative and become more similar to the intercellular space?

I didn't even want to address this question because it ate up like 2 hours of my time one day. The short answer I got was, Calcium interacts with, and blocks, sodium channels. Decreased blocking/interaction of the sodium channels thereby lowers the threshold for initiating an action potential. It can drop it so much that it causes spontaneous firing of the neurons. I didn't get a good explanation as to the exact mechanism as to how it interacts with and blocks the channel. Wikiepedia didn't do a good job explaining it, it basically just said what I said.

"The neuromuscular symptoms of hypocalcemia are caused by a positive bathmotropic effect due to the decreased interaction of calcium withsodium channels. Since calcium blocks sodium channels and inhibits depolarization of nerve and muscle fibers,[clarification needed] diminished calcium lowers the threshold for depolarization"

I even looked in a Medical Physiology Book (a highly regarded book) and I didn't get much more out of it:

Everyone just sort of glazes over how exactly Calcium does this. Therefore my conclusion was that a question like this simply must be answered by deductive reasoning (as Hanguk and MedPR pointed out)

 

[dyspareunia]

Calcium has a tendency to compete with sodium on membrane channels. In hypocalcemia this competition is reduced and Na+ influx increases. Contrary to what OP says, Na+ leaks into cells all the time. This is one of the many reasons why the Na/K pump is so important.

This is way beyond the scope of the MCAT.

 

[Endoxifen]

I know this posted a long time ago, but is the blocked sodium channel a passive channel or the M-gate of the voltage-gated channel? It sounds like it is the voltage-gated channel because the previous posters stated that the resting membrane potential does not increase. I just want to be sure though. Thank you!