AAMC BIO q pack 1 #74 reasoning

[Spectar]

Hello everyone quick question on this AAMC question (74 on BIO Q PACK 1)-

"What would be the result of complete removal of the Parathyroid Glands?

A: Severe neural and muscular problems due to deficiency of calcium in the plasma
B: An increase in calcitonin production to compensate for calcium deficiency in plasma
C: A drastic change in the ratio of mineral to matrix ratio in bones
D: Calcification of some organs due to accumulation of calcium in the plasma"

The correct answer is A which I got and understand. However I am confused about the reasoning the AAMC gave: "Removal of the parathyroid gland would lead to hypocalcemia, a condition of low blood calcium, resulting from the lack of parathyroid hormone. This would cause increased neuromuscular excitability because of the change in membrane potential, which under normal physiological conditions is partially kept in balance with extracellular calcium. The person will typically die from severe respiratory muscle spasms"

I always thought that high calcium concentrations would increase neuromuscular excitability? I know that calcium can flow into the neuron and cause release of neurotransmitters at the junction- and that calcium itself can be released at some neuromuscular junctions- but how would low calcium cause excitability?

Thanks

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[NextStepTutor_3]

Hi there - great question!

The AAMC explanation referenced membrane potential, so let's think about that first. To put it simply, we can correlate cell depolarization with increased excitability, and we can associate hyperpolarization with decreased excitability. (Just think about what you know about neurons - depolarization past the threshold leads to an action potential, while a hyperpolarized neuron is relatively unlikely to fire.)

Next, remember what you may have read about the nervous system! In a typical cell, the cell interior is much more negative than the extracellular environment. This is where we get the idea that resting membrane potential is approximately -70 mV (although that varies). If the extracellular environment were made to be less positive, it would no longer be much more positive than the cell interior. In other words, the cell interior would no longer be as much more negative than the exterior environment, so the membrane potential would be depolarized (less negative).

This is what this explanation is saying about calcium! Less extracellular calcium -> a less positive extracellular environment -> the interior of the cell is no longer as negative relative to its exterior conditions -> membrane potential is relatively depolarized, closer to its threshold, making it require a smaller stimulus to fire. Of course, this actual process is complex (if you're interested, you can read a significant amount of research on it), but this is a simplified explanation.

If this is still confusing, I'm happy to clarify further!

 

[Spectar]

Hi there - great question!

The AAMC explanation referenced membrane potential, so let's think about that first. To put it simply, we can correlate cell depolarization with increased excitability, and we can associate hyperpolarization with decreased excitability. (Just think about what you know about neurons - depolarization past the threshold leads to an action potential, while a hyperpolarized neuron is relatively unlikely to fire.)

Next, remember what you may have read about the nervous system! In a typical cell, the cell interior is much more negative than the extracellular environment. This is where we get the idea that resting membrane potential is approximately -70 mV (although that varies). If the extracellular environment were made to be less positive, it would no longer be much more positive than the cell interior. In other words, the cell interior would no longer be as much more negative than the exterior environment, so the membrane potential would be depolarized (less negative).

This is what this explanation is saying about calcium! Less extracellular calcium -> a less positive extracellular environment -> the interior of the cell is no longer as negative relative to its exterior conditions -> membrane potential is relatively depolarized, closer to its threshold, making it require a smaller stimulus to fire. Of course, this actual process is complex (if you're interested, you can read a significant amount of research on it), but this is a simplified explanation.

If this is still confusing, I'm happy to clarify further!

This is a perfect explanation thank you so much