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TBR Bio Endo and Immuno Psg VIII #53

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erythrocyte666

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Answer is C.
I disagreed with choice III because Ca doesn't play much of a role in neural excitability. If it does at all, I would have though decreased excitability because when extracellular Ca is low (as in the case of hypoparathyroidism), there is a lower electrochemical driving force when the voltage-gated Ca channels open to allow subsequent vesicular fusion; hence, I'd expect less neurotransmitter to be released and hence less excitability.
Any input?
 

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Cawolf

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http://www.merckmanuals.com/home/ho...lcemia_low_level_of_calcium_in_the_blood.html

"An extremely low calcium level may cause tingling (often in the lips, tongue, fingers, and feet), muscle aches, spasms of the muscles in the throat (leading to difficulty breathing), stiffening and spasms of muscles (tetany), seizures, and abnormal heart rhythms."

These symptoms are related to increased neuronal excitability.
 

Cawolf

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Sure.

Calcium in the plasma increases the threshold potential - you could say it stabilizes the membranes. The mechanism has nothing to do with neurotransmitter release.

When plasma calcium is low, the threshold potential is lowered and the neurons are more easily depolarized - increased excitability.


It did not. :/
 

erythrocyte666

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but how does Ca lower threshold potential - isn't that inherent to the cell? i'd expect low plasma Ca to have a lower electrochemical driving force and hence it should lower resting membrane potential, no? this would technically decrease excitability.

I heard you guys had a tough PS; but hey, at the worst I'd expect you to score mid-30s, so with a generous curve who knows?
 

Cawolf

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Think about it as a gradient of charges.

Inside is negative and outside is positive.

Calcium is a 2+ cation. The more calcium outside the cell, the more positive the outside is, the greater charge differential, the harder it will be to depolarize the inside relative to the outside.

If we have less positive charge outside, then the resting potential difference is less and it will be easier to depolarize.

Yah hopefully.
 
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