TPRH Bio: Why does hypocalcemia cause convulsions?

[bear2roo]

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I have the old TPRH SW from 2005 so I don't think my passage # corresponds to the SW of today. It's the passage about the irradiatiion of mouse organs and injections of pulverized organs and monitoring the results of several hormones/ion levels in plasma.

Anyways why would Hypocalcemia (low plasma [Ca2+]) cause convulsions aka muscle contractions? I know that Ca2+ causes increased Na+ permeability --> depolarization --> muscle contraction

But wouldn't that need increased Ca2+ levels? Or am I thinking of this wrong? B/c if there was LOW [Ca2+] wouldn't that mean LESS Na+ Permeability? And therefore LESS muscle contraction?

Thanks!

 

[ScatmanCrothers]

I ran into a similar problem in my BR bio book the other day (passage 3-11 if you have it). However, in mine, the passage itself stated that low extracellular calcium concentration causes muscle spasms. The question went something like, "According to the passage, it can be concluded that:
a) blah
b) blah blah
c) blah blah blah
d) the effect of extracellular calcium concentration on membranes is distinct from its role as a mediator of muscle contraction (correct answer)

Useful excerpts from the answer explanation:
"We often think of calcium as being involved in the troponin-tropomyosin complex... in that situation a low calcium concentration would lead to decreased nerve and muscle excitability. However, the effects of calcium concentration on the membrane is distinct from its role as mediator of contraction. In fact, these effects relate to calcium's ability to bind to plasma membrane proteins that function as ion channels. The binding alters the state of these channels.

Hope this helps. I doubt there will be a question about this on the actual MCAT unless it is stated in a passage like this BR problem was.