TBR Bio, Section 3, Passage 11, Question 70

[sillyjoe]

The question reads:

70. According to information in the passage, it can be concluded that:

A. the concentration of extracellular calcium in no way effects neuromuscular excitability.
B. persons with low extracellular calcium concentrations may suffer total skeletal muscle spasms.
C. persons with increased extracellular calcium concentrations may suffer from frequent muscle spasms.
D. the effect of extracellular calcium concentration on membranes is distinct from its role as a mediator of muscle contraction.

D is correct, the effect of the extracellular calcium concentration on membrane is distinct from its role as a mediator of muscle contraction. Wc are told from the passage that a low extracellular calcium concentration leads to an increase in nerve and muscle cell excitability. In fact, a person with a low calcium concentration may suffer from muscle spasms. We often think of calcium as being involved with the troponin-tropomyosin complex, where calcium is needed to initiate contraction. In that situation, a low calcium concentration would lead to decreased excitability or contraction. We are told that in this case, a low extracellular concentration leads to increased nerve and muscle excitability. Based on this apparent contradiction, we can conclude that the effect of calcium concentration on the membrane is distinct from its role as a mediator of contraction. In fact, these effects reflect calcium's ability to bind to plasma membrane proteins that function as ion channels. The binding alters the state of the these channels. The correct choice is D.

Would someone be so kind and explain why B is not also correct?

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[sillyjoe]

Bump

 

[y123]

We are told from the passage that a low extracellular calcium concentration leads to an increase in nerve and muscle cell excitability. In fact, a person with a low calcium concentration may suffer from muscle spasms.
I have the same question.

 

[exacto]

I have a question off of your question now lol...lets suppose the extracellular calcium is the same for muscle contractions and choice D is incorrect, wouldn't choice C fit better than B because having to much calcium will constantly bind to the troponin causing the tropomyosin to alter and allows the mysoin to bind to the actin and to contract the fiber causing spasms?

 

[kraskadva]

There are 2 separate groups of Ca++ ions, intracellular (usually sequestered in the sarcoplasmic reticulum) and extracellular.

When an action potential comes along, the extracellular Ca++ can enter the cell through voltage-gated channels and act as a messenger to release the intracellular Ca++ from the SR. This rush of intracellular Ca++ is what binds to troponin and stimulates contraction, not the influx of extracellular Ca++, since that is not enough to change either intra- or extra- concentrations.

The relative concentrations of intra- vs extra- Ca++ also are part of determining the membrane potential (google the Goldman Equation). If the [Ca++]ex is low, then the ratio of [Ca++]ex : [Ca++]in is altered, which makes the membrane potential less negative, and thus more easily depolarized.
More easily depolarized = more muscle contractions --> spasms.

Ans B in the OP would not be correct because the spasm still requires an action potential to occur. Muscles are enervated in motor units, and low extra- Ca++ does not generate action potentials, so will not cause all motor neurons to activate all motor units simultaneously.
C would be incorrect for similar reasons, and also because increased extra- Ca++ would make the membrane potential more negative and less easily depolarized.