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MCAT Question A Day - 8/3/13 - Answer!

Before depolarization of a muscle cell, where will calcium ions primarily be found?

A.  In extracellular fluid just outside the sarcolemma.
B.  In the axon endings of a motor neuron.
C.  In the intracellular fluid just inside the sarcolemma.
D.  In the sarcoplasmic reticulum.

The correct answer is (D). Calcium is stored in the sarcoplasmic reticulum until depolarization. Choice (B) is incorrect because it is the neurotransmitter substance that leaves the axon endings of the motor neuron. This neurotransmitter disturbs the muscle cell membrane, causing Na+ to move inside from the extracellular fluid. Thus, choice (A) is also incorrect. Choice (C) is incorrect because it is K+ that moves from the intracellular fluid to the outside.