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MCAT Question A Day - 7/16/13 - Answer!

Which sequence of depolarization best describes the order in which a muscle contracts?

A.  Sarcoplasmic reticulum, t-tubules, sarcolemma
B.  Sarcoplasmic reticulum, sarcolemma, t-tubules
C.  Sarcolemma, t-tubules, sarcoplasmic reticulum
D.  Sarcolemma, sarcoplasmic reticulum, t-tubules

The correct answer is (C). When acetylcholine is released from the motor neuron to the neuromuscular junction, the sarcolemma depolarizes. Depolarization then spreads along the outside, through the t-tubules to the inside, and finally to the sarcoplasmic reticulum, where calcium ions are released.