Which of the following situations occurs in electrically excitable cells?
A. Depolarization causes voltage-gated sodium ion channels to open.
B. Sodium ion channels are opened by high extracellular calcium levels.
C. The sodium-potassium pump moves sodium into the cell.
D. When Na+ ion channels open, K+ ion channels close.
E. The opening and closing of ligand-gated channels cause depolarization.
Answer: A
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