The plateau portion of the action potential in contractile cardiac muscle cells is due to:
A. an increased potassium permeability.
B. an influx of calcium ions.
C. an influx of sodium ions.
D. exit of calcium ions from the sarcoplasmic reticulum.
Ans: B. an influx of calcium ions.
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B) mucosa
C) villi
D) mucous epithelium
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