MCAT Question A Day - 9/12/14 - Answer!
When left alone, certain specialized cardiac muscle cells have the capacity for self-excitation. The SA node is a collection of such cells. The SA node is innervated by the vagus nerve. The frequency of self excitation of the cardiac cells of the SA Node is likely to be:
A. slower than a normal heartbeat because excitation by the vagus nerve decreases the heart rate.
B. slower than a normal heartbeat because excitation by the vagus nerve increases the heart rate.
C. faster than a normal heartbeat because excitation by the vagus nerve decreases the heart rate.
D. faster than a normal heartbeat because excitation by the vagus nerve increases the heart rate.
C is correct. You should know that the vagus nerve is a parasympathetic nerve. Heart rate is set by the SA node, and the SA node is innervated by the parasympathetic vagus nerve. The pace of the SA node is faster than the normal heart beats, but the parasympathetic vagus nerve tonically slows the contractions of the heart to its resting pace. Without tonic inhibition from the vagus, the heart would normally beat at 100 - 120 beats per minute.
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C is correct. You should know that the vagus nerve is a parasympathetic nerve. Heart rate is set by the SA node, and the SA node is innervated by the parasympathetic vagus nerve. The pace of the SA node is faster than the normal heart beats, but the parasympathetic vagus nerve tonically slows the contractions of the heart to its resting pace. Without tonic inhibition from the vagus, the heart would normally beat at 100 - 120 beats per minute.
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