The intrinsic firing rate of the cardiac pacemaker (SA node) is approximately 90

ID: 3518499 • Letter: T

Question

The intrinsic firing rate of the cardiac pacemaker (SA node) is approximately 90/min. Why then is resting heart rate closer to 65 bpm?

A. Extracellular Na concentrations are too low.

B. Inefficient coupling to purkinje fibers.

C. Baseline acetylcholine release from cardiac parasympathetic fibers.

D. Baseline norepinephrine release from cardiac sympathetic nerve fibers.

E. The delay that occurs in the A-V-node.

A. Extracellular Na concentrations are too low.

B. Inefficient coupling to purkinje fibers.

C. Baseline acetylcholine release from cardiac parasympathetic fibers.

D. Baseline norepinephrine release from cardiac sympathetic nerve fibers.

E. The delay that occurs in the A-V-node.

Baseline norepinephrine release from cardiac sympathetic fibers causes the resting heart rate to 65 bpm.

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