In the presence of a drug that blocks all effects of norepinephrine and epinephrine on the heart, the autonomic nervous system can:

Ans. B. Lower the heart rate below its intrinsic rateActivation of parasympathetic nerves to the heart would lower the heart rate below its intrinsic rate. However, with all effects of norepinephrine and epinephrine blocked, the sympathetic nervous system cannot raise the heart rate above its intrinsic rate. The withdrawal of parasympathetic nerve tone could only raise the heart rate to the intrinsic rate.