In the presence of a drug that blocks all effects of norepinephrine and epinephrine on the hea, the autonomic nervous system can
**Core Concept**
The autonomic nervous system (ANS) regulates various bodily functions, including heart rate and blood pressure, through sympathetic and parasympathetic branches. Norepinephrine and epinephrine are key neurotransmitters involved in sympathetic transmission, affecting the heart by increasing heart rate and contractility.
**Why the Correct Answer is Right**
In the presence of a drug that blocks all effects of norepinephrine and epinephrine on the heart, the autonomic nervous system can still regulate heart rate through parasympathetic transmission. The parasympathetic nervous system, primarily mediated by the neurotransmitter acetylcholine, exerts a negative chronotropic effect on the heart, meaning it slows down the heart rate. This is an example of a physiological mechanism called "autonomic nervous system compensation," where one branch of the ANS compensates for the blockade of the other branch.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is too vague and does not specify a mechanism of action that would allow the ANS to compensate for the blockade of norepinephrine and epinephrine.
**Option B:** This option is incorrect because it implies that the blockade of norepinephrine and epinephrine would have no effect on the ANS, which is not true.
**Option C:** This option is incorrect because it is too broad and does not specifically address the effects of the blockade on the ANS.
**Clinical Pearl / High-Yield Fact**
The parasympathetic nervous system can compensate for the blockade of sympathetic transmission through the use of acetylcholine, a neurotransmitter that has a negative chronotropic effect on the heart.
**Correct Answer:** C.