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

A. Increase heart rate
B. Decrease heart rate
C. Have no effect on heart rate
D. Cause tachycardia

**Core Concept:** The autonomic nervous system (ANS) innervates the heart through sympathetic and parasympathetic branches. Sympathetic stimulation leads to increased heart rate (tachycardia) due to activation of beta-adrenergic receptors, while parasympathetic stimulation results in decreased heart rate (bradycardia) via activation of alpha-adrenergic receptors. Norepinephrine and epinephrine are the primary catecholamines involved in these responses.

**Why the Correct Answer is Right:** When a drug blocks all effects of norepinephrine and epinephrine on the heart, it prevents the sympathetic stimulation and subsequent tachycardia. As a result, the heart rate remains unchanged or may even decrease due to the lack of inhibitory effect of sympathetic blockade.

**Why Each Wrong Option is Incorrect:**

A. Increase in heart rate: This is incorrect because the drug blocks the actions of norepinephrine and epinephrine, which normally lead to tachycardia (increased heart rate).
B. Decrease in heart rate: This is incorrect as well. The drug blocks the actions of norepinephrine and epinephrine, which typically cause bradycardia (decreased heart rate) via parasympathetic stimulation.
C. No effect on heart rate: This is incorrect because the drug blocks the actions of norepinephrine and epinephrine, preventing their influence on the heart rate.
D. Cause tachycardia: This is incorrect, as the drug blocks the actions of norepinephrine and epinephrine, preventing sympathetic activation and resulting tachycardia.

**Core Concept:**

Norepinephrine and epinephrine are catecholamines involved in the regulation of heart rate via the autonomic nervous system. They act on beta-adrenergic receptors to cause tachycardia (increased heart rate) and on alpha-adrenergic receptors to cause bradycardia (decreased heart rate). The drug blocking all effects of norepinephrine and epinephrine ensures that the heart rate remains unchanged or may even decrease due to the lack of inhibitory effects of sympathetic blockade.

**Why Each Wrong Option is Incorrect:**

A. Increase in heart rate: The drug blocks the actions of norepinephrine and epinephrine, preventing sympathetic activation and resulting tachycardia.
B. Decrease in heart rate: The drug blocks the actions of norepinephrine and epinephrine, preventing sympathetic activation and resulting bradycardia.
C. No effect on heart rate: The drug blocks the actions of norepinephrine and epinephrine, ensuring that the heart rate remains unchanged or may even decrease due to the lack of inhibitory effects of sympathetic blockade.
D. Cause tachy