**Question:** Section of the vagus nerve results in

A. bradycardia
B. increased salivation
C. decreased salivation
D. decreased respiratory rate

**Core Concept:** The vagus nerve is a cranial nerve that plays a crucial role in regulating various physiological processes. It has both sympathetic and parasympathetic branches, with the latter being further divided into cardiac and respiratory divisions.

**Why the Correct Answer is Right:** When the cardiac division of the vagus nerve is severed, it predominantly affects the parasympathetic tone, leading to bradycardia (slow heart rate) due to the lack of parasympathetic stimulation.

**Why Each Wrong Option is Incorrect:**

A. **Bradycardia:** While sectioning the vagus nerve can cause bradycardia, option A is incorrect because it is the sympathetic nervous system that primarily controls heart rate, not the vagus nerve.

B. **Increased salivation:** This is incorrect because the parasympathetic division of the vagus nerve stimulates salivary glands and increased salivation would be expected, not decreased.

C. **Decreased salivation:** Similar to option B, this is incorrect as the parasympathetic division of the vagus nerve stimulates salivary glands, leading to increased salivation.

D. **Decreased respiratory rate:** Sectioning the vagus nerve can potentially affect respiratory rate, but option D is incorrect as it is the sympathetic nervous system, specifically the cardiac branch of the vagus nerve, that influences respiratory rate, not the vagus nerve itself.

**Clinical Pearl:** The vagus nerve plays a vital role in maintaining homeostasis by regulating heart rate, cough reflex, and saliva production, among other functions. However, it is essential to differentiate between the sympathetic and parasympathetic divisions of the vagus nerve as well as the cardiac and respiratory branches. Understanding these distinctions is crucial for accurate diagnosis and management of various cardiovascular and respiratory disorders.