**Core Concept**
Vagal stimulation of the heart triggers a reflex that modulates heart rate and cardiac function through the autonomic nervous system. This reflex involves the release of neurotransmitters from the vagus nerve, which bind to specific receptors on the heart, leading to changes in heart rate and contractility.

**Why the Correct Answer is Right**
The vagus nerve stimulates the sinoatrial (SA) node, the natural pacemaker of the heart, to slow down its firing rate, resulting in a decrease in heart rate. This is mediated by the release of acetylcholine (ACh) from the vagus nerve, which binds to muscarinic receptors (M2 subtype) on the SA node. The binding of ACh to these receptors opens potassium channels, leading to an influx of potassium ions into the cell, which hyperpolarizes the SA node and reduces its firing rate. This response is known as the vagal or cholinergic tone on the heart.

**Why Each Wrong Option is Incorrect**
* **Option A:** Vagal stimulation does not increase heart rate; in fact, it decreases heart rate.
* **Option B:** The baroreceptor reflex is a different mechanism that involves changes in blood pressure and is not directly related to vagal stimulation of the heart.
* **Option C:** The sympathetic nervous system, not the vagus nerve, stimulates the heart to increase heart rate and contractility.

**Clinical Pearl / High-Yield Fact**
Remember that the vagus nerve has a "vagal tone" on the heart, which means that it continuously modulates heart rate and function, and this tone can be overridden by sympathetic stimulation during times of stress or exercise.

**Correct Answer: C. The sympathetic nervous system, not the vagus nerve, stimulates the heart to increase heart rate and contractility.**