Heart rate is decreased by:
## **Core Concept**
The question tests the understanding of autonomic nervous system (ANS) effects on heart rate. The ANS, through its sympathetic and parasympathetic branches, regulates heart rate. Sympathetic stimulation increases heart rate, while parasympathetic stimulation decreases it.
## **Why the Correct Answer is Right**
The correct answer, . (Vagus nerve stimulation), is right because the vagus nerve is a key component of the parasympathetic nervous system. When stimulated, it releases acetylcholine, which acts on the sinoatrial (SA) node of the heart to decrease heart rate. This is mediated through the activation of muscarinic receptors (specifically M2 receptors) on the SA node, leading to a decrease in cyclic AMP (cAMP) and an increase in potassium conductance, hyperpolarizing the cell and reducing the firing rate of the SA node.
## **Why Each Wrong Option is Incorrect**
- **Option A:** (Isoproterenol) is incorrect because isoproterenol is a beta-adrenergic agonist that increases heart rate by stimulating beta-1 receptors in the heart, increasing cAMP, and thus enhancing the firing rate of the SA node.
- **Option B:** (Epinephrine) is incorrect because epinephrine (adrenaline) is a catecholamine that stimulates both alpha and beta receptors. Its effect on the heart is primarily through beta-1 receptors, which increases heart rate and contractility.
- **Option D:** (Atropine) is incorrect because atropine is an anticholinergic agent that blocks muscarinic receptors. By blocking these receptors, atropine prevents the parasympathetic nervous system from decreasing heart rate, effectively increasing heart rate or preventing a decrease in heart rate.
## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the **vagus nerve** plays a crucial role in decreasing heart rate through the release of acetylcholine. Clinically, this is significant in procedures like the vagal maneuver to convert supraventricular tachycardia (SVT) back to a normal sinus rhythm.
## **Correct Answer:** . Vagus nerve stimulation