During inhibition of SA node by vagal stimulation, which of the following occurs:
**Core Concept**
The autonomic nervous system, specifically the parasympathetic division, modulates the sinoatrial (SA) node's intrinsic pacemaker activity through the release of acetylcholine (ACh). This process is mediated by the vagus nerve, which stimulates the SA node's muscarinic receptors.
**Why the Correct Answer is Right**
When the vagus nerve stimulates the SA node, it releases ACh, which binds to M2 muscarinic receptors on the SA node cells. This binding causes a decrease in the SA node's firing rate, resulting in a slower heart rate. The mechanism involves the inhibition of adenylate cyclase, a key enzyme in the SA node's signaling pathway, thereby reducing cAMP levels and the subsequent activation of protein kinase A. This ultimately leads to a decrease in the SA node's excitability and a slower heart rate.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the question, as it does not address the SA node's response to vagal stimulation.
**Option B:** This option is incorrect because it suggests an increase in heart rate, which is the opposite of what occurs during vagal stimulation of the SA node.
**Option C:** This option is incorrect because it suggests an increase in contractility, which is not directly related to the SA node's response to vagal stimulation.
**Clinical Pearl / High-Yield Fact**
The vagus nerve's stimulation of the SA node is a classic example of a negative chronotropic effect, where the heart rate is slowed. This is a key concept in understanding the autonomic nervous system's control over the cardiovascular system.
**Correct Answer: D**. Decreased heart rate.