A research physiologist is performing an experiment in which he stimulates sympathetic cholinergic neurons. Which of the following responses is expected?
**Core Concept**
The sympathetic nervous system (SNS) is a division of the autonomic nervous system (ANS) that is responsible for the body's 'fight or flight' response. It primarily uses norepinephrine as its neurotransmitter, but it also contains a smaller subset of cholinergic neurons that use acetylcholine as their neurotransmitter.
**Why the Correct Answer is Right**
When sympathetic cholinergic neurons are stimulated, they release acetylcholine onto effector organs, which in this case are smooth muscle cells in the blood vessels. This causes vasodilation, leading to an increase in blood flow to the affected area. The correct answer is vasodilation, which is a characteristic response of sympathetic cholinergic neurons.
**Why Each Wrong Option is Incorrect**
* **Option A:** Bradycardia is not a typical response of sympathetic cholinergic neurons, as they do not directly stimulate the sinoatrial node. Instead, they cause vasodilation, which can lead to a decrease in blood pressure and potentially a reflex increase in heart rate.
* **Option B:** Vasoconstriction is the opposite of the expected response, as sympathetic cholinergic neurons cause vasodilation. This option is incorrect because it describes a response typically associated with sympathetic adrenergic neurons, not cholinergic neurons.
* **Option D:** Increased heart rate is not a direct response of sympathetic cholinergic neurons, as they do not directly stimulate the sinoatrial node. While vasodilation can lead to a reflex increase in heart rate, this is not the primary response of sympathetic cholinergic neurons.
**Clinical Pearl / High-Yield Fact**
The existence of sympathetic cholinergic neurons is a classic exam trap, as many students assume that the SNS only uses norepinephrine as its neurotransmitter. However, this subset of neurons plays a crucial role in regulating blood vessel diameter and has implications for various clinical conditions, such as hypertension and shock.
**Correct Answer:** C.