## **Core Concept**
The autonomic nervous system (ANS) consists of two main divisions: the sympathetic nervous system and the parasympathetic nervous system. The parasympathetic division is responsible for promoting 'rest and digest' functions. Neurotransmitters play a crucial role in transmitting signals across synapses in the ANS.

## **Why the Correct Answer is Right**
The correct answer, **acetylcholine (ACh)**, is the primary neurotransmitter released at the end organs of the parasympathetic division of the ANS. ACh acts on muscarinic receptors in the end organs to produce various effects such as decreased heart rate, increased salivation, and increased gastrointestinal motility. The release of ACh from the postganglionic neurons of the parasympathetic division is a key mechanism by which the parasympathetic nervous system exerts its effects on various organs.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Norepinephrine is the primary neurotransmitter released by the postganglionic neurons of the sympathetic nervous system, not the parasympathetic division. It acts on adrenergic receptors in various organs.
- **Option B:** Epinephrine (adrenaline) is a catecholamine released by the adrenal medulla and is involved in the 'fight or flight' response. It is not a neurotransmitter released at the end organs of the parasympathetic division.
- **Option D:** Dopamine is a neurotransmitter involved in various functions, including mood regulation, reward, and movement. While it does play a role in the autonomic nervous system, particularly in certain sympathetic ganglia, it is not the primary neurotransmitter released at the end organs of the parasympathetic division.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **atropine**, an anticholinergic medication, blocks muscarinic acetylcholine receptors. This results in various effects, including tachycardia, decreased salivation, and decreased gastrointestinal motility, highlighting the role of acetylcholine in parasympathetic functions.

## **Correct Answer: C. Acetylcholine**