## **Core Concept**
The autonomic nervous system (ANS) consists of two main divisions: the sympathetic nervous system (SNS) and the parasympathetic nervous system (PNS). The SNS is often associated with the 'fight or flight' response. Neurotransmitters play a crucial role in transmitting signals from neurons to effectors like muscles or glands.

## **Why the Correct Answer is Right**
The major neurotransmitter released at the end organ effectors of the sympathetic division is **norepinephrine (NE)**, also known as noradrenaline. This neurotransmitter acts on various adrenergic receptors (α1, α2, β1, β2, β3) in different organs, leading to 'fight or flight' responses such as increased heart rate, blood pressure, and energy mobilization. Norepinephrine is the primary neurotransmitter released by the postganglionic neurons of the sympathetic nervous system, except for sweat glands, which are innervated by cholinergic fibers.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Acetylcholine* is incorrect because, although it is a neurotransmitter used in the autonomic nervous system, it is primarily associated with the parasympathetic nervous system and the preganglionic neurons of both the sympathetic and parasympathetic divisions.
- **Option B:** *Epinephrine* is incorrect because, while epinephrine (adrenaline) is indeed involved in the 'fight or flight' response and is released by the adrenal medulla (which is innervated by preganglionic sympathetic fibers), it is not the primary neurotransmitter released at the end organ effectors of the sympathetic division.
- **Option D:** *GABA* is incorrect because GABA (gamma-aminobutyric acid) is an inhibitory neurotransmitter primarily found in the central nervous system, not typically associated with the autonomic nervous system's end organ effectors.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the sympathetic nervous system uses **norepinephrine** as its primary neurotransmitter at end organ effectors, with **acetylcholine** being used by the parasympathetic nervous system and the preganglionic neurons of both divisions. An exception in the sympathetic division is the sweat glands, which are stimulated by **acetylcholine**.

## **Correct Answer:** C. norepinephrine