Major neurotransmitter released at end organ effectors of the sympathetic division of the autonomic nervous system is
**Core Concept**
The sympathetic division of the autonomic nervous system (ANS) is a key component of the body's "fight or flight" response. It regulates various physiological processes, including heart rate, blood pressure, and respiration, through the release of neurotransmitters at end organs.
**Why the Correct Answer is Right**
The major neurotransmitter released at end organs of the sympathetic division is **norepinephrine (noradrenaline)**. Norepinephrine is synthesized from tyrosine through a series of enzyme-catalyzed reactions, ultimately involving the activity of dopamine beta-hydroxylase. At the end organs, norepinephrine binds to adrenergic receptors, such as alpha-1 (α1) and beta-1 (β1) receptors, triggering a cascade of intracellular signaling events that lead to the desired physiological effects.
**Why Each Wrong Option is Incorrect**
* **Option A:** Acetylcholine is the primary neurotransmitter released by the parasympathetic division of the ANS, not the sympathetic division.
* **Option B:** Dopamine is an important neurotransmitter in the brain, but it is not the primary neurotransmitter released at end organs of the sympathetic division.
* **Option D:** Serotonin is a neurotransmitter involved in various physiological processes, including mood regulation and appetite control, but it is not the primary neurotransmitter released at end organs of the sympathetic division.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the sympathetic division of the ANS is often referred to as the "fight or flight" response, and its activation leads to increased heart rate, blood pressure, and respiration, preparing the body for physical activity or stress.
**Correct Answer: C. Norepinephrine**