Which ANS fibers release acetylcholine? Which release norepinephrine?

Verified step by step guidance

1. Understand that the autonomic nervous system (ANS) is divided into two main branches: the sympathetic and parasympathetic nervous systems, each with distinct neurotransmitters.
2. Identify that parasympathetic fibers release acetylcholine (ACh) at both their preganglionic and postganglionic synapses, meaning both sets of fibers in this division are cholinergic.
3. Recognize that sympathetic fibers release acetylcholine at their preganglionic synapses, but most postganglionic sympathetic fibers release norepinephrine (NE), making them adrenergic.
4. Note the exception that some sympathetic postganglionic fibers, such as those innervating sweat glands, also release acetylcholine instead of norepinephrine.
5. Summarize: Parasympathetic pre- and postganglionic fibers release acetylcholine; sympathetic preganglionic fibers release acetylcholine; sympathetic postganglionic fibers mostly release norepinephrine, with some exceptions releasing acetylcholine.

Key Concepts

Autonomic Nervous System (ANS) Divisions

The ANS is divided into the sympathetic and parasympathetic systems, each controlling involuntary body functions. Understanding these divisions is essential because they differ in the types of neurotransmitters their fibers release and their physiological effects.

Cholinergic Fibers and Acetylcholine

Cholinergic fibers are nerve fibers that release acetylcholine as their neurotransmitter. In the ANS, all preganglionic fibers (both sympathetic and parasympathetic) and parasympathetic postganglionic fibers release acetylcholine to transmit signals to target organs.

Adrenergic Fibers and Norepinephrine

Adrenergic fibers release norepinephrine as their neurotransmitter. In the ANS, most sympathetic postganglionic fibers are adrenergic, meaning they release norepinephrine to activate target tissues, contributing to the 'fight or flight' response.