Classification of Neurotransmitters

The autonomic nervous system contains cholinergic and adrenergic neurons, classified by their neurotransmitters. Cholinergic neurons release acetylcholine, while adrenergic neurons release noradrenaline. All preganglionic neurons are cholinergic, but postganglionic neurons vary: parasympathetic postganglionic neurons are cholinergic, whereas sympathetic postganglionic neurons are adrenergic.

Acetylcholine facilitates signal transmission between preganglionic neurons and autonomic ganglia in both sympathetic and parasympathetic divisions. It also mediates neuromuscular transmission, inducing voluntary muscle contraction through the somatic nervous system. Additionally, all postganglionic parasympathetic neurons release acetylcholine at their target organs.

Postganglionic sympathetic neurons release noradrenaline at their nerve terminals. In contrast, the adrenal medulla releases a mixture of both adrenaline and noradrenaline directly into the bloodstream. This dual source allows the sympathetic nervous system to produce both localized and systemic effects during fight-or-flight responses.

Nonadrenergic noncholinergic (NANC) transmitters include vasoactive intestinal peptide (VIP), ATP, neuropeptide Y, nitric oxide, and others like GABA and dopamine. These transmitters function at postganglionic nerve terminals and in ganglionic transmission, providing additional layers of autonomic regulation beyond acetylcholine and noradrenaline signaling.

Co-transmitters are released alongside primary neurotransmitters to enhance signal transmission. For example, VIP is co-transmitted with acetylcholine in cholinergic neurons, while ATP accompanies noradrenaline in adrenergic neurons. Co-transmitters help reach remote targets, produce sustained effects, and are typically stored in separate vesicles from the primary neurotransmitter.

Each neurotransmitter stimulates specific receptors on target cells, modulating pre- or postsynaptic effects. This receptor specificity allows acetylcholine, noradrenaline, and NANC transmitters to produce distinct physiological responses despite acting within the same nervous system. Most neurons release multiple neurotransmitters simultaneously to fine-tune these responses.

Classifying neurotransmitters by type—cholinergic, adrenergic, or NANC—reveals how drugs can selectively target specific neural pathways. Direct acting cholinergic agonists and indirect acting cholinergic agonists work by enhancing acetylcholine signaling, while cholinergic antagonists block it. This classification framework enables precise pharmacological intervention in autonomic dysfunction.