Neurotransmitters

Cholinergic neurons release acetylcholine as their primary neurotransmitter and include all preganglionic fibers of the sympathetic nervous system and pre- and postganglionic fibers of the parasympathetic nervous system. Adrenergic neurons employ noradrenaline as their main neurotransmitter, primarily found in postganglionic sympathetic neurons. The functional divisions of the nervous system determine which neurotransmitter type is used.

Acetylcholine transmits signals between preganglionic neurons and autonomic ganglia in both sympathetic and parasympathetic divisions. It also mediates neuromuscular transmission, inducing voluntary muscle contraction. Postganglionic parasympathetic neurons are cholinergic and release acetylcholine to activate target organs, enabling coordinated autonomic responses.

The adrenal medulla releases a mixture of adrenaline and noradrenaline directly into the bloodstream, amplifying sympathetic nervous system effects throughout the body. This dual hormone release enhances the fight-or-flight response by reaching distant target tissues that postganglionic neurons alone cannot access efficiently.

Co-transmitters are secondary neurotransmitters released alongside primary neurotransmitters to help reach remote targets and produce sustained effects. For example, vasoactive intestinal peptide (VIP) is co-transmitted with acetylcholine in cholinergic neurons, while ATP accompanies noradrenaline. Co-transmitters are typically stored in separate vesicles and can exert trophic effects on synapses.

Nonadrenergic noncholinergic (NANC) transmitters include vasoactive intestinal peptide (VIP), ATP, neuropeptide Y (NPY), and nitric oxide, functioning at postganglionic nerve terminals. These transmitters modulate autonomic transmission independently of acetylcholine and noradrenaline, providing additional layers of neural control in the autonomic nervous system.

Each neurotransmitter stimulates specific receptors on target cells, modulating pre- or postsynaptic effects to produce distinct physiological responses. Neurons often release multiple neurotransmitters simultaneously, allowing complex signaling patterns. This receptor specificity ensures that acetylcholine, noradrenaline, and NANC transmitters each trigger appropriate autonomic responses.

Most neurons release more than one neurotransmitter simultaneously to enhance signaling efficiency and reach distant targets. Co-transmitters provide sustained effects and alternative pathways for signal transmission. This multi-transmitter strategy allows neurons to fine-tune responses and ensure reliable communication across the autonomic nervous system.