Any specific chemical agent (including acetylcholine, 5 amines, 4 amino acids, 2 purines, and more than 28 peptides) released by a presynaptic cell, upon excitation, that crosses the synapse to stimulate or inhibit the postsynaptic cell. More than one may be released at any given synapse. The n.'s released by presynaptic cells may modulate transmitter release from presynaptic cells. NO may be a retrograde n., released from postsynaptic cells, to act on presynaptic cells. [neuro- + L. transmitto, to send across]
adrenergic n. a n. formed in sympathetic postganglionic synapses (e.g., norepinephrine).
cholinergic n. a n. formed in pre- and postganglionic synapses of the parasympathetic nervous system (e.g., acetylcholine).
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