The precipitating event in the formation of saliva is the binding of neurotransmitter molecules to cell surface receptor proteins. The principal neurotransmitters involved are acetylcholine and norepinephrine that bind, respectively, to muscarinic-cholinergic, and a- and β-adrenergic receptors. The transduction of the extracellular signal requires an integral membrane protein capable of binding GTP, a G protein, that specifically interacts with the receptor. The components of G protein transduction systems are fairly well studied, but the pathways by which signals are routed are just being recognized. Delineation of such routing pathways is essential to understanding the regulation of saliva formation.
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