How saliva production is controlled

Salivating is not a decision: it is driven by the autonomic nervous system, the part of the nervous system that works without you thinking about it. Both of its branches stimulate the salivary glands, but not equally: the effects of the parasympathetic branch are stronger and last longer. They are two hands on the same faucet: one opens it wide and the other barely cracks it open.

Each branch arrives by its own route. The parasympathetic travels along two cranial nerves, the facial and the glossopharyngeal, and acts on M3 muscarinic receptors, which are the locks on the cell that its messenger fits into. The sympathetic releases norepinephrine onto different locks, the adrenergic ones.

Saliva is not secreted in finished form, either. It is made in two steps: the acinus, the little secretory sac at the bottom of the gland, produces an isotonic primary saliva .. with the same salt concentration as blood .. and then the striated duct modifies it as it passes through.

This explains something familiar. Medications are a common cause of dry mouth, presumably because they interrupt that parasympathetic signal.