There are three large salivary glands and up to 1000 smaller ones. The largest producers of saliva are the parotid glands, the submandibular glands, and the sublingual glands.

The salivary glands receive both sympathetic and parasympathetic input, although the effects of the parasympathetic nervous system are much stronger. Both types of innervation arrive through cranial nerve VII (the facial nerve) and cranial nerve IX (the glossopharyngeal nerve). Cranial nerve VII innervates the sublingual and submandibular glands, while cranial nerve IX innervates the parotid gland.

Cell bodies of preganglionic parasympathetic neurons are found in the superior salivatory nucleus (cranial nerve VII) or inferior salivatory nucleus (cranial nerve IX). They send axons to the intramural ganglia.

These cells synapse onto postganglionic parasympathetic neurons in the submandibular ganglion (cranial nerve VII) whose axons form the lingual nerve, innervating the sublingual and submandibular glands. Those from cranial nerve IX synapse onto postganglionic neurons that reside in the otic ganglion and whose axons make up part of the auriculotemporal nerve, innervating the parotid gland.

Release of acetylcholine from postganglionic parasympathetic nerves stimulates M₃ muscarinic receptors in the glands. This, in turn, activates phospholipase C in the glandular cells, which splits the lipid phosphatidylinositol diphosphate (PIP₂) into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP₃). IP₃ binds to a ryanodine receptor and releases calcium from intracellular stores. The increased calcium leads to production and release of saliva. M₁ receptors also contribute to this response; they have a similar transduction mechanism to the M₃ receptor, resulting in increased intracellular Ca²⁺.

The peptide neurotransmitter substance P is also involved. Substance P increases the production and release of the enzyme amylase in saliva. Amylase is an enzyme that breaks down starch into simple sugars and helps begin the process of digestion.

The sympathetic innervation comes from preganglionic cell bodies in the upper thoracic levels of spinal cord sending a short axon to end in on synapses onto postganglionic cell bodies in the superior cervical ganglion. The axons travel with blood vessels up to the face, where they release norepinephrine to act on adrenergic receptors in the glands. The effects of sympathetic nervous system activation are shorter in duration and weaker. Because of the role of saliva in digestion, parasympathetic innervation produces a much longer-lasting and more robust response.

About the author

name: Jim Hutchins, PhD

institution: Colorado School of Mines

website: https://jim.hutchins.name

Dr. Jim Hutchins is an adjunct instructor at Colorado School of Mines with 45 years of teaching experience spanning K-12 through medical school. He earned his PhD in Neuroscience from Baylor College of Medicine, where his dissertation focused on acetylcholine as a neurotransmitter in the human retina, followed by postdoctoral research at Vanderbilt University on visual system development. His research contributions include highly cited work on mitochondrial superoxide dismutase and synaptic pruning in the retinogeniculate system. Dr. Hutchins has authored multiple open-access textbooks in neuroscience, medical terminology, and anatomy & physiology, creating freely accessible educational resources that have saved students over $5 million in textbook costs. He is committed to open educational resources and Creative Commons licensing, believing that knowledge should be freely available to all learners.