Salivary glands

Glandulae salivariae

Three pairs of major salivary glands (parotid, submandibular and sublingual) and hundreds of minor glands secrete about 1–1.5 L of saliva a day into the mouth. The autonomic nervous system controls them almost entirely.

Overview

Daily saliva≈1–1.5 L

Unstimulated shareSubmandibular ≈65–70% · parotid ≈20–25% · sublingual ≈5–8%

Stimulated shareParotid rises to ≈50%

All salivary glands share the same plan. Secretory end-pieces called acini feed a branching duct system. Serous acini secrete a watery fluid rich in protein, including α-amylase, from apical zymogen granules. Mucous acini secrete viscous mucins. Mixed glands have mucous tubules capped by serous demilunes. Contractile myoepithelial cells embrace the acini and intercalated ducts.

Saliva is made in two stages. The acini produce an isotonic, plasma-like primary fluid. The striated ducts then reabsorb Na⁺ and Cl⁻ and secrete K⁺ and HCO₃⁻. They are relatively impermeable to water, so the final saliva is hypotonic. At high flow rates there is less time for this modification, so saliva becomes richer in Na⁺, Cl⁻ and HCO₃⁻.

Unusually, both autonomic divisions stimulate secretion. Parasympathetic activity is the main driver and produces copious watery, enzyme-rich saliva. It comes from the superior salivatory nucleus via VII to the submandibular and sublingual glands, and from the inferior salivatory nucleus via IX to the parotid. Sympathetic activity, from T1–T3 via the superior cervical ganglion, produces a small volume of viscous, protein-rich saliva. Salivary secretion is purely neurally controlled; no hormone triggers it. Taste, chewing, smell and conditioned reflexes (Pavlov) increase it. Sleep, fear, dehydration and anticholinergic drugs reduce it.

Location

Parotid: in front of and below the ear. Submandibular: in the submandibular triangle. Sublingual: in the floor of the mouth. Minor glands: throughout the oral mucosa (lips, cheeks, palate, tongue).

Parts & subdivisions

Parotid gland

Largest. Purely serous. Duct opens opposite the upper second molar.

Submandibular gland

Mixed, mainly serous. Produces most unstimulated saliva.

Sublingual gland

Smallest. Mainly mucous.

Minor salivary glands

Several hundred small, mostly mucous glands in the labial, buccal, palatine and lingual mucosa, including the serous von Ebner glands of the vallate papillae.

Saliva

The combined secretion.

Anatomy

Innervation

Parasympathetic secretomotor fibres: submandibular and sublingual glands via facial nerve → chorda tympani → lingual nerve → submandibular ganglion; parotid via glossopharyngeal nerve → lesser petrosal nerve → otic ganglion → auriculotemporal nerve. Sympathetic fibres come from the superior cervical ganglion along arterial plexuses.

Histology

Compound tubulo-acinar glands divided into lobules. Duct sequence: intercalated → striated (basal infoldings packed with mitochondria) → interlobular excretory ducts.

Function

Lubricate food and form the bolus.

Begin starch digestion (α-amylase) and fat digestion (lingual lipase).

Dissolve tastants so they can reach taste receptors.

Protect the mouth and teeth through buffering, antimicrobial proteins, IgA and remineralisation.

Moisten the oral mucosa for speech.

Regulation

Neural only. Salivary nuclei in the medulla receive input from taste, oral touch, smell, higher centres and the hypothalamus. Aldosterone modifies duct Na⁺/K⁺ handling but does not control the volume secreted.

Clinical significance

Sialolithiasis

About 80–90% of salivary stones form in the submandibular gland. Its duct is long and runs uphill, and its saliva is more viscous, more alkaline and richer in calcium. The classic symptom is painful swelling at mealtimes.

Salivary gland tumours

"The smaller the gland, the more likely a tumour is malignant." Most tumours arise in the parotid, and about 80% of those are benign; pleomorphic adenoma is the commonest and Warthin tumour occurs in smokers. Tumours of the sublingual and minor glands are usually malignant.

Xerostomia and Sjögren syndrome

Dry mouth is caused by anticholinergic drugs, head-and-neck radiotherapy, dehydration or autoimmune Sjögren syndrome. Sjögren syndrome involves lymphocytic destruction of salivary and lacrimal glands, anti-Ro/La antibodies and a raised risk of lymphoma. Loss of saliva leads to rampant caries and candidiasis.