Salivary Glands: Anatomy, Histology, and Function

Core Review

The Major Salivary Glands

Parotid Gland

The parotid is the largest salivary gland, located in the preauricular region, extending from the zygomatic arch superiorly to the angle of the mandible inferiorly, and from the external ear posteriorly to the masseter muscle anteriorly. The facial nerve (CN VII) traverses through the substance of the parotid gland but does NOT provide parasympathetic innervation to it — a critical NBDHE distinction.

• Duct: Stensen's duct (parotid duct). It courses anteriorly across the masseter muscle, pierces the buccinator muscle, and opens into the oral cavity through a small papilla opposite the maxillary second molar. The duct is approximately 5-7 cm long.
• Secretion type: SEROUS (watery, enzyme-rich). The parotid produces approximately 25% of the total salivary volume at rest but becomes the dominant contributor (up to 60-70%) during stimulated (eating) conditions.
• Parasympathetic innervation: CN IX (glossopharyngeal nerve) via the otic ganglion. Preganglionic fibers travel via the tympanic nerve (Jacobson's nerve) and lesser petrosal nerve to the otic ganglion. Postganglionic fibers join the auriculotemporal nerve (V3) to reach the gland.
• Sympathetic innervation: Superior cervical ganglion via the external carotid plexus.
• Clinical notes:
  • Mumps (paramyxovirus) typically affects the parotid glands, causing bilateral swelling (parotitis)
  • Parotid swellings are painful because the parotid fascia (capsule) is tightly bound, limiting expansion
  • The parotid is the most common site for salivary gland tumors (~80%), the majority of which are benign pleomorphic adenomas
  • During surgical procedures, the facial nerve and its branches must be carefully preserved

Submandibular Gland

A mixed gland approximately the size of a walnut, located in the submandibular triangle of the neck, partially deep and partially superficial to the mylohyoid muscle.

• Duct: Wharton's duct (submandibular duct). Approximately 5 cm long, it courses anteriorly between the sublingual gland and the genioglossus muscle, opening at the sublingual caruncle (papilla) at the base of the lingual frenulum. Its long, uphill trajectory predisposes it to sialolithiasis (stone formation).
• Secretion type: MIXED — predominantly serous (~90% serous, ~10% mucous). The submandibular gland is the primary contributor to resting (unstimulated) salivary flow, producing approximately 60-65% of the total volume.
• Parasympathetic innervation: CN VII (facial nerve) via the chorda tympani → joins lingual nerve (V3) → synapses in the submandibular ganglion → postganglionic fibers directly innervate the gland.
• Sympathetic innervation: Superior cervical ganglion.
• Clinical notes:
  • Sialolithiasis (salivary stones) most commonly affects the submandibular gland (80-90% of cases) due to the long, tortuous duct with uphill drainage against gravity, plus the calcium-rich, mucoid nature of the secretion
  • Wharton's duct can be palpated bimanually in the floor of the mouth
  • Submandibular gland swelling during meals (meal-time syndrome) suggests an obstructive stone

Sublingual Gland

The smallest of the major salivary glands, located in the floor of the mouth, superior to the mylohyoid muscle, immediately beneath the oral mucosa of the sublingual fold.

• Ducts: Multiple small ducts — the ducts of Rivinus (8-20 small ducts opening along the sublingual fold) and occasionally a larger duct, Bartholin's duct, which may join Wharton's duct.
• Secretion type: MIXED — predominantly mucous (~80% mucous, ~20% serous). Produces approximately 5% of total salivary volume.
• Parasympathetic innervation: CN VII via the chorda tympani → submandibular ganglion (same as submandibular gland).
• Clinical notes:
  • A ranula is a mucous retention cyst (pseudocyst) of the sublingual gland, presenting as a bluish, fluctuant swelling in the floor of the mouth
  • A plunging ranula extends through the mylohyoid muscle into the neck

Minor Salivary Glands

Approximately 600-1,000 minor salivary glands are distributed throughout the oral mucosa (except the gingiva and anterior hard palate). They are primarily mucous (except von Ebner's glands, which are serous and associated with circumvallate papillae). Locations include labial, buccal, palatal, lingual, and retromolar regions. Their parasympathetic innervation comes from CN VII (palatal glands via the greater petrosal nerve, and anterior lingual and floor-of-mouth glands via the chorda tympani).

Histology: Serous vs. Mucous Acini

A basic understanding of salivary gland histology helps in identifying gland types:

Serous demilunes are crescent-shaped caps of serous cells capping mucous acini in mixed glands (especially submandibular). They secrete lysozyme and other serous products.

Saliva: Composition and Functions

Saliva is not simply water — it is a complex fluid containing electrolytes, enzymes, immunoglobulins, mucins, and other proteins. Normal daily production is approximately 0.5-1.5 liters.

Major components of saliva:

• Water (~99%): Solvent and vehicle
• Electrolytes: Na+, K+, Cl-, HCO3-, Ca2+, PO4^3-, F- (in trace amounts)
• Enzymes: Alpha-amylase (ptyalin — initiates starch digestion), lingual lipase
• Mucins (MG1, MG2): Glycoproteins providing lubrication and tissue coating
• Antimicrobial proteins: Lysozyme (cell wall degradation), lactoferrin (iron sequestration), peroxidase (bacterial metabolism inhibition), histatins, cystatins
• Immunoglobulins: Predominantly secretory IgA (sIgA) — the primary immunological defense
• Buffers: Bicarbonate (HCO3-), phosphate, and proteins — critical for pH neutralization after acid challenges

Functions of saliva:

1. Lubrication and tissue protection: Mucins coat oral surfaces, facilitating speech, swallowing, and protecting mucosa from desiccation and mechanical irritation.

1. Buffering and pH regulation: Bicarbonate and phosphate buffer systems neutralize acids produced by cariogenic bacteria and from dietary sources, helping maintain oral pH. This is the primary mechanism opposing enamel demineralization.

1. Antimicrobial defense: sIgA, lysozyme, lactoferrin, peroxidase, histatins, and defensins collectively inhibit bacterial, fungal, and viral colonization.

1. Remineralization: Saliva provides calcium and phosphate ions and serves as the delivery medium for fluoride. It maintains supersaturation with respect to hydroxyapatite, promoting remineralization of early carious lesions.

1. Digestion: Alpha-amylase initiates starch breakdown (hydrolyzing alpha-1,4 glycosidic bonds). Lingual lipase begins fat digestion.

1. Taste: Saliva acts as a solvent, dissolving tastants so they can reach taste receptors.

1. Pellicle formation: Salivary proteins selectively adsorb onto enamel surfaces, forming the acquired enamel pellicle — a protective layer that also serves as the initial substrate for bacterial adhesion.

1. Clearance (oral clearance): The continuous flow of saliva helps clear food debris, sugars, and microorganisms from the oral cavity.

Salivary Flow Regulation

Unstimulated (resting) flow: Approximately 0.3-0.4 mL/min. Dominated by the submandibular gland (~65%). At rest, the sublingual and minor salivary glands provide continuous mucin-rich lubrication.

Stimulated flow: Can reach 4-7 mL/min. Dominated by the parotid gland (~50-60%). Stimuli include taste (especially sour/acidic), chewing (masticatory stimulus), smell, and even the thought of food (psychic stimulus).

Neural control:

• Parasympathetic (primary stimulator): Produces copious, watery (serous) saliva. Mediated by ACh acting on M3 muscarinic receptors.
• Sympathetic: Produces small volumes of viscous, protein-rich saliva. Mediated by norepinephrine acting on beta-adrenergic receptors.

This explains why anticholinergic medications cause xerostomia — they block parasympathetic (M3) stimulation of salivary secretion.

Xerostomia: Clinical Consequences

Xerostomia (subjective sensation of dry mouth) may result from:

• Medications (anticholinergics, antihistamines, antidepressants, antihypertensives, diuretics — the most common cause)
• Radiation therapy to the head and neck (damaging salivary gland parenchyma)
• Sjögren's syndrome (autoimmune destruction of salivary and lacrimal glands)
• Diabetes mellitus, dehydration, anxiety
• Aging (primarily due to increased polypharmacy, not aging per se)

Oral consequences of salivary hypofunction:

• Rampant dental caries (especially cervical/root caries)
• Increased susceptibility to oral candidiasis
• Difficulty swallowing (dysphagia), speaking, and wearing dentures
• Burning mouth sensation, altered taste (dysgeusia)
• Halitosis, mucosal friability, and fissuring
• Increased dental erosion (loss of buffering capacity)

Clinical Application

The dental hygienist routinely assesses salivary flow and quality. The clinical observation of pooled saliva in the floor of the mouth (submandibular/sublingual flow) and the ability to express saliva from Stensen's duct (parotid flow) are part of the oral examination. Understanding salivary function explains why caries risk assessment includes evaluation of xerostomia as a major risk factor. Patients undergoing head and neck radiation require aggressive caries prevention including daily fluoride application, as radiation-induced xerostomia is often permanent and dramatically increases caries risk.

Common Traps

• Confusing parotid innervation (CN IX) with submandibular/sublingual (CN VII)
• Thinking the parotid is mucous or mixed — it is purely serous
• Thinking the sublingual gland has a single major duct like the parotid — it has multiple small ducts
• Confusing Stensen's duct (parotid) with Wharton's duct (submandibular)
• Forgetting that the facial nerve passes THROUGH the parotid but does NOT innervate it