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Oral Cavity and Salivary Glands — Anatomy of the GI Tract

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1. Overview of the Oral Cavity

• Initial processing of food (mastication + salivary digestion)
• Speech — modulates sounds produced by the larynx
• Breathing — opens into the pharynx, a common air/food pathway

Boundaries

Two Regions

1. Oral vestibule — horseshoe-shaped space between the teeth and the cheeks/lips
2. Oral cavity proper — the space enclosed within the dental arches

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2. Skeletal Framework

• Maxillae — palatine processes form the anterior ⅔ of the hard palate; alveolar processes carry the upper teeth
• Palatine bones — horizontal plates form the posterior ⅓ of the hard palate; transmit the greater palatine nerve and vessels via the greater palatine foramen and lesser palatine nerve via the lesser palatine foramen
• Mandible — carries the lower teeth; articulates at the temporomandibular joint
• Sphenoid — pterygoid processes anchor soft palate muscles
• Hyoid bone — provides attachment for tongue muscles

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3. The Palate

Hard Palate

• Formed anteriorly by the palatine processes of the maxillae (anterior ¾) and posteriorly by the horizontal plates of the palatine bones (posterior ¼)
• Mucosa bears transverse palatine rugae and a median palatine raphe ending in the incisive papilla anteriorly

Soft Palate

• Continues posteriorly from the hard palate as a movable fibromuscular flap
• Elevated → separates nasopharynx from oropharynx (during swallowing)
• Depressed → closes the oropharyngeal isthmus
• Has a pendant projection posteriorly — the uvula

Five Muscles of the Soft Palate

Key rule: All soft palate muscles are innervated by vagus nerve [X], EXCEPT tensor veli palatini (→ V3).

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4. The Tongue

Surfaces

• Oral (anterior ⅔) — lies in the horizontal plane; bears papillae
• Pharyngeal (posterior ⅓) — curves inferiorly; bears lymphoid nodules (lingual tonsil)
• Separated by the V-shaped terminal sulcus, at whose apex is the foramen cecum (embryological origin of the thyroid gland)

Papillae (on the dorsal/oral surface)

Inferior Surface

• Frenulum of tongue — median mucosal fold; tethers tongue to floor of mouth
• Lingual veins visible on either side
• Fimbriated folds lateral to the veins

Muscles of the Tongue

• Superior/inferior longitudinal, transverse, and vertical

Key rule: All tongue muscles are innervated by hypoglossal nerve [XII], EXCEPT palatoglossus (→ vagus [X]).

Innervation Summary

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5. Innervation of the Oral Cavity

• Upper parts (palate, upper teeth) → Maxillary nerve [V2]
• Lower parts (lower teeth, floor, anterior tongue) → Mandibular nerve [V3]

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6. Salivary Glands

• Numerous small minor glands in the submucosa (lips, palate, tongue, cheeks) — contribute ~10% of total saliva
• Three major paired glands — contribute ~90% of saliva

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6.1 Parotid Gland

• Largest of the three major salivary glands
• Lies in a triangular trench bounded by:
  • Behind — sternocleidomastoid muscle
  • In front — ramus of mandible
  • Above — external acoustic meatus + zygomatic arch
• Extends anteriorly over the masseter muscle
• Parotid duct (Stensen's duct) passes anteriorly over masseter, turns medially to penetrate buccinator, and opens into the oral vestibule opposite the crown of the 2nd upper molar tooth
• Encloses: external carotid artery, retromandibular vein, and facial nerve [VII] (important surgical landmark)
• Secretion: Purely serous (watery, enzyme-rich)
• Blood supply: External carotid artery and branches
• Venous drainage: External jugular vein
• Lymphatics: Parotid nodes → superficial and deep cervical nodes
• Parasympathetic innervation: Glossopharyngeal nerve [IX] → lesser petrosal nerve → otic ganglion → auriculotemporal nerve (V3) → parotid gland

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6.2 Submandibular Gland

• Hook-shaped (intermediate in size; smaller than parotid, larger than sublingual)
• Superficial (larger) part — lies in the submandibular triangle, against the submandibular fossa of the mandible, inferior to the mylohyoid line and below the mylohyoid muscle
• Deep (smaller) part — loops around the posterior margin of mylohyoid to lie in the floor of the oral cavity, lateral to the root of the tongue on the hyoglossus muscle
• Submandibular duct (Wharton's duct) emerges from the deep part and opens on the sublingual caruncle (papilla) beside the base of the frenulum of the tongue
• The lingual nerve loops under and around the submandibular duct (crosses its lateral then medial side)
• Secretion: Seromucous (mixed serous + mucous)
• Blood supply: Facial and lingual arteries
• Venous drainage: Lingual and facial veins
• Lymphatics: Submandibular nodes → deep cervical nodes (especially jugulo-omohyoid node)
• Parasympathetic innervation: Facial nerve [VII] → chorda tympani → lingual nerve → submandibular ganglion → postganglionic fibers to gland

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6.3 Sublingual Gland

• Smallest of the three paired major salivary glands
• Almond-shaped; lies in the floor of the oral cavity, immediately lateral to the submandibular duct and lingual nerve
• Lies against the medial surface of mandible (sublingual fossa, above the anterior ⅓ of mylohyoid line)
• Its superior margin raises the sublingual fold of mucosa, running from the posterolateral floor to the sublingual caruncle anteriorly
• Drainage: Via numerous minor sublingual ducts opening onto the sublingual fold; occasionally a major sublingual duct joins the submandibular duct at the sublingual caruncle
• Secretion: Predominantly mucous
• Blood supply: Facial and lingual arteries
• Venous drainage: Lingual and facial veins
• Lymphatics: Submandibular nodes → deep cervical nodes
• Parasympathetic innervation: Same pathway as submandibular — chorda tympani → submandibular ganglion

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Summary Table — Three Major Salivary Glands

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7. Parasympathetic Innervation of Salivary Glands (High-Yield)

• All glands above the level of the oral fissure → innervated by greater petrosal nerve (branch of CN VII) → pterygopalatine ganglion
• All glands below the level of the oral fissure (submandibular, sublingual, lingual minor glands) → innervated by chorda tympani (branch of CN VII) → submandibular ganglion
• Parotid gland (exception) → innervated by CN IX → lesser petrosal nerve → otic ganglion → auriculotemporal nerve

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8. Histology of Salivary Glands (Junqueira's)

1. Serous cells — pyramidal; round nuclei; prominent RER; apical secretory granules; form spherical acini with a small central lumen; secrete enzymes (e.g., amylase) and proteins
2. Mucous cells — flattened/basal nuclei; condensed chromatin; pale cytoplasm; form tubuloacinar units; produce mucin glycoproteins
3. Myoepithelial cells — lie between secretory cells and their basal lamina; contraction expels secretion into ducts

• Intercalated ducts → low cuboidal epithelium
• Striated ducts → columnar cells with basal infoldings and mitochondria (for ion transport, modify saliva electrolytes)
• Excretory ducts → pseudostratified or stratified epithelium

Clinical note: Xerostomia (dry mouth) results from decreased salivary production — causes include mumps (parotitis), radiation therapy to the head and neck, and drugs such as antihistamines and anticholinergics.

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