Digestive System – Mouth & Salivary Structures

Digestive Tract Road-Map

• Course structure from here on: one organ at a time, starting at the mouth and proceeding distally (pharynx → esophagus → stomach → small intestine → large intestine).

• Accessory organs (liver, gallbladder, pancreas, salivary glands) connect to, but are not part of, the tube; they will be discussed whenever their ducts meet the tract.

Mouth – General Functions

• Ingestion site: entry point of food & drink.

• Begins mechanical digestion (mastication with teeth, tongue, cheeks).

• Begins chemical digestion (salivary enzymes).

• Negligible absorption:

  • Lining = moist (non-keratinized) stratified squamous epithelium → superb protection, poor transport.

  • Only small, lipophilic molecules diffuse across.

Epithelial Character

• “Moist / non-keratinized” ≠ zero keratin; thin keratinized cap compared with skin’s epidermis.

• Stratification essential for withstanding abrasion (chips, crusts, accidental tongue bites, etc.).

Spatial Anatomy of the Mouth

Two major spaces

• Oral cavity proper: posterior & medial to teeth; houses the tongue.

• Vestibules: between teeth & lips/cheeks.

  • Superior vestibule (above dental arcade).

  • Inferior vestibule (below).

Guarding Structures – Lips (Labia / Labium)

• External skin continuous with epidermis but thinner; rich vascular bed → red hue.

• Internal surface = moist stratified squamous epithelium.

• Packed sensory endings: warn against thermal & mechanical extremes.

• Skeletal muscle core (orbicularis oris) → voluntary closing, pouting, phonation; circular arrangement classifies it as a sphincter.

Cheeks

• Lateral walls of the oral cavity.

• Skeletal muscle = buccinator (from Hebrew root for “trumpet-player”).

• Works with masseter for mastication.

Roof – The Palate

• Hard palate (anterior): maxilla + palatine bones.

• Soft palate (posterior): skeletal muscle; mobile.

  • Elevates during swallowing → seals nasopharynx.

  • Terminates inferiorly in the uvula (contains mucosa-associated lymphoid tissue, MALT).

Floor – The Tongue

• Covered by moist stratified squamous epithelium; skeletal muscle core → fully voluntary.

• Two regions:

  • Body: mobile anterior 2⁄3.

  • Root: posterior 1⁄3 anchored to hyoid & fascia.

• Lymphatic companions:

  • Lingual tonsils on superior root.

  • Palatine tonsils lateral to root, near soft palate.

• Lingual papillae (tiny projections)

  • Roughen surface → better food grip, bolus formation.

  • Taste buds embedded on papilla sides, accessed by taste pores.

  • Gustatory cells = chemoreceptors transmitting via cranial nerves VII, IX, X.

• Histology snapshot:

  • Epithelium stains purple, underlying CT blue, skeletal muscle gray.

Teeth

Classes

1. Incisors – chisel-like; cut.

2. Cuspids / Canines – pointed cone; tear & grip.

3. Bicuspids / Premolars – two cusps; crush.

4. Molars – broad, multiple cusps; grind.

Generic Tooth Anatomy

• Enamel (white cap)

  • Acellular; densely crystalline like hydroxyapatite Ca{10}(PO4)6(OH)2.

  • Hardest body substance; cannot self-repair.

• Dentin (deep orange in diagram)

  • Cellular, bone-like; can undergo limited repair.

• Regions

  • Anatomic crown: all enamel-covered parts (includes a short intra-gingival segment).

  • Clinical crown: only enamel visible above gingiva; relative height used to assess gum recession.

  • Neck: junction between crown & root.

  • Roots: anchor in bony alveoli of maxilla/mandible.

• Pulp cavity & root canal

  • Contains pulp (loose CT, blood vessels, nerves).

  • Apical foramen at tip admits neurovascular bundle.

• Cementum: thin layer on root; attachment site for periodontal ligaments (PDL).

• PDL: dense regular CT fibers radiating 360° between cementum & alveolar bone → “suspension bridge” shock-absorber.

Clinical Connections

• Gingivitis: bacterial acid erodes gingiva → clinical crown appears “longer.”

• Periodontal disease: deeper infection destroys PDL; tooth loosens & may fall out.

• Untreated spread → bloodstream → endocarditis or sepsis.

Salivary Glands

• Six total; bilateral symmetry.

1. Parotid (largest) – ante-inferior to ear; duct pierces buccinator into vestibule.

2. Submandibular – beneath body of mandible; duct opens beside lingual frenulum.

3. Sublingual – cluster of small glands under tongue; multiple small ducts.

Microscopic Layout

• Compound tubulo-alveolar glands:

  • Serous alveoli → watery fluid rich in enzymes & HCO_3^-.

  • Mucus alveoli → viscous mucus.

• Myoepithelial (basket) cells wrap alveoli; when stimulated, contract → squeeze secretions out (Boyle’s Law P1V1 = P2V2).

Saliva Composition & Roles

• Water (~99%): dissolves food chemicals → taste; medium for enzymes.

• Mucus: agglutinates food → soft, cohesive bolus for swallowing.

• Enzymes

  • Salivary amylase: starts carbohydrate catabolism

  • \text{Starch} \xrightarrow[amylase]{} \text{Maltose (disaccharide)}

  • Only partial; final breakdown to monosaccharides happens later in small intestine.

  • Lingual lipase: initiates triglyceride digestion; most activity continues in stomach due to acidic activation.

• HCO_3^- (bicarbonate): buffers oral pH ≈ 6.8–7.0 →

  • Protects enamel, gingiva, PDL from acid.

  • Maintains optimal pH for amylase & lipase (proteins denature if pH drifts).

• IgA, lysozyme, defensins (not emphasized in lecture but implied by innate immunity): antimicrobial.

Applied Physiology & Integrated Themes

• Mastication synergy: teeth (mechanical), tongue (positioning), cheeks/lips (containment) + saliva (lubrication) → creates bolus & triggers swallowing reflex.

• Accessory organs’ preview: even in the mouth, pattern emerges—glands outside lumen secrete via ducts into tract lumen. Same theme recurs (liver → bile ducts; pancreas → pancreatic duct).

• Boyle’s Law in secretion: contraction of myoepithelial cells ↑ pressure, forcing saliva into ducts: ΔP = F/A → F = ΔP \times A (conceptual).

• Structural theme – protection vs absorption: stratified epithelium shields but hampers uptake; later gut segments switch to simple columnar to favor absorption.

• Evolutionary morphology of dentition: incisor–canine–premolar–molar arrangement ideal for omnivory (cut → tear → grind).

• Ethical/Preventive angle: oral hygiene not mere cosmetic; prevents periodontal-triggered systemic infections (e.g., bacterial endocarditis).

Quick Numerical / Chemical References

• Enamel ≈ 96% mineral (mostly hydroxyapatite) vs. bone ≈ 70%.

• Saliva production ≈ 1–1.5 L day⁻¹ in a healthy adult.

• pH buffering by bicarbonate: CO2 + H2O \rightleftharpoons H2CO3 \rightleftharpoons H^+ + HCO3^- (Le Châtelier shift toward HCO3^- when acids accumulate).

Lab & Real-World Links

• Upcoming lab worksheet: trace carbohydrate digestion from mouth (amylase) → stomach (pause) → small intestine (pancreatic & brush-border enzymes).

• Clinical dentistry messages: measure “clinical crown” height; look for gingival pockets; x-ray root canal patency.

• Real-world: trumpet players & wind-instrument musicians hypertrophy buccinator & orbicularis oris (etymological insight).

At this stage you possess a full map of structures, tissues, enzymes, and clinical tie-ins for the oral phase of digestion—sufficient to replace re-watching the source video and to tackle associated lab and exam questions.