Upper Digestive Tract – Comprehensive Study Notes

Function & General Overview of the Digestive System

  • Vital, unified role: preparation of ingested nutrients for absorption and cellular use.
  • Digestive tract (alimentary canal) = continuous tube from mouth ➔ anus; food in its lumen is technically outside the body’s internal environment.
  • Accessory organs empty into or lie within the GI tract: salivary glands, teeth, tongue, pancreas, liver, gall-bladder, etc.

Sequential Components of the Alimentary Canal

  • Mouth (oral/buccal cavity)
  • Pharynx (nasopharynx ➔ oropharynx ➔ laryngopharynx)
  • Esophagus
  • Stomach (cardia ➔ fundus ➔ body ➔ pylorus)
  • Small intestine (duodenum ➔ jejunum ➔ ileum) ❮beyond current lecture❯
  • Large intestine (cecum ➔ colon ➔ rectum ➔ anal canal) ❮beyond current lecture❯

Layered Structure of the GI Wall

  • Mucosa
    • Epithelium varies by region: stratified squamous (protection) in mouth–esophagus; simple columnar (secretion/absorption) in stomach onward.
    • Lamina propria (loose connective tissue) + muscularis mucosae (thin smooth muscle).
  • Submucosa
    • Dense connective tissue containing larger blood/lymph vessels; submucosal (Meissner) nerve plexus; exocrine glands may project here.
  • Muscularis (muscularis externa)
    • Two smooth-muscle layers (inner circular, outer longitudinal); stomach adds an inner oblique layer, giving three sublayers.
    • Myenteric (Auerbach) plexus lies between muscle layers, forming part of the combined intramural plexus that governs peristalsis & secretory tone.
  • Serosa (or adventitia outside the peritoneum)
    • Visceral peritoneum + thin connective tissue; mesenteries are double-layered serosa suspending GI organs.

Key Modifications

  • Extensive mucus-secreting goblet cells in simple columnar regions.
  • Gastric pits/glands in stomach mucosa; intestinal villi, plicae, microvilli in later segments (preview).

Oral (Buccal) Cavity

External/Boundary Structures

  • Lips: skin externally, mucosa internally; meet at oral fissure; philtrum = midline skin groove.
  • Cheeks: lateral walls; mainly buccinator muscle + adipose (forms buccal fat pad in infants for suckling).
  • Fauces: arch-like openings leading to oropharynx; palatine tonsils sit in lateral folds.

Palates

  • Hard palate: anterior bony partition (formed by 2 maxillae + 2 palatine bones).
  • Soft palate: muscular posterior partition; elevates to seal nasopharynx during swallowing; midpoint bears uvula.

Tongue

  • Three regions: tip (apex), body, root (anchored to hyoid).
  • Anchoring: lingual frenulum attaches ventral surface to floor; shortened frenulum = ankyloglossia ("tongue-tie").
  • Intrinsic muscles (superior & inferior longitudinal, transverse, vertical) alter shape for speech/mastication.
  • Extrinsic muscles (genioglossus, hyoglossus, styloglossus, palatoglossus) move tongue position for deglutition & articulation.
  • Vascular/nerve supply: deep lingual artery & vein; motor via CN XII (hypoglossal).

Papillae & Taste Buds

  • Filiform: most numerous, keratinized, provide friction (no taste buds).
  • Fungiform: mushroom-shaped; contain scattered taste buds.
  • Circumvallate (vallate): 8\text{–}12 large papillae in V-shaped row anterior to sulcus terminalis; moats with many taste buds.
  • Foliate: leaf-like ridges on posterolateral edges; taste buds degenerate in childhood.
  • Taste bud microanatomy: taste pore opening through stratified squamous epithelium ⇢ gustatory cells ⇢ afferent fibers (CN VII, IX, X).

Floor of Mouth

  • Rich capillary network → rapid drug absorption (e.g., nitroglycerin).
  • Sublingual papillae: openings of submandibular ducts beside frenulum; multiple small ducts (of Rivinus) for sublingual gland.

Salivary Glands

  • Produce ≈ 1\ \text{L day}^{-1} saliva (water, electrolytes, lysozyme, IgA, amylase, lipase, mucus).
  • Parotid (largest; anterior to ear)
    • Purely serous; secretes amylase-rich, watery juice via Stensen duct opening opposite upper 2^{\text{nd}} molar.
  • Submandibular (medial to mandible body)
    • Mixed (serous > mucus); Wharton duct opens beside frenulum.
  • Sublingual (below mucosa of mouth floor)
    • Predominantly mucous; many small ducts along plica sublingualis; thick, protective secretion.
  • Accessory buccal & palatal glands maintain mucosal moisture between meals.

Dentition

Tooth Anatomy

  • Crown (visible, enamel-covered), neck, root (within alveolar socket; secured by periodontal ligament).
  • Internal tissues: dentin (major bulk; similar to bone but avascular), pulp (loose CT, neurovascular), cementum (covers root; attaches to ligament).

Dental Formulae

  • Deciduous (primary) teeth: 20 total; erupt ≈ 6 mo – 2 yr; shed 6\text{–}13 yr.
    • Formula (per quadrant): 2 I, 1 C, 0 PM, 2 M.
  • Permanent teeth: 32 total; replace deciduous & add premolars + third molars.
    • Formula: 2 I, 1 C, 2 PM, 3 M.

Deglutition (Swallowing)

  • Coordinated muscular reflex moving bolus mouth ➔ pharynx ➔ esophagus.
    1. Voluntary oral phase
    2. Pharyngeal phase (involuntary; soft palate seals nasopharynx, epiglottis covers larynx)
    3. Esophageal phase (peristaltic wave to stomach).

Pharynx

  • Common passage for food/air; digestive participation limited to oropharynx & laryngopharynx.
  • Skeletal muscle walls + mucosal lining; propels bolus during second swallowing phase.

Esophagus

  • Muscular tube (~25 cm) connecting pharynx to stomach.
  • Segments: cervical (with upper esophageal sphincter, UES), thoracic, abdominal (ends at lower esophageal sphincter, LES/cardiac sphincter).
  • Wall: stratified squamous mucosa; submucosal mucus glands ease passage; upper third skeletal muscle → middle mixed → lower third smooth.
  • Adventitia (not serosa) in thorax; short serosal covering below diaphragm.

Stomach

Size & Location

  • Collapsed (empty) ≈ size of large sausage; capacity 1\text{–}1.5\ \text{L} in adults.
  • Lies under diaphragm, mostly in LUQ, partly epigastric region.

Gross Divisions

  • Cardia (receives esophagus)
  • Fundus (dome superior to LES; gas reservoir)
  • Body (central bulk)
  • Pylorus: antrum + canal leading to pyloric sphincter.

Curvatures & Attachments

  • Lesser curvature (concave medial) — lesser omentum.
  • Greater curvature (convex lateral) — greater omentum drapes intestines.

Sphincters

  • LES (cardiac): prevents reflux; failure → GERD.
  • Pyloric sphincter: regulates gastric emptying into duodenum; pylorospasm/stenosis impairs passage.

Wall Specializations

  • Gastric mucosa: thick, folded rugae expand volume; contains gastric pits leading to:
    • Chief (zymogenic) cells → pepsinogen, gastric lipase.
    • Parietal (oxyntic) cells → HCl (pH≈1), intrinsic factor (B$_{12}$ absorption).
    • Mucous neck cells → alkaline mucus.
    • Endocrine cells (G cells) → gastrin; others → ghrelin (hunger hormone).
  • Gastric muscularis: unique third oblique layer enabling multidirectional churning.

Physiologic Functions

  • Mechanical breakdown & churning.
  • Chemical digestion: initiates protein hydrolysis via pepsin.
  • Secretion of intrinsic factor indispensable for erythropoiesis.
  • Limited absorption (water, alcohol, some drugs).
  • Immunologic barrier: acidic milieu kills many microbes.

Selected Disorders of the Upper Digestive Tract

  • Mouth/Salivary
    • Sjögren syndrome: autoimmune exocrinopathy; xerostomia & keratoconjunctivitis sicca.
    • Mumps: paramyxovirus infection of parotid; painful swelling, risk of orchitis.
    • Dental: caries, gingivitis (gum inflammation), periodontitis (deeper infection), leukoplakia (premalignant white patches), malocclusion.
  • Esophagus
    • GERD: chronic LES incompetence → reflux, esophagitis, Barrett changes, risk of adenocarcinoma.
  • Stomach
    • Gastroenteritis: infectious/inflammatory.
    • Anorexia, nausea, emesis — nonspecific symptoms signifying gastric/central triggers.
    • Pylorospasm/stenosis: spasmodic or congenital narrowing of pyloric canal; projectile vomiting in infants.

Key Integrative/Clinical Connections

  • Neural control: enteric nervous system (submucosal + myenteric plexuses) autonomously coordinates motility; modulated by parasympathetic (vagus) & sympathetic fibers.
  • Vascular network in submucosa critical for nutrient pickup; portal circulation directs venous blood from stomach & intestines to liver for first-pass metabolism.
  • Intrinsic factor deficiency (e.g., autoimmune gastritis) → pernicious anemia, underscoring need for normal gastric mucosa.
  • Dentist/physician collaboration: oral health (periodontitis) linked to cardiovascular & systemic inflammation.