Digestive System – Mouth, Saliva, and Swallowing Notes

Mouth and Associated Organs

Also called the oral (buccal) cavity; bounded by lips (anterior), cheeks (lateral), palate (superior), and tongue (inferior).
Walls lined with stratified squamous epithelium.
In the mouth: food is chewed, mixed with enzyme-containing saliva, beginning digestion; swallowing initiated.
Associated organs: Teeth, Tongue, Salivary glands.

Teeth lie in sockets in the gum margins of mandible and maxilla.
Crown made of enamel — the hardest substance in the body.
Classification by function: incisors (cutting), canines (tearing), premolars/bicuspids (grinding), molars (best grinders).
Permanent teeth: humans = 32; different species have different counts (e.g., pigs 44, dogs 42, ruminants 32).

Occupies floor of mouth; composed of interlacing skeletal muscle.
Functions: gripping/repositioning/mixing food, forming bolus, initiating swallowing, speech, and taste.
Papillae: filiform (roughness; no taste buds), fungiform, vallate (circumvallate), foliate (taste buds).

Mechanical breakdown of food; includes indigestible cellulose membranes.
Muscles innervated mainly by motor branch of the cranial nerve V (CN V).
Rhythmical chewing controlled by brain stem; taste centres and other CNS areas influence chewing.

Chewing is partly voluntary and partly reflexive.
Lower jaw held closed by a stretch reflex; reflex inhibition allows jaw drop.
Rhythm, force, and degree of occlusion shaped by reflexes; salivary flow stimulated; food mixed with saliva.

Intrinsic (minor) glands (buccal) scattered in oral mucosa; secrete small amounts to keep cavity moist.
Major extrinsic glands: parotid, submandibular, and sublingual.
Saliva production: humans 0.5-1.5 \,\text{L/day}; horses ~12 \,\text{L/day}; lactating cows ≤200 \,\text{L/day}.
Ducts: parotid duct, submandibular duct, sublingual gland ducts.

Mucus secretions contain mucins for lubrication.
Serous secretions: watery fluid with electrolytes and enzymes.
Parotid glands: serous secretions; Submandibular: serous + mucoid; Sublingual: serous + mucoid; Buccal glands: mucoid.

Glands possess acini (clusters of secretory cells) with primary secretion similar in composition to plasma.
Ducts modify electrolyte composition; saliva entering the mouth is typically hypotonic.
Na⁺ and Cl⁻ reabsorbed in ducts; K⁺ and HCO₃⁻ remain higher in saliva; overall final saliva is hypotonic due to reabsorption.
Membrane potential: luminal Na⁺ reabsorption and Na⁺/K⁺-ATPase activity create a gradient that drives Cl⁻ reabsorption.

Primarily controlled by the parasympathetic nervous system.
Chemo- and mechanoreceptors in the mouth trigger salivatory nuclei in the brain stem; facial (VII) and glossopharyngeal (IX) nerves increase secretion.
Sympathetic stimulation can increase a limited mucus-rich secretion; generally SNS inhibits saliva (dry mouth).
Secretion rate modulated by external stimuli (smell, sight), conditioned/learned reflexes, and congenital reflexes.

Parasympathetic response: abundant/copious saliva, sustained flow, protein-poor, high in HCO₃⁻ and K⁺.
Sympathetic response: limited saliva, protein-rich, lower in HCO₃⁻ and K⁺, transient/several small bursts.

Oropharynx and laryngopharynx allow passage of food and fluids to the esophagus and air to the trachea.
Lined with stratified squamous epithelium; contains mucus glands.
Two skeletal muscle layers for peristalsis: inner longitudinal and outer pharyngeal constrictors.

Muscular tube from the laryngopharynx to the stomach; traverses the thorax and passes through the diaphragm to join the stomach at the gastroesophageal (cardiac) sphincter.
Mucosa: non-keratinized stratified squamous; glands secrete mucus as a bolus moves.
Muscularis externa: circular and longitudinal layers; transitions from skeletal muscle superiorly to smooth muscle inferiorly.
Species differences in muscle composition noted (e.g., primates: more striated; dogs/pigs mixed; ruminants mostly smooth).

Involves coordinated activity of tongue, soft palate, pharynx, and esophagus (about 22 muscle groups).
Three phases exist: buccal phase, pharyngeal-esophageal phase, and esophageal peristalsis.
Buccal phase: voluntary; tongue pushes bolus to oropharynx; upper esophageal sphincter (UES) contracted.
Pharyngeal-esophageal phase: tongue blocks mouth; soft palate and uvula rise to close nasopharynx; epiglottis covers larynx; UES relaxes; food enters esophagus; pharyngeal constrictors push bolus inferiorly; UES contracts after entry.
Peristalsis moves bolus down the esophagus; gastroesophageal sphincter opens to allow entry into stomach and then closes to prevent regurgitation.

Lower esophageal sphincter (LES) opens during swallowing and closes afterward to prevent reflux of acidic gastric contents.
LES becomes fully developed after birth.
Reflux esophagitis: inflammation from reflux of stomach acids.
GERD: erosion of esophageal tissue, scarring, luminal narrowing, and increased cancer risk.

Roles of the tongue, teeth, and salivary glands in forming a bolus.
Three substances found in saliva and their functions: mucin (lubrication), amylase (starch digestion), IgA/lysozyme/peroxidase (defense).
How the esophageal tunics differ from the default GI tunic pattern (mucosa = non-keratinized stratified squamous; muscularis externa transitions from skeletal to smooth).
Phases of swallowing and how the bolus moves from the oral cavity to the stomach: buccal phase, pharyngeal-esophageal phase, esophageal peristalsis with LES opening and closing.
How the respiratory passages are blocked during swallowing: soft palate/uvula elevate to close nasopharynx; epiglottis covers the trachea; respiration is inhibited during the swallow.