10 - Cephalic, Oral, and Esophageal Phase

cephalic phase

sensory inputs (thinking, olfactory, visual, auditory stimuli) that sends afferents to CNS for activation of sympathetic and parasympathetic efferents (neurocrine pathway)

• CNVII and CNIX → stimulate salivary secretion

• CNX → stimulate gastric, bile, and pancreatic secretions

• responses primarily mediated by neurocrine pathway

salivary glands

• parotid gland → serous

  • largest pair of glands

  • innervated by CNIX

  • makes 25% of saliva

  • contain ⍺-amylase

• sublingual gland → mucous

  • smallest pair

  • innervated by CNVII

  • makes 5% of saliva

  • contain lingual lipase

• submandibular gland → mixed

  • innervated by CNVII

  • makes 70% of saliva

major components of saliva

• ⍺-amylase → starch digestion

• lingual lipase → lipid/fat digestion

• mucin and pellicle → glycoprotein for lubrication and teeth protection

• proline-rich proteins → bind tannins

• lysozyme, lactoferrin, IgA → immune function

• R-protein → binds vitamin B₁₂ in stomach

• epidermal and nerve growth factors → mucosal growth and protection

• HCO₃^- → buffers acid and other electrolytes

• H₂O → buffers temperature, oral comfort, speech, cleansing

salivary gland structure

• acinar cells → amylase-containing primary secretion

  • isotonic secretion containing organic and inorganic components

  • similar to plasma concentration

• ductal cells → secondary/final secretion of ductal system

  • hypotonic secretion after modification of ionic content

  • water movement restricted, causing water accumulation

electrolyte secretion by acinar cells

primary isotonic secretion

• apical/luminal side

  • Cl^-/HCO₃^- efflux → establishes electrochemical gradient

  • K⁺ efflux → maintains isotonicity 

• basolateral side 

  • Na⁺/2Cl^-/K⁺ influx → activation based on ionic concentration gradient

• paracellular

  • Na⁺ with water entry via aquaporin-5

electrolyte secretion by ductal cells

secondary hypotonic secretion

• apical/luminal side → increased K⁺ and HCO₃^- with NaCl absorption and alkalinization of saliva

  • HCO₃^-/Cl^- counter-transporter

  • H⁺/Na⁺ counter-transporter

  • K⁺/H⁺ counter-transporter

  • HCO₃^- + H⁺ → H₂CO₃ → H₂O + CO₂ for hypotonic secretions 

• luminal side → maintain electrochemical balance for Na⁺, K⁺, and Cl^- on two sides of membrane

  • Na⁺/K⁺ counter-transporter

  • K⁺ efflux

  • Cl^- efflux

  • H⁺/Na⁺ counter-transporter

effects of saliva flow rate

low rate with pH of 6 and hypotonic; increased rate with pH of 8 and less hypotonic

• pH → exchange of HCO₃^- with Cl^- 

  • active secretion of HCO₃^- in stimulated conditions, causing pH to increase

• osmolality → less hypotonic in response to increased secretion rate

  • less time for ionic modification and water accumulation in high secretion rates

regulation of saliva

saliva secretion is a neurocrine-mediated response

• parasympathetic (CNVII, CNIX)

  • Ach → increased Ca²⁺ → increased watery secretion

  • vasodilation → increased saliva volume

• sympathetic (T1 - T3)

  • NE → ⍺ receptors → increased Ca²⁺ → secretion

  • NE → β receptors → increased cAMP → increased thick secretions

blood flow of salivary glands

salivary glands produce large amounts of saliva, as a function of high blood supply → 1.5L/day

• high blood perfusion provides high metabolic support

• protease from saliva converts plasma kininogen to bradykinin → vasodilator that increases blood perfusion

• Ach and VIP → vasodilation to increase tissue/salivary gland perfusion

oral phase

sensory inputs and mouth stimulations with food

• CNVII and CNIX → stimulate salivary secretion

• CNX → stimulate gastric and pancreatic secretions

• carbohydrate digestion via ⍺-amylase

chewing / mastication

food positioned by tongue and cheek between teeth for grinding, where jaws bring teeth into intermittent contact to repeatedly occlude and open

• muscles → masseter, temporalis, medial pterygoid, lateral pterygoid

• innervation → CN V3 (mandibular branch of trigeminal nerve)

• feedback from proprioceptive nerve in teeth and TMJ

• role in digestion → carbohydrate digestion begins, lipid digestion

pharyngeal phase

swallowing → bolus transfer from mouth to esophagus 

• nasopharynx closure → prevent food into nose

• UES relaxation → force food to be received

• pharyngeal contraction → peristalsis pushes bolus to esophagus

• LES relaxation → prevent food into trachea

  • relaxation mediated by VIP and NO

  • contraction mediated by Ach

esophageal phase

transportation from mouth to stomach, and protection

• anatomy → two muscle layers of circular and longitudinal portions with striated to smooth muscle transition

• innervation → differs between striated and smooth

  • striated (upper 1/3)→ controlled by somatic motoneurons

  • smooth (lower 1/3) → controlled by visceral motoneurons (dorsal motor nucleus) with parasympathetic CNIX and CNX efferent fibers synapsing with ENS

esophageal function

• propulsive → peristalsis with UES and LES relaxation 

  • ENS stimulates motor neuron via Ach release

• protective → closure of UES and LES 

  • ENS inhibits motor neuron via VIP and NO release

LES position

normal lower esophageal sphincter (LES) is below diaphragm, so abdominal pressure keeps it tightly closed 

• if LES above diaphragm → pressure drops and increases risk of reflux

histal hernia

LES moves into thorax and increases reflux

• aggravated by pregnancy, obesity, chronic increased abdominal pressure

• reduced LES tone due to smooth muscle relaxation from progesterone-caused relaxation during pregnancy