Salivation, Swallowing, and Mouth Physiology

Salivation and Swallowing

• The session aims to consider the role and properties of saliva, including control of secretion.
• The session will provide an understanding of the neurological control of swallowing and food movement down the esophagus.

Physiology of the Mouth

• The mouth serves as the entrance to the gastrointestinal tract
• Disruption of food and mixing with saliva form boluses for swallowing.
• A major function of saliva is to protect the oral environment by ensuring a moist, chemically appropriate environment with a healthy bacterial flora is maintained.
• Physical disruption of food is achieved through mastication.
• The oral mucosa and teeth are vulnerable to physical, chemical damage, and infections.
• Masseter muscles, innervated by a branch of the trigeminal nerve, generate forceful forces transmitted via the teeth.

Saliva

• Saliva moistens and lubricates food for swallowing
• Saliva contains enzymes that start the digestion of carbohydrates.
• The autonomic nervous system mainly controls salivary secretion.
• Parasympathetic nerves from the otic ganglion stimulate acinar cells to produce primary secretion.
• Extra HCO3 is added to saliva by duct cells.
• Daily saliva secretion is about 1-1.5 liters, with a large proportion secreted at mealtime.
• Ordinary mixed saliva is composed of 99.5% water and 0.5% solids.
• The solid materials in saliva are organic and inorganic.
• Saliva is a hypotonic solution with relatively low concentrations of Na+ and Cl-, but with excess concentrations of Ca2+ and I-.
• While resting saliva is neutral, it becomes alkaline when stimulated.
• Saliva contains significant mucus.
• The parotid glands secrete about 25% of saliva by volume, which is serous with a mixture of water, electrolytes, and enzymes.
• Sublingual glands secrete about 5% of saliva by volume, rich in mucus, known as mucous saliva.
• Submaxillary glands secrete about 70% of saliva by volume, containing both serous and mucous components.

Secretions in the Mouth

• Salivary Glands: There are 3 pairs of them.
• Parotid
• Sublingual
• Submandibular
• Saliva functions by:
  • Lubricating and cleaning the oral cavity.
  • Dissolving chemicals.
  • Suppressing bacterial growth.
  • Digesting starch with amylase.
• Saliva is always hypotonic to plasma, but first is secreted as an isotonic solution that removes ions.
• Acinar cells secrete an isotonic fluid with enzymes.
• Duct cells remove Na+ and Cl- and add HCO-.
• Tight gaps between duct cells prevent water from following the osmotic gradient, keeping saliva hypertonic.
• At low flow rates, duct cells remove most sodium, making saliva very hypotonic.
• The capacity of duct cells to modify saliva is limited.
• At high flow rates, a smaller fraction of ions is removed, making saliva less hypertonic.
• Secretion promotes HCO3- secretion by duct cells, making saliva more alkaline.
• The rate of ductal recovery of Na+ is increased by aldosterone from the adrenal cortex.

Swallowing

• Begins once saliva mixes with chewed food to form a bolus.
• Swallowing occurs in 3 phases:
  • Voluntary phase: the bolus is moved onto the pharynx.
  • Pharyngeal phase.
  • Oesophageal phase.
• During the pharyngeal phase, afferent information from receptors in the pharynx reaches the swallowing center in the brain stem and triggers:
  • Inhibition of breathing
  • Raising of the larynx
  • Closure of the glottis
  • Opening of the entrance to the oesophagus.
• The upper third of the oesophagus contains voluntary striated muscle under somatic nerve control.
• The lower two thirds contain smooth muscle under parasympathetic nervous system control.
• In the oesophageal phase, a peristaltic wave sweeps down the oesophagus.
• This propels the bolus to the stomach in about 9 seconds.
• During the voluntary stage, food is ready for swallowing and pushed into the pharynx, but can no longer be stopped.
• Once a food bolus reaches the pharynx, actions initiated include:
  • Closure of the nasal aperture.
  • The soft palate is pulled upward to close the posterior nares.
  • Closure of the trachea.
  • Vocal cords are closely approximated.
  • The larynx is pulled upward and anteriorly by neck muscles.
  • Swinging back of the epiglottis.
  • Opening of the esophagus.
• Swallowing: Stages
  • Voluntary Stage
    • Initiates Swallowing process
  • Involuntary Stage
    • Pharyngeal stage
    • Esophageal stage
  • Transport of food from pharynx to stomach