physiology of the mouth, pharynx and oesophagus

week 1 ctb

saliva function

• lubricates and wets food: helps create bolus for swallowing

• helps with taste

• begins digestion of starch (via α-amylase) and lipids (via lingual lipase)

• protects oral environment

  • washes away bacteria and food particles

  • keeps mucosa moist

  • cools hot foods

  • contents destroy bacteria

  • maintains alkaline environment

key features of saliva

• hypotonic solution

• 800-1500ml produced each day

• pH 6.2-8

composition of saliva

• water

• high conc of some electrolytes

  • K⁺ : compared to plasma/initial saliva

  • HCO₃^- : maintains alkaline environment

• relatively low conc of some electrolytes (compared to plasma/initial saliva)

  • Na⁺

  • Cl^-

• calcium phosphates: prevent demineralisation of teeth

• mucus: lubrication

• digestive enzymes: salivary alpha amylase, lingual lipase

• antibacterial agents: proteolytic enzymes (lysozyme), ABs (IgA)

salivary glands

• exocrine glands 

• parotid gland: serous saliva, watery and rich in alpha amylase

• sublingual gland: mostly mucous saliva

• submandibular gland: mixed serous and mucous saliva

• many tiny buccal glands

• von Ebner’s glands of tongue: lingual lipase

histological structure of major salivary glands

• compound (branched) tubuloacinar glands

• secretory portion: acinar cells

  • serous cells: alpha amylase and immune components

  • mucous cells: mucin

• myoepithelial cells: contract to compress acinus → saliva forced into ducts

• highly vascularised

branching duct system

• small intercalated ducts: lined by simple cuboidal epithelium and myoepithelial cells

• straited ducts lined by simple cuboidal to complue columnar epithelium

• terminal (principle) duct → oral cavity

saliva production: primary secretion

• acinar cells secrete initial saliva

• initial saliva is isotonic and has a similar electrolyte concentration to plasma

saliva production: ductal modification

• transporters on luminal and basolateral membranes of ductal cells enable modification of initial saliva

• absorption of NaCl is greater than secretion of potassium and bicarb → net absorption of solute

• ductal cells are relatively impermeable to water → hypotonic solution

effect of flow rate

• degree of modification is dependent on flow rate

• bicarb (HCO₃^-) secretion is selectively stimulated so its conc increases with increasing flow rate

resting saliva vs stimulated saliva

control of saliva secretion

• the dominant neural input to the salibary glands is the parasympathetic nervous system (PNS): rest and digest

• stimulation results in:

  • increased saliva production

  • increased bicarb and enzyme secretions

  • contraction of myoepithelial cells

  • increased blood flow (PNS)

xerostomia

• dry mouth from reduced/absent salivary secretion or change in the compositionof saliva

• many potential causes including:

  • dehydration

  • anxiety

  • damage to salivary glands or their innervation

  • medication side effect

  • Sjogren’s syndrome

signs and symptoms of reduced saliva secretion

• dry and painful throat

• dry and rough tongue

• dry and cracked lips

• problems with swallowing and speaking

• difficulty keeping dentures in place

• altered taste

• halitosis (bad breath)

• dental caries and periodontal disease

• signs of oral infections

taste (gustation)

• 5 classifications: sweet, sour, bitter, salty, umami

• taste buds are found on the tongue, palate, larynx and pharynx

• taste buds in tongue are located in taste papillae

• taste buds:

  • taste receptor cells

  • supporting cells

  • basal cells

taste receptor cells

• taste receptor cells = chemoreceptors

• they detect chemical signals and transfuce into electrical signals

• microvilli provide a large surface area

• tastant molecules bind to receptors or enter taste receptor cells → depolarisation → AP generation in afferent nerves

• appreciation of flavour involves olfaction

mastication

• physical digestion

  • breaks down food into small pieces 

  • increases surface area for enzyme action

  • mix food with saliva

  • create bolus for swallowing 

• structures involved:

  • teeth

  • tongue

  • mandible

  • temperomandibular joint

  • muscles of mastication

pharynx

• muscular tube that connects nasal cavity, oral cavity, larynx, oesophagus

3 parts:

• nasopharynx: posterior to nasal cavity

• oropharynx: posterior to the oral cavity

• laryngopharynx: posterior to the larynx

muscle of the pharynx

• inner longitudinal layer

  • shortens, elevates and widens the pharynx during swallowing

• external circular layer (pharyngeal constrictors)

  • contract sequentially to force bolus through pharynx and into oesophagus

  • cricopharyngeus: upper oesophageal sphincter

lower oesophageal sphincter (LOS)

• physiology sphincter at gastro-oesophageal junction

• prevents reflux of gastric contents into oesophagus

components of LOS

• intrinsic components

  • smooth muscle

• extrinsic component

  • right crus of the diaphragm

• other components:

  • acute angle at which the oesophagus enters stomach

  • muscosal folds present at the gastro-oesophageal junction

gastro-oesophageal refulx disease (GORD)

• refulx of stomach contents through the LOS into the oesophagus

• occurs due to the impairment of normal anti-reflux mechanisms

  • increased frequency of transient lower oesophageal sphincter relaxations

  • increased intra-abdominal pressure (pregnancy, obesity)

  • low LOS pressure

  • hiatus hernia

• can cause inflammation of the oesophageal mucosa

• symptoms:

  • heartburn

  • acid brash

  • regurgitation

GORD complications

• oesophageal strincture

  • scarring and narrowing of the oesophagus

Barrett’s oesophagus

• metaplasia of squamous epithelium of oesophagus to gastric mucosa (columnar epithelium)

• associated with an increased risk of oesophageal cancer

swallowing

• oral phase

  • voluntary

• pharyngeal phase 

  • involuntary

• oesophageal phase

  • involuntary

oral phase

• tongue moves bolus back towards oropharynx

• somatosensory receptors, including mechanoreceptors, send afferent information to the swallowing centre in medulla (in brainsteam)

  • via vagus (CN X) and glossopharyngeal (CN IX) nerves

• involuntary swallowing reflex initiated

  • motor information sent to muscles of pharynx and upper oesophagus

pharyngeal phase

• respiratory tract protected

  • soft palate elevates

  • glottis closes abnd larynx elevates

  • respiration inhibited

  • epiglottis tilts to cover larynx

• upper oesophageal sphincter relaxes

• peristaltic wave of contraction 

oesophageal phase

• upper oesophageal sphincter closes

• larynx falls, glottis opens and respiration recommences

• primary peristatic wave

  • mediated by swallowing reflux

• lower oesophageal sphincter relaxes

• secondary peristatic wave

  • stimulated by mechanoreceptors in wall of oesophagus

  • mediated by ENS

dysphagia

• difficulty swallowing

• causes: 

  neurological: dementia, stroke, head injury

  stomach: gastric cancer

  mouth: cleft lip/palate, mouth cancer

  pharynx: tonsilitis, pharyngeal pouch, pharyngeal cancer

  oesophagus: mediastinal tumour, achalasia, GORD

assessment of swallowing

• history and examination

• SALT

  • clinical assessment

  • bedside swallow test

  • instrumental assessment

• investigations:

  • endoscopy

  • barium swallow

  • manometry