Salivary Secretion & Glands

Salivary Secretion

• Saliva production is about 1.5 L per day and is hypotonic with a pH between 6.3 and 6.8
• Three pairs of salivary glands contribute to saliva secretion
• The sympathetic along with the parasympathetic nervous system stimulates salivation, but the latter increases salivation more

Salivary Glands

• Parotid glands contribute 20% of saliva, and secrete serous acini for secretion (watery & rich in enzymes), it's supplied by the glossopharyngeal nerve.
• Submaxillary glands produce 75% of saliva and secrete water that is mixed, and they are supplied by the facial nerve.
• Sublingual glands produce 5% of saliva, secrete mucus acini (thick, rich in mucin), and are supplied by the facial nerve.
• Some scattered salivary glands called Ebner's glands are present in the mucosa of the dorsum of the tongue and secrete lipases

Saliva Composition

• Saliva is 99.5% water and 0.5% solids
• Saliva contains 0.3% organic components which include enzymes like amylase, lipase, lysozymes, and mucus
• Saliva contains 0.2% inorganic components
• Bicarbonate and phosphate act as buffers
• Calcium salts prevent decalcification of teeth
• Electrolytes like Na+, Cl-, Hco3-, and K+ act as coenzymes for salivary amylase

Saliva Function

• Saliva helps facilitate speech and deglutition
• Saliva cleans the mouth through washing, and has antibacterial effects from lysozymes and immunoglobulin A
• Saliva buffers using bicarbonate and phosphate to maintain pH around 7.0 and neutralizes gastric secretion in gastroesophageal reflux
• Ptyalin (salivary α-amylase) digests starch to maltose at pH 6.9 but it is inhibited in the stomach
• Lingual lipase digests 30% of lipids and is secreted by Ebner’s glands on the tongue
• Saliva excretes lead, mercury, fluoride, morphine, and alcohol
• Saliva facilitates taste sensation
• Saliva regulates water balance; decreased saliva gives a thirst sensation

Stages of Salivary Secretion

• The 1st stage occurs in the salivary acini (Primary), produces saliva with similar composition to plasma, and is isotonic
• The 2nd stage occurs in the salivary duct (secondary), modifies secretion under the influence of aldosterone hormone, which reabsorbs Na+, Cl- & Hco3- and secretes K+, and becomes hypotonic to plasma

Innervation of Salivary Glands - Parasympathetic

• Arises from the superior salivatory nucleus in the pons
• The chorda tympani is a branch of the facial nerve that leads to the submandibular ganglion and then the submandibular and sublingual glands.
• The inferior salivatory nucleus in the medulla oblongata connects via the glossopharyngeal nerve
• The lesser superficial petrosal nerve branches to the otic ganglion then connects to the parotid gland
• True secretion has a large volume, is watery, and is rich in enzymes, Na+, CL-, and Hco3.
• Parasympathetic stimulation causes vasodilation of blood vessels in salivary glands

Innervation of Salivary Glands - Sympathetic

• Arises from lateral horn cells of the upper two thoracic segments
• Relays in the superior cervical sympathetic ganglia before innervating salivary glands
• Results in trophic secretion: little in volume, viscus, and rich in mucin
• Causes vasoconstriction of blood vessels in salivary glands
• High Na+, CL-, Hco3- & low K+ concentrations occur with parasympathetic stimulation if the flow of salivary secretion increases

Control of Salivary Secretion

• Saliva secretion is controlled only by nervous reflexes - conditioned and unconditioned

Unconditioned Reflex

• An inborn reflex that does not need prior learning
• Stimuli include direct contact with food, and chewing
• Receptors include taste receptors and receptors in the GIT wall
• Afferent nerves include the chorda tympani from the anterior 2/3 of the tongue, the glossopharyngeal from the posterior 1/3 of the tongue, the lingual nerve with movement of the tongue, and Vagus nerve from the epiglottis
• The center is located within the superior and inferior salivatory nuclei in the brain stem
• Efferent nerves include the chordae tympani and glossopharyngeal
• This increases salivary gland secretion -> True secretion

Conditioned Reflex

• Acquired reflexes needing previous learning
• The stimuli are the sight, smell, hearing, and thought of food.
• Receptors include special sense receptors.
• Afferent nerves are optic, olfactory, and auditory nerves.
• The center is located in the cerebral cortex then connects to salivatory nuclei.
• The efferent and response occur as an unconditioned reflex

Pharynx and Esophagus

• The pharynx is a common pathway for the respiratory and digestive systems
• It has a swallowing receptor area
• Primary peristalsis waves originate from it
• The pharynx is separated from the esophagus by the upper esophageal sphincter
• Swallowing (deglutition) propels food bolus from the mouth to the stomach and is controlled by the swallowing center in the medulla

Swallowing Phases

• There are 3 phases of swallowing: buccal (voluntary), pharyngeal (involuntary) and esophageal (involuntary)

Buccal Phase

• Also called the oral phase
• The elevation and retraction of the tongue propels the bolus to the pharynx using the hard palate

Pharyngeal Phase

• This step is very rapid and involuntary, occurring reflexively through the swallowing reflex
• Receptors are in the oropharynx (tonsillar pillars)
• Sensory fibers of the 5th, 9th, and 10th cranial nerves are afferent
• The center is located in the medulla oblongata (swallowing center)
• The motor fibers of the 5th, 7th, 9th, 10th, and 12th cranial nerves are efferent and result in protective reflexes to prevent entry of food into the air passages.
• The soft palate elevates to closes the nasal cavity
• The palate-pharyngeal folds approximate to sagittal slit to regulate the passage of food
• The glottis (opening of larynx) closes by elevation of the larynx and folding of the epiglottis
• Swallowing apnea inhibits breathing
• The pharyngo-esophageal sphincter relaxes and the superior pharyngeal muscle contracts

Esophageal Phase

• The pharyngo-esophageal junction is normally closed by striated muscle tone to prevent air entry into the stomach
• The sphincter relaxes reflexively during swallowing and then recloses after swallowing
• After swallowing, the bolus travels along the esophagus

Primary Peristaltic Waves

• Initiates with the entry of food bolus into the esophagus
• Starts at the upper end of esophagus
• Is a continuation of the pharyngeal peristalsis
• It travels at 2-4 cm/sec, and is increased by gravity

Secondary Peristaltic Waves

• These initiate at the site of bolus when there is a presence of a bolus in the esophagus
• These waves repeat until the food bolus is driven down to the stomach.
• Vago-vagal reflex coordinates peristaltic movements in the upper part of the esophagus
• Local enteric reflex coordinates it in the lower part

Differences Between Upper and Lower Esophagus

• The upper esophagus has striated musculature and the lower is smooth
• The upper esophagus receives vagus nerve supply only, while the lower uses the vagus nerve and the enteric nervous system
• The upper esophagus has rapid movement, while the lower has slow movement
• Because of the slower rate in the lower part, this will allow time for stomach acid to damage or ulcerate the lower part
• Bilateral vagotomy causes complete paralysis in the upper esophagus, but secondary peristalsis persists in the lower

Lower Esophageal Sphincter (LES)

• The LES is also called the cardiac sphincter
• It is the lower 3-5 cm of the esophagus
• It has a high resting tone (high-pressure zone) exerting a pressure of 15-30 cm H2O above the intra-abdominal pressure to prevent reflux
• It relaxes with a delay when a food bolus reaches it, making it susceptible to damage or ulceration by cold, hot, and spicy foods
• It is contracted by: Sympathetic alpha adrenergic receptors, and gastrin hormone will also increase LES contraction
• It is relaxed via: Inhibitory vagal effect via VIP secretion, and Nitric Oxide. Also by fatty foods, chocolate, alcohol and coffee.

Achalasia

• Failure of LES relaxation during swallowing
• Caused by a decrease in the myenteric nerve plexus, high sensitivity to gastrin hormone, or lesions of the vagus nerve
• Complications include mega-esophagus from food accumulation, increased risk of esophageal ulcer and carcinoma, and recurrent pneumonia from aspiration

Gastric Reflux Prevention

• A high-pressure zone sphincter is present
• The intra-abdominal portion of the esophagus is squeezed by increased intra-abdominal pressure
• The esophagus enters the stomach at an angle
• Gastrin hormone increases tone

Gastroesophageal Reflux

• Return of gastric contents into the esophagus due to weak sphincter pressure
• Increases with pregnancy, smoking, coffee, alcohol, and obesity
• Leads to ulcer of the lower esophagus, heartburn, and stricture of the cardiac sphincter. Barrett’s esophagus is premalignant