Physiology Of Salivary Secretion Notes

Saliva

Exocrine salivary glands play a major role in maintenance of oral health. Saliva secreted from the salivary glands mostly (99%) consists of water, but the small part of organic and inorganic constituents have many roles. Salivary glands are tubuloacinar glands whose ducts open into the oral cavity.

Functional Anatomy And Histology Of Salivary Glands

There are three pairs of Major Salivary Glands:

1. Major Salivary Glands Parotid,
2. Major Salivary Glands Submandibular and
3. Major Salivary Glands Sublingual glands.

Read And Learn More: Oral Physiology Notes

• These glands have acini which may be serous (enzyme rich secretions) or mucus (mucin rich secretions) or mixed (having both serous and mucus in the secretion). all the acini have secretory granules assembled near apical regions of the cell.
  • In serous acinus, granules are smaller and consist of amylase.
  • In mucus acinus, the granules are larger and consist of mucin.
  • The acini are surrounded by myoepithelial cells. Myoepithelial cells are contractile in nature and facilitate flow along the duct system.

Types of Salivary Glands and their Secretions:

Nerve supply to the salivary glands:

Characteristics and composition of saliva:

Mechanism of secretion of saliva:

Salivary secretion involves two stages: Primary secretion and secondary secretion.

• Primary secretion is by acinar cells whose composition is almost equal to that of plasma and as this flows along the duct system.
• It gets modified by secretion or reabsorption of electrolytes which is known as secondary secretion.
• As the primary secretion flows through the ducts, there will be a significant modification in the ionic composition of the fluid in the saliva.
• Sodium ions are actively reabsorbed in exchange with potassium ions into the ductular lumen resulting in increase in the potassium concentration and decrease in the sodium concentration in tubular fluid. The sodium reabsorption is in excess of potassium secretion; chloride ions are passively reabsorbed.
• Bicarbonate ions are secreted by tubular cells, partly by active transport and partly by passive transport in exchange with chloride ions.

Hence, at rest, the saliva is hypotonic, with low sodium and chloride level. When stimulated, the flow rate increases and time available for the absorption of sodium and chloride at the duct is reduced resulting in saliva rich in sodium and chloride. However, the saliva remains hypotonic to plasma. Since bicarbonate levels are also increased when the flow rate is increased, the pH of saliva becomes more alkaline.

Functions of saliva:

Saliva:

1. Keeps the mouth clean and moist.
2. Facilitates speech.
3. Helps in food bolus formation.
4. Stimulate taste buds when food dissolves in saliva.
5. Maintains oral pH.
6. Helps in deglutition.
7. Constant flow of saliva washes food debris and prevents bacterial growth.

Amylase (ptyalin):

• Hydrolyses α1 – 4 glycosidic linkages, hence complex starch in the food is broken down into maltose, before entering the stomach. This enzyme has its maximum activity at pH 7.

Function Of Salivary Glands

Lysozymes have:

• Lysozymes have antimicrobial property and help in maintenance of a balanced oral microflora.
• Lysozymes cleave β-N-acetylmuramic acid residues in bacterial cell walls. They also enhances the activity of immunoglobulins. They work on Gram-negative and Gram-positive organisms.

Salivary mucins:

• They are a large family of glycoproteins. Due to their high negative charge, salivary mucin acquires an expanded structure and help in binding water molecules. This aids in the formation of bolus, forms a coating over bolus, helps in swallowing and keeping the mouth moist.

Saliva consists of small molecular proteins:

• Inhibit growth of calcium phosphate crystals
• Function in remineralization of the enamel
• Bind tannins

The salivary gland synthesizes proteins like IgA, which has a bacteriostatic role. The buffering substances in saliva include bicarbonates and phosphates.

• They help in maintenance of oral pH.
• (If the pH falls – calcium is removed from the tooth – predisposes to caries formation
• If the pH increases – tartar formation destroys the alveo-gingival margin.)

Function Of Salivary Glands

Regulation of salivary secretion:

• There is basal secretion of saliva, which keeps the mouth moist.
• Salivary secretion is regulated by autonomic nerve supply to the gland.
• Parasympathetic nerve supply to the glands is by the VII and IX cranial nerves, which secrete acetyl choline resulting in increase in enzyme-rich salivary secretion.
• Sympathetic nerve supply to the glands is by the upper thoracic segments, which decrease the salivary secretion.

Unconditioned and conditioned reflexes bring about salivary secretion.

• Unconditioned reflex: It is present from birth. The secretion of saliva is followed by stimuli such as
  • Presence of food in the mouth
  • Dental manipulation
  • Stimulation of mechanoreceptors present in the oropharynx.
• Conditioned reflex: This type of salivary secretion is acquired after birth. Aroma of food, thought of food, etc. can bring about salivary secretion without food being in mouth, provided if we have eaten that food before.
  • Russian Physiologist Ivan Pavlov proved this in his experiments on dogs. He rang the bell and gave food to the dog and he repeated the same procedure for a few days. Later on he discovered that ringing the bell alone without food could make the dog to salivate.

Clinical Considerations

Xerostomia (dry mouth):

A condition associated with absence of saliva in the mouth is called Xerostomia.

• Causes: Old age, side effects of certain drugs, radiation, chemotherapy, tobacco use, dehydration
• Features: Dry mouth, cracked lips, sticky and viscous saliva, halitosis (bad breath), problem with speech, dysphagia (difficulty in swallowing), infections in the mouth.

Sialorrhoea (hypersalivation):

A condition associated with increase in the salivary secretion is called Sialorrhoea.

• Causes: Neuromuscular dysfunction/sensory dysfunction, or anatomic (motor) dysfunction, teething, dental caries; in children with cerebral palsy, mental retardation, Parkinson’s disease

Salivary secretion:

Salivary secretion can be blocked by atropine (atropine blocks the receptors on which acetylcholine will act). Atropine is used as a premedication in patients undergoing general anaesthesia as this prevents choking of saliva into their lungs.

Function Of Salivary Glands

Saliva Synopsis

• Exocrine salivary glands play a major role in maintenance of oral health. Saliva secreted from the salivary glands mostly (99%) consists of water, but the small part of organic and inorganic composition has many roles.
• Salivary glands are tubuloacinar glands whose ducts open into the oral cavity.
• There are three pairs of major salivary glands. They are parotid, submandibular and sublingual glands.
• These glands have acini which may be serous (enzyme-rich secretions) or mucus (mucin-rich secretions) or mixed (having both serous and mucus in the secretion). All the acini have secretory granules assembled near the apical regions of the cell.
• The volume of saliva secreted per day is 600–1000 mL and the pH is alkaline.
• Salivary secretion involves two stages, i.e. primary secretion and secondary secretion.
• At rest the saliva is hypotonic, with low sodium and chloride level. When stimulated, the flow rate increases and time available for the absorption of sodium and chloride at the duct is reduced resulting in saliva rich in sodium and chloride.
• Saliva keeps the mouth clean and moist, facilitates speech, helps in food bolus formation, stimulates taste buds when food dissolves in saliva, maintains oral pH, helps in deglutition, constant flow washes food debris and prevents bacterial growth.
• Salivary secretion is regulated by the autonomic nerve supply to the gland.
• Parasympathetic nerve supply to the glands is by the VII and IX cranial nerve supply, which secrete acetyl choline resulting in increase in enzyme rich salivary secretion.
• Sympathetic supply to the glands is by the upper thoracic segments, which decrease the salivary secretion.
• Unconditioned and conditioned reflexes bring about salivary secretion.