Salivary Secretion and Glands

• Salivary secretion involves the production and release of saliva.
• Approximately 1500 ml of saliva are secreted per day.
• Saliva contains proteins (mucin and enzymes like ptyalin), IgA, lysozymes, and electrolytes (K+, HCO3-, Na+, Cl-).
• The pH of saliva is around 7 at rest.

Salivary Glands

• There are several salivary glands.
• The parotid gland secretes serous saliva, accounting for 25% of the total saliva production.
• The submandibular gland secretes mixed saliva (more serous), contributing to 70% of saliva.
• The sublingual gland secretes mixed saliva (more mucous), making up 5% of saliva.
• Buccal glands secrete mucous only.

Stages and Mechanisms of Salivary Secretion

• Salivary secretion occurs in two stages: in the acini and in the ducts.
• In the acini (first stage), the primary secretion contains ptyalin and/or mucin.
• The ionic composition of the primary secretion is similar to plasma (ECF) and is isotonic.
• In the ducts (second stage), the primary secretion is modified to produce hypotonic saliva.
• Na+ is actively reabsorbed, and K+ is actively secreted (regulated by aldosterone).
• Cl- is passively reabsorbed due to the higher rate of Na+ reabsorption compared to K+ secretion.
• HCO3- is actively secreted into the ducts.
• The ducts are relatively impermeable to water, resulting in hypotonic saliva.
• Saliva is hypotonic to plasma, with decreased Na+ and Cl- and increased K+ and HCO3-.

Saliva Secretion During Maximal Flow

• During maximal (rapid) salivary secretion, stimulated by parasympathetic activity, there is not enough time for ductal modification.
• Saliva produced in high flow rates is closer to plasma composition than normal saliva.
• All saliva secreted is absorbed from the gastrointestinal tract and interstitial fluid.
• Loss of saliva outside the body can lead to hypokalemia.

Functions of Saliva

• Saliva protects the oral mucosa.
• It cools hot foods and warms cold foods.
• Saliva neutralizes acids, acting as a buffer to maintain oral pH at about 7.0.
• Swallowing saliva neutralizes gastric acid, relieving heartburn if gastric juice is regurgitated.
• Saliva maintains healthy oral mucosa through:
  • Washing away pathogenic bacteria and food remnants.
  • Lysozyme attacking bacterial walls.
  • Antibodies (IgA) destroying oral pathogenic bacteria.
  • Lactoferrin being bacteriostatic.
• Saliva protects teeth against dental caries.
  • At oral pH 7.0, saliva is saturated with Ca2+, preventing teeth from losing Ca2+.
  • Ca2+ dissolves in acidic environments and precipitates in alkaline environments.
  • Loss of Ca2+ from teeth enamel leads to dental caries.
  • Fluoride protects teeth enamel and is added in drinking water and excreted in saliva.
  • Proline-rich proteins protect teeth enamel and bind toxic tannins.
• Absence of salivation (xerostomia) results in dry, ulcerated, and infected oral mucosa, increasing the incidence of dental caries.
• Saliva plays a role in digestion at the mouth.
  • Ptyalin (salivary α-amylase) digests starch, though this is not essential.
  • Ptyalin is inactivated by low pH in the stomach and reactivated by high pH in the small intestine.
• Saliva provides lubrication and wetting.
  • Swallowing: Mucins (glycoproteins) lubricate food, facilitating swallowing.
  • Speech: Moistens the mouth, facilitating movements of lips and tongue.
  • Taste: Acts as a solvent, stimulating taste receptors.
• Saliva has other functions.
  • Excretion of several drugs like mercury, lead, fluorides, and some organisms.
  • Growth and healing of buccal ulcers by secreting growth hormone.
  • Water regulation: Mouth dryness stimulates thirst.
  • Heat regulation (in panting animals): Evaporation of saliva leads to heat loss.

Control of Salivary Secretion

• Salivary secretion is continuous and increases when food is in the mouth, or with the sight, smell, or thought of food.
• Stimulation of salivary secretion is entirely under neural control (autonomic nervous system).

Innervation of Salivary Glands

• Sympathetic and parasympathetic innervation are complementary and increase salivation.
• Parasympathetic efferent nerves:
  • Produce true watery, profuse secretion with increased electrolytes and decreased organic material.
  • Vasodilation (VD) is due to VIP (co-transmitter with ACh). Atropine decreases saliva production.
• Facial Nerve (chorda tympani):
  • Arises from the superior salivary nucleus.
  • Relays in the submandibular ganglia.
  • Supplies the submandibular and sublingual salivary glands.
• Glossopharyngeal Nerve:
  • Arises from the inferior salivary nucleus.
  • Relays in the otic ganglia.
  • Supplies the parotid salivary gland.
• Sympathetic efferent nerves:
  • Produce viscous and small secretions rich in organic constituents.
  • From the lower cervical and upper thoracic spinal segments (T1 & T2) reach the superior cervical ganglion, then to salivary glands.

Control of Salivary Secretion: Nervous Control Only

• Control of salivary secretion is purely nervous (no hormonal control).
• Unconditioned reflexes (inborn reflexes, no previous learning):
  • Food in the mouth stimulates receptors, leading to increased salivary nuclei activity and salivary secretion.
  • Taste (chemical) receptors respond to salt, sweet, bitter, and sour (acid) stimuli.
  • Tactile (touch) and thermal receptors also play a role.
  • Irritation of the stomach and upper intestine can cause vomiting and salivation.
• Conditioned reflexes (acquired and needing previous training and involving the cerebral cortex):
  • Stimulation of receptors outside the GI tract (proven by Pavlov's experiment).
  • Sight, smell, hearing, preparation, or thinking of food (without presence of food in the mouth) stimulate the cerebral cortex, leading to increased salivary nuclei activity and salivary secretion.
  • Taste and smell areas of the cerebral cortex or amygdala stimulate the appetite area in the hypothalamus, leading to increased salivary nuclei activity and salivary secretion.