Saliva: Mechanism of Saliva Formation

Introduction

• The nervous system controls salivary secretion. The smell or taste of food, and masticatory stimuli affect the amount of salivary secretion. Secretion of saliva can be divided into two phases – the primary phase which involves secretory acini, and the secondary phase which occurs in excretory ducts.

You can read more about the classification and composition of saliva here.

Saliva – Neural Control

• The autonomic nervous system affects the composition and flow of saliva from glands.
• Parasympathetic Nerves:

• • Primary control of salivary glands is through parasympathetic nerves. These nerves are responsible for secretion of watery, electrolyte-rich and low protein saliva.
  • The parotid gland is primarily stimulated and partly the submandibular gland. Hence, while eating, serous salivary secretion is stimulated.
  • The most effective stimulus for an increase in salivary secretion is a sour taste followed by a salty taste.

• Sympathetic Nerves:

• • In situations of fear, anger, stress, or vigorous exercise where sympathetic nerves are stimulated, the salivary glands produce thicker, protein-rich saliva. This occurs due to a modification in vascular supply of gland by the sympathetic nerves.
  • The sublingual gland and partly the submandibular gland are responsible for an increase in mucous content of salivary secretion.

Salivary Reflex

• Unconditioned reflex: It is an inborn reflex, which induces secretion of saliva when any substance is placed in oral cavity.
• Conditioned reflex: It is a reflex acquired by experience. Even a thought of your favourite food initiates salivary secretion. 

Physiological Process of Formation:

• Primary secretory phase:
  • Formation of protein component:

• • Formation of electrolyte component:

• Secondary/Modification phase of secretion
  • Saliva produced by secretory acini contain electrolytes such as sodium and chloride. Prevention the loss of sodium and chloride ions from the body is necessary. The excretory ducts play a key role in reabsorbing sodium and chloride secreted in saliva from acini. Thus, the end product is hypotonic to plasma.
  • The striated duct modifies secreted saliva from acini by reabsorbing sodium and chloride ions and simultaneously excreting potassium and bicarbonate ions.
  • The autonomic nervous system and mineralocorticoids control the modification step.

Summary

• Salivary secretion is controlled by both the parasympathetic and sympathetic nervous systems.
• The parasympathetic system is typically responsible in normal day-to-day function.
• The sympathetic system usually works when our ‘fight or flight’ response is triggered.
• Saliva can be divided in to a protein component and an electrolyte component. These vary in composition based on the nervous system controlling production.
• Further modification of the saliva occurs in the ducts.

Recommended Reading

• • Essentials of Medical Physiology
  • Master Dentistry Volume 3 Oral Biology: Oral Anatomy, Histology, Physiology and Biochemistry, 1e