Salivary Glands: Types, Salivary Secretion, Function Mechanism of Secretion by Nurses Note

 Salivary Glands

The secretion from the salivary glands is called salivary secretion. It is secreted from a heterogeneous group of salivary glands located in and around the mouth cavity. Salivary secretion is distinctive for its exclusive neural regulation.

Salivary Glands 

Types 

There are major and minor salivary glands. 

1. Major salivary glands are three pairs: parotid, sublingual and submandibular glands.

2. There are many minor salivary glands located in the mucosa of oral cavity, at the pharyngeal outlet, in the palates and in buccal pouches.

Based on nature of secretion: Salivary glands may be serous that exclusively release watery secretions, mucous that secrete viscous secretion mainly containing mucus and mixed that secrete moderately viscous secretions.

Parotid Glands 

Parotid glands are the largest salivary glands. 

1. They are entirely serous glands. Serous cells are usually arranged in the form of rounded acini.

2. The parotid secretion is rich in water and electrolytes, and lacks mucins. 

3. They pour their secretion into the mouth cavity by means of parotid duct (duct of Stensen), which opens into the oral cavity at the level of second molar tooth.

Sublingual Glands 

These glands are situated below the tongue in the floor of the mouth. 

1. Secretions from these glands drain directly into the mouth by means of sublingual ducts (ducts of Rivinus).

2. There are about 10 sublingual ducts. Sublingual glands are predominantly mucous glands.

3. They secrete thick viscous saliva.

Submandibular Glands 

Submandibular, also called submaxillary glands are situated below the inner ramus of mandible on both sides.

1. They pour their secretion into the mouth cavity by means of submandibular ducts (ducts of Wharton).

2 These glands are mixed type (both serous and mucous).

Histology of Salivary Glands 

Salivary gland consists of base units called salivon. Each salivon consists of acinus, intercalated duct and striated duct.

Acinus 

Acinus is a sac-like structure containing many pyramidal cells. Myoepithelial cells are present at places surrounding the pyramidal cells. 

1. Serous cells of acinus contain many endoplasmic reticulum and zymogen granules, and secrete the digestive enzyme, whereas mucous cells contain mucin droplets and secrete mucin.

2. The secretion in the acinus is called primary secretion. Electrolyte composition of primary secretion is similar to that of plasma.

Intercalated and Striated Ducts 

These ducts are lined by cuboidal cells. Secretion from these duct epithelial cells modifies the ionic composition of secretion from acinus and determines the final composition of saliva. Secretion coming out of duct is called modified or final secretion.

Innervation of Salivary Glands 

Salivary glands are innervated by both the divisions of autonomic nervous system. In normal situation, parasympathetic innervation is the major neural factor for salivary secretion.

Parasympathetic Innervation 

 The centers for parasympathetic fibers are located in medulla.

1. Preganglionic fibers for parotid gland are present in 9th cranial nerve that originate in inferior salivary nucleus and terminate in otic ganglion from where postganglionic fibers originate and innervate the gland.

2. Fibers for submandibular and sublingual glands are present in the 7th cranial nerve that originate from superior salivary nucleus and terminate in submandibular ganglion from where postganglionic fibers come out and supply the glands.

3. In general, parasympathetic stimulation is excitatory.

Sympathetic Innervation 

Sympathetic fibers originate from upper cervical segments and terminate in superior cervical ganglion. Postganglionic fibers leave the ganglion and innervate acini, duct and blood vessels. In general, sympathetic stimulation is inhibitory.

Salivary Blood Flow 

Salivary glands have a high rate of metabolism and a high blood flow. Blood flow and metabolism are proportionate to the rate of saliva formation. 

1. The rate of salivary secretion in human is about 50 mL/min/100 g of salivary tissue. 

2. Blood flow to salivary glands is about 10 times the blood flow to that of active skeletal muscles.

3. Stimulation of parasympathetic nerve to salivary gland increases blood flow by about 10 times by causing vasodilation, which occurs due to secretion of VIP, bradykinin, and acetylcholine at nerve terminals. 

4. Sympathetic stimulation decreases blood flow.

Salivary Secretion 

Rate of Secretion 

About 1.5 liters (1,000–1,800 mL) of saliva is secreted every day. Normally, we never realize the amount of saliva secreted as it is a continuous and slow process. Salivary secretion even continues in minimum amount during sleep. However, an orexigenic stimulus, especially sight, smell or thought of food causes immediate and profuse salivation. We realize the importance of salivation when the secretion becomes less and mouth becomes dry. Salivary secretion increases in response to feeding.

Composition of Saliva 

Saliva contains mainly water (99.5%) and some solids (0.5%). Solids are organic and inorganic.

Organic Solids 

Organic solids are mainly enzymes such as ptyalin, lysozyme, lactoperoxidase, carbonic anhydrase, lingual lipase, RNAase, and DNAase. Other organic solids include kallikrein, blood group substances, secretory immunoglobulin (IgA), and nerve growth factor.

Inorganic Solids 

Cations like sodium, calcium, potassium, and magnesium ions, and anions like chloride, bicarbonate, phosphate, sulfate, and bromide ions constitute the inorganic solids.

Tonicity of Saliva 

In human beings, saliva is always hypotonic to plasma. The concentration of sodium and chloride ions in saliva is less than that of plasma. The tonicity of saliva depends on the rate at which saliva is produced. Greater the rate of secretion, higher is the tonicity. The tonicity of saliva is about 70% of that of plasma.

The pH and K+ Content of Saliva 

The pH of saliva is alkaline.

1. The pH of the original secretion in the salivary gland is slightly acidic. But as saliva flows down the salivary. ducts, it becomes alkaline because of secretion of bicarbonate from the duct epithelium. 

2. The decrease in flow rate in duct increases bicarbonate concentration as time to add more bicarbonate increases, and therefore, this increases pH. 

3. However, if the increase in flow is due to parasympathetic stimulation, secretion of bicarbonate from duct cells is more that increases salivary content of bicarbonate.

4. Therefore, invariably with increased flow rate, bicarbonate content becomes high.

5. The concentration of K+ in saliva is always greater than that in plasma, but with increase in flow rate, K+ concentration decreases.

Functions of Saliva 

Saliva performs many important digestive and non-digestive functions.

1. Saliva contains ptyalin. This is an enzyme called salivary amylase, which causes splitting of starch. Salivary amylase reduces starch to oligosaccharide molecules. However, digestion of starch is not impaired in the absence of salivary amylase (if pancreas is intact). Action of salivary amylase is maximum at pH 6.8. But, digestion by ptyalin takes place in the stomach, as food remains for a short duration in mouth. In the stomach, digestion occurs at the center of food bolus which is still alkaline, until the food is thoroughly mixed with the acidic gastric secretion of the stomach.

2. Saliva has many protective functions. Saliva keeps the mouth clean and therefore prevents oral infection. Saliva contains lysozyme, which is anti-bacterial. IgA in saliva provides local immunity and lactoferrin in saliva is bacteriostatic.

3. Saliva keeps the mouth cavity and tongue moist, which facilitates speech. The importance of this function of saliva is realized when mouth becomes dry due to decreased salivary secretion that impairs speech. Dryness of mouth is usually felt before appearing for an interview, especially for learners.

4. Saliva increases the taste of food. Taste is perceived by the taste buds present in the tongue. For taste of food to be well appreciated, food particles should better be present in solution. Saliva dissolves the food and makes the solution of food.

5. Saliva helps in mastication and swallowing. This is due to lubricant property of saliva. The mucin in saliva lubricates food. In the absence of saliva, deglutition becomes impossible, especially for dry foods.

6. Saliva contains bicarbonate which buffers gastric acid to some extent in the stomach, and therefore reduces heart burn.

7. Proline rich protein in saliva protects enamel of the teeth and thus provides them strength. Proline also binds with tannin & reduces its toxicity.

8. Saliva prevents injury to buccal mucosa by diluting the hot and irritant food materials.

9. In animals, salivation (panting) is an important process of dissipation of heat and therefore, has contribution in temperature regulation.

10. Saliva excretes certain heavy metals, thiocyanate and morphine from the body.

Mechanism of Secretion 

Salivary secretion occurs in two stages: secretion in the acinus and secretion in the duct.

Secretion in Acinus of Gland 

In the gland acini, the secretion is called primary secretion in which amylase concentration is more.

1. The enzyme is produced and stored in the zymogen granules.

2. On stimulation, granules are exocytosed and the enzyme is released.

3. The electrolytes secreted from the epithelial cells of the end pieces are Na+, K+, HCO3–, and Cl–.

4.  The fluid of primary secretion is almost isotonic to that of plasma.

Secretion in Ducts

The composition of secretion in the ducts when the fluid passes through the intercalated and striated ducts, is modified. This is called modified or final secretion. 

1. This is because Na+ and Cl–  are extracted from the fluid and K+ and HCO3– are added to the fluid.

2. The ducts do not change the volume of saliva but only modify the composition of the primary secretion.

Stimuli for Salivary Secretion

 

Saliva is continuously secreted. However, a large number of stimuli increase the secretion. The important stimuli are anticipation, thought, sight and smell of food, discussion on food, and presence of food in the mouth cavity. Chewing is also an important stimulus for salivary secretion.

Control of Salivary Secretion 

Salivary secretion is controlled exclusively by neural mechanisms. Both sympathetic and parasympathetic stimuli influence salivary secretion.

Neural Control 

Parasympathetic Stimulation 

Parasympathetic stimulation occurs by sight, smell, and taste of food. 

1. Chewing of food is a potent stimulus for salivary secretion. These factors act through salivary nucleus in the medulla.

2. Parasympathetic activation increases salivary secretion rich in enzymes, and mucin. 

3. It increases secretion by causing vasodilation (via bradykinin, VIP, and acetylcholine), and stimulation of glandular tissue.

Sympathetic Stimulation 

Stimulation of sympathetic fibers (sympathetic fibers to salivary gland originate from superior cervical ganglion) temporarily increases secretion but finally decreases it.

1. The transient increase is due to contraction of myoepithelial cells of the glandular tissue. 

2. However, sympathetic stimulation causes vasoconstriction that decreases saliva formation and makes the secretion thick.

Reflex Secretion 

Saliva is secreted reflexly by contact of food with the mouth cavity. This reflex secretion is unconditioned as this is present since birth (does not need learning). However, salivary secretion due to smell or thought of food is a condition reflex as these are learnt by social and environmental factors. Conditioned salivary secretion was described by Pavlov.

Salivary secretion exclusively occurs in the cephalic phase (sight, smell and thought of food, presence of food in mouth, chewing food, etc.). Secretion is almost nil in gastric and intestinal phase of digestion of food. Hormonal regulation of saliva is negligible.