Salivary gland and its secretion
Salivon is the functional salivary unit. It begins most proximally as several Acini which converge into a intercalated duct.
Numerous intercalated ducts unite to form a striated duct. A group of these ducts directly drain their content into glandular tubule which pour their content into fewer excretory ducts. Finally, the excretory ducts converge to form a single excretory duct, which leads to oral cavity.
Salivary gland and secretion
Saliva is the mixed secretion of three pair of main salivary glands, namely parotid, sub maxillary or sub mandibular and sublingual and also many small glands found in the mucous membrane of the mouth.
Glands in general are divided into serous, mucous and mixed types.
Serous glands give rise to thin, watery secretion containing protein without mucin, mucous glands produce a secretion containing the glycoprotein in mucin ; mixed glands produce both serous and mucous secretions.
Cells that gives rise to a serous secretion also secrete enzymes. Zymogens granules are the precursors of enzyme stored in the serous cells.
Mucous gland is collected in a system of ducts which discharge their content through larger ducts into the mouth.
The absorptive and secretary activities of the cells lining the ducts affect the composition of the saliva.
In most of the mammals, parotid glands are serous. In some animals the secretion of this glands is devoid of enzymes .
The sub maxillary glands is mixed in ungulates, dogs and cats but it is serous in rodents. The sublingual gland of the horse, ox, pig, dog and cat is a mixed gland and that of rodents is mucous.
Salivary glands of ruminants
Parotid, submaxillary and sublingual are the major salivary glands of ruminants. In addition, sheep and cattle have two inferior molar glands, small and numerous buccal and labial glands in cheek and lips, palatine glands in hard and soft palate and pharyngeal glands in the pharynx and roof of the tongue. (Inferior molar is a serous gland, Buccal, Pharyngeal and Palatine are mucous glands and Labial is a mixed gland)
- Based on the composition of saliva salivary glands are classified as
- Alkaligenic glands (Parotid, Inferior molar, Buccal and Palatine) secrete more of HCO3 and low content of mucin.
- Mucogenic glands (Submaxillary, Sublingual and Pharyngeal) secrete more of mucin with low concentration of HCO3
Pattern of secretion of salivary glands
The parotid glands secrete spontaneously and continuously. Its secretion is rapid during feeding and rumination on the side of bolus chewing. Its flow is about 2ml/min. at rest and 30 to 50ml/min. during rumination. Mechanical stimulation of mouth, cardia, reticulo-omasal orifice, lips of oesophageal groove and walls of reticulum reflexly stimulate parotid secretion.
Secretion of inferior molar occur only during feeding, whereas buccal and palatine glands shows slow resting rate of secretion. Flow of saliva in cow is 60 to 160litres/day; in sheep 6.0 to 16litres/day.
In dogs, submaxillary and sublingual glands show free flow of saliva during chewing of normal meat (no secretion by parotid), whereas dry meat powder excites abundant secretion from the parotid.
In horse, parotid secretion occurs only during feed intake, whereas in ruminants, parotid secretion is continuous.
Sheep show slight increase in parotid secretion as a result of feeding.
Numerous small glands like inferior molar, buccal, labial, palatine and pharyngeal do contribute to the quantity of ruminant saliva. Volume of saliva produced by adult cattle is 90-190 liter per day. Salivary secretion is continuous and Flow of saliva varies with activity and increase with feeding and rumination to aid deglutition.
Type of ration influences rate of salivation. Saliva is predominantly of two types i.e. serous and mucus.
Serous saliva is rich in carbonate ions and is secreted continuously. Mucus saliva helps to reduce foaming by increasing surface tension and is secreted when animal is feeding and is rich in mucopolysaccharides.
Methods of studying salivary secretion
- Mixed saliva may be obtained directly from the mouth.
- By means of cannulation of the appropriate salivary ducts.
Spontaneous secretion of saliva
Some salivary glands possess the innate ability to secrete in the absence of any from of stimulation and this type of secretion is known as spontaneous secretion. This mechanism exists for maintaining the moist mucous membrane of the mouth and pharynx. In ruminant, the parotid salivary glands secrete spontaneously.
Stimulation of salivary buccal receptors by the entry of the food in the mouth reflexly stimulate copious salivary secretion. Normal foodstuffs cause salivary secretion rich in mucin and enzymes to facilitate easy swallowing, while dry or noxious materials cause watery secretion with little mucin.
Composition and formation of saliva
Saliva like all digestive secretions, is formed from the blood. Mixed saliva obtained from the mouth is a colorless, slightly opalescent liquid containing small amounts of electrolytes, protein and in some animals, a carbohydrate splitting enzymes- salivary alpha amylase, desquamated cells from buccal mucosa and lymphocytes derived from the lymphoid tissues in the mouth and pharynx are also present.
Dog parotid saliva is highly fluid and thin like water,whereas dog sub maxillary glands saliva is highly viscous. The protein of saliva are complex and the term mucin is the galactosamine containing muco proteins. In some animals the resting serous cells of salivary glands contains numerous zymogen granules which are the precursors of the salivary enzymes, alpha amylase, and which disappear after prolonged secretion.
During para sympathetic stimulation, the rate of secretion of both the sub maxillary and parotid glands is almost linear to the rate of stimulation, some of the inorganic constituents of the saliva change their concentration as the rate of secretion increases, the sodium and chloride concentration in the saliva are greater at high rates of secretion than at lower rates whereas the concentration of bicarbonates and potassium are higher at higher secretory rates.
Composition of Ruminant saliva
Inorganic salts | Nacl, KCl, CaCO3, NaHCO3, PO4 |
Organic components | Mucin, Ptyalin, Urea,Uric acid, Creatinine and Amino acids. |
Suspended organic matters | Leukocytes and epithelial cells |
Gases | CO2, O2, H2, N2 and water vapours |
Reaction of saliva
Slightly acidic in man, slightly alkaline in most of the domestic animals (except ruminants) and distinctly alkaline in ruminants.
Amount of saliva secretion
- Amount of salivary secretion is inversely related to moisture content in the food. Quantity of salivary secretion:
- Horse- 50 ml / minute during mastication
- cow- 100 to 200 L/ day
- single parotid gland of sheep- 930 to 1840 ml/24hrs.