Salivary glands are found in the mouth. The secretion of salivary glands is called saliva. Saliva is slightly alkaline in nature containing about 99% water. It consists of proteins like mucin which helps in lubricating and passage of the food along the alimentary canal. It also contains the enzymes like salivary amylase and lysozyme. Amylase enzyme incompletely digests starch (tasteless polysaccharide) into maltose (sweet tasting disaccharide). Thus amylase is very helpful for the animals with starch rich diet.
For example, pigs as they feed on tubers and roots. It is important to note that amylase is absent in the saliva of domestic herbivore animals like cows and buffaloes and also in carnivore animals like tiger, lion, etc. The other enzyme present in saliva is lysozyme. It helps to kill the harmful bacteria.
There are two types of salivary glands:
- The major salivary glands
- The minor salivary glands
Major Salivary Glands
The major salivary glands are the largest and most important salivary glands. They produce most of the saliva in your mouth.
There are three pairs of major salivary glands: the parotid glands, the submandibular glands, and the sublingual glands.
Parotid Glands
The parotid glands are the largest salivary glands. They are located just in front of the ears. The saliva produced in these glands is secreted into the mouth from a duct near your upper second molar.
Each parotid gland has two parts, or lobes: the superficial lobe and the deep lobe. Between the two lobes is the facial nerve. The facial nerve is important because it controls your ability to close your eyes, raise your eyebrows, and smile.
Other critical structures near the parotid glands include the external carotid artery, which is a major supplier of blood to the head and neck region, and the retromandibular vein, a branch of the jugular vein.
Surgery to treat a parotid gland tumor is called a parotidectomy. It requires great precision because the surgeon has to locate and operate around these important structures.
Submandibular Glands
About the size of a walnut, the submandibular glands are located below the jaw. The saliva produced in these glands is secreted into the mouth from under the tongue.
Like the parotid glands, the submandibular glands have two parts called the superficial lobe and the deep lobe. Nearby structures include:
- the marginal mandibular nerve, which helps you smile
- the platysma muscle, which helps you move your lower lip
- the lingual nerve, which allows sensation in your tongue
- the hypoglossal nerve, which allows movement in the part of your tongue that helps with speech and swallowing
Sublingual Glands
The sublingual glands are the smallest of the major salivary glands. These almond-shaped structures are located under the floor of the mouth and below either side of the tongue.
Minor Salivary Glands
There are hundreds of minor salivary glands throughout the mouth and the aerodigestive tract. Unlike the major salivary glands, these glands are too small to be seen without a microscope. Several small salivary glands are distributed inside the mouth on inner lining of the lips, cheeks, palate, pharynx and tongue. The important salivary glands which are present outside the mouth are parotid, submandibular and submaxillary glands. All of these glands discharge their secretions into the mouth through ducts.
How Salivary Glands Work
The food enters the digestive tract through mouth or oral cavity. The food is chewed or masticated with the help of teeth. The masticated food is mixed up with the salivary juice secreted by the salivary glands. In man three types of salivary glands are present in pairs. They are Parotid glands in upper jaw, sub lingual and sub maxillary in lower jaw. The saliva contains 99.4% of water and remaining 0.5% of proteins like mucin which is a glycoprotein. The saliva is slightly is acidic in nature having the pH of about 6.8. Number of buccal glands present in the mouth secrete mucous into saliva. The secretion and flow of saliva is stimulated by
- The food in the mouth stimulates salivary secretion
- Dry food secretes more saliva than the moist food
- The thought, sight and smell of the food stimulates saliva
- Acids, salts and many other chemical agents stimulate the salivary secretion.
The digestive action of the saliva depends up on the salivary amylase or ptylin. This salivary amylase is the mixture of two enzymes α and β amylase. Salivary amylase acts on starch converting it into maltose.
| Insoluble starch→ soluble starch→ Erythro dextrin→ Achrodextrin→ maltose |
Maltose is the end product of starch in salivary digestion while insoluble starch is the starting component of digestion.
Salivary amylase activity
- It acts rapidly at the normal body temperature
- It acts more rapidly at 50 to 55o C
- At 75 o C it gets destroyed
- Amylase activity in the stomach is inhibited when acidity of the stomach reaches a pH of less than two.
- The chloride ion is necessary for amylase activity. When the food in the mouth is mixed well with the saliva, the pepsin activity in the stomach is rapid.
When the food is masticated in the mouth in the presence of saliva and mucous, it becomes sticky. When swallowed, this sticky food reaches the stomach through the oesophagus. This phenomenon is called deglutition. This process is assisted by the peristaltic action of the muscles of oesophagus. Because of the absence of the digestive glands, no digestion occurs in oesophagus.
Functions of Saliva
What then are the important functions of saliva? Actually, saliva serves many roles, some of which are important to all species, and others to only a few:
Lubrication and binding: The mucus in the saliva is extremely effective in binding masticated food into a slippery bolus that (usually) slides easily through the esophagus without inflicting damage to the mucosa. Saliva also coats the oral cavity and esophagus, and food basically never directly touches the epithelial cells of those tissues.
Solubilises dry food: In order to be tasted, the molecules in food must be solubilized.
Oral hygiene: The oral cavity is almost constantly flushed with saliva, which floats away food debris and keeps the mouth relatively clean. Flow of saliva diminishes considerably during sleep, allow populations of bacteria to build up in the mouth -- the result is dragon breath in the morning. Saliva also contains lysozyme, an enzyme that lyses many bacteria and prevents overgrowth of oral microbial populations.
Initiates starch digestion: In most species, the serous acinar cells secrete an alpha-amylase which can begin to digest dietary starch into maltose. Amylase does not occur in the saliva of carnivores or cattle.
Provides alkaline buffering and fluid: This is of great importance in ruminants, which have non-secretory forestomachs.
Evaporative cooling: Clearly of importance in dogs, which have very poorly developed sweat glands. Look at a dog panting after a long run, and this function will be clear.
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