Saliva is the first digestive juice to come in contact with food.
Saliva is secreted by three major salivary glands namely:
ii. Submandibular (sub-maxillary)
Apart from these, there are minor salivary glands in the floor of the mouth, pharynx, tongue and cheeks.
Composition of Saliva:
On an average, the rate of secretion of saliva is about 1 ml/minute.
i. Total volume/day = 1-1.5 liters
ii. pH—slightly acidic
iii. Hypotonic to plasma
iv. 99.5% water
v. 0.5% solids
vi. Electrolytes: Na+, K+, Ca++, CI–, HC03–, PO4
i. Salivary—ptyalin (salivary amylayse)
v. Blood group substance
vi. Amino acids
viii. Uric acid
x. Cell debris
Mechanism of Secretion of Saliva:
There are two theories that try to explain the mechanism of salivary secretion:
a. Active secretion:
Secretion of saliva is an active process because it involves increased metabolism, utilization of energy substrate and increased oxygen consumption. The secretion from the acini is called the primary secretion. It contains water and electrolytes similar in composition to plasma besides the other salivary proteins.
Unlike other glands, the duct system of salivary glands is actively involved in modifying the composition of saliva, e.g. in the striate duct of the epithelium reabsorbs Na+ in exchange for K+. This epithelium also actively secretes HCO3–. Therefore, the final saliva reaching mouth has more K+ and HCO3– and less of Na+ and CI– compared to the primary secretion.
According to this theory, saliva is an ultra filtrate of plasma, very much like glomerular filtrate in the kidney. The pressure for filtration is provided by arterial blood pressure. This theory is not accepted because even when the pressure in the salivary gland is higher than the arterial blood pressure, the salivary secretion continues.
The saliva that is collected from the mouth is contributed by all the salivary glands and the fraction from the different glands will be 70% from submandibular, 25% from parotid and 5% from sublingual.
Regulation of Secretion of Saliva:
Salivary secretion regulation is brought about by both neural and the hormone (hormonal) mechanisms. Salivary secretion is a spontaneous process subject to modification by other factors. Effect of stimulation of nerves supplying salivary glands.
a. Parasympathetic secretomotor fibers:
As the name secretomotor implies, parasympathetic stimulation increases the volume of salivary secretion profoundly.
The secretion is rich is ptyalin.
This is because of two reasons:
i. The parasympathetic nerve fibers stimulate the acinar cells directly.
ii. They cause vasodilatation in the salivary gland increasing the blood flow.
a. The mechanism of vasodilatation by the parasympathetic nerves in salivary gland is brought about by acetylcholine.
b. Parasympathetic stimulation also liberates VIP (vasoactive intestinal polypeptide). This can also cause vasodilatation.
c. Parasympathetic stimulation also secretes kallikrien, an enzyme found in the acini, kallikrien acts on two globulins of plasma and forms bradykinin which is a very powerful local vasodilator. Increase in the blood flow is not abolished by atropine suggesting that acetylcholine is not responsible for vasodilatation.
b. Sympathetic stimulation:
Sympathetic stimulation causes vasoconstriction and reduction in blood supply to the salivary glands.
As far as the direct action on acini is concerned, the sympathetic stimulation has no effect on parotid gland in man. In the case of submandibular gland, in man there is increase in secretion but the amount is small and the secretion is thick and viscous due to high mucin content. Sympathetic stimulation causes contraction of myoepithelial cells.
Salivary secretion is brought about by reflex action:
a. Unconditioned reflex:
This reflex is present at birth. This is due to the stimulation of receptors in the mouth by chemical substances which are present in food and even mechanical stimulation brought about by food in mouth.
Presence of food in the mouth brings about immediate secretion. Exclusive mechanical stimulation of oral cavity by any means also stimulates salivary secretion. For example, maneuver of oral cavity by dentists, movement of tongue thereby coming in contact with cheeks.
The most important stimulus is the presence of food in the mouth. The details of the influence of unconditioned stimulus on salivary secretion is detailed in Fig. 5.6.
b. Conditioned reflex:
This reflex is acquired during the life. Here, the stimulation process originates not from the mouth but from the organs of special senses, especially sight and smell, to a certain extent even hearing.
In the case of human beings, the previous experiences associated with the supply of food like the sight, smell can give rise to secretion of saliva. In animals, the conditioned reflex for secretion of saliva can be experimentally produced for sight, sound or smell of food.
Pavlov demonstrated conditioned salivary secretion in dogs. Every time a dog was served with food, after ringing of bell. After few days, just the ringing of the bell alone without food being served causes secretion of saliva. During the training period, the dog learns to associate ringing (sound) of bell with supply of food.
Paralytic Salivary Secretion:
When the chorda tympani nerve supplying the sub- mandibular and sublingual glands is cut, although these fibers are the secretomotor fibres, still the two glands secrete saliva initially less in volume but it starts increasing to a maximum by about the 7th or 8th day after the nerve is cut. This rate of secretion remains constant for about 3 weeks and by about 6 weeks the secretion almost stops.
The reasons for the paralytic salivary secretion are:
1. Chorda tympani:
Chorda tympani, the parasympathetic nerve cutting, means cutting the preganglionic fibers. The postganglionic fibers remain active and secrete acetylcholine for some time.
2. Increased excitability of sympathetic fibers:
On cutting the parasympathetic nerve, the excitability of sympathetic fibers increases and they get stimulated more easily.
3. Denervation hypersensitivity:
Cutting of the parasympathetic chorda tympani nerve makes the gland more sensitive to circulating chemical substances, like acetylcholine.
Augmented Salivary Secretion:
Stimulation of sympathetic and parasympathetic nerve fiber supplying the salivary glands simultaneously causes an increase in secretion which is far more than the summated effects of individual nerve stimulation added.
Taste is not essential for salivation. Substances which are insipid also stimulate the secretion of saliva. But the rate of secretion of saliva is affected by taste. Substances having sour taste bring about a greater volume of secretion. Composition of saliva whether it is rich in enzymes, mucin or water can be varied according to the food that is presented.
Disorders of Salivary Secretion:
Hyposalivation is decrease in the volume of salivary secretion. It can be caused by anxiety, excitement or irradiation.
b. Hypersalivation or sialorrhea:
This can occur in pregnancy and parotitis (e.g. mumps), tumors in oral cavity.
c. Xerostomia (dry mouth):
This is a rare condition in which the salivary glands are deficient or absent right from birth. So there may be scanty secretion of saliva.
d. Chorda tympani syndrome:
Sometimes the chorda tympani nerve may be cut accidentally. When the fibers regenerate, they miss their normal target and instead innervate the sweat gland of skin in the submandibular region.
In such a case, whenever there is stimulation for salivation, say while eating, there is also significant sweating of skin in sub mandibular region. It must be noted that now the sweat glands are also supplied by some parasympathetic nerve fibers that were supplied to salivary glands earlier.