In this article we will discuss about:- 1. Nerve Supply of Salivary Glands 2. Mechanical Effects of Salivary Secretion 3. Observations 4. Rate of Flow and Composition 5. Adaptability 6. Disturbances.
Nerve Supply of Salivary Glands:
The salivary centre consists of superior and inferior salivary nuclei in the reticular formation of the medulla.
The salivary glands receive double nerve supply—both from the sympathetic and the parasympathetic. The parasympathetic fibres to the sub-maxillary (submandibular) and sublingual glands arise from the superior salivary nucleus (dorsal nucleus of the VIIth cranial nerve) in the medulla as nervus intermedins and by-passing the geniculate ganglion descend downwards through the facial (VIIth cranial) nerve and then through the chorda tympanic branch of the facial nerve.
The chorda tympanic nerve descends downwards and reaching the cavity of the mouth meets the lingual nerve. Then the secretory fibres leave the lingual nerve and end in the sub-maxillary (submandibular) ganglion (Langley’s ganglion in animals). From the sub-maxillary ganglion the postganglionic fibres arise and reach the sub-maxillary and sublingual glands and supply them with secretory and dilator fibres.
The parasympathetic or bulbar fibres to the parotid gland arise from the inferior salivary nucleus (dorsal nucleus of IXth nerve) in the medulla and descend downwards through the glossopharyngeal (IXth) nerve and being separated as the tympanic branch pass through the tympanic plexus and then through the lesser superficial petrosal nerve end ultimately in the otic ganglion. From this the postganglionic fibres arise and reach the parotid gland through the auriculotemporal branch of the fifth nerve to supply it with secretory and dilator fibres.
The sympathetic fibres to all these glands is derived from first and second thoracic segments of the spinal cord and come out through the first three or four anterior thoracic nerve roots and end in the superior cervical ganglion.
The postganglionic fibres arise from this ganglion, pass along the walls of the arteries and supply all the salivary glands (Fig. 9.28). The sympathetic fibres are believed to end in the serous gland or in the serous part of the mixed gland and supply vasoconstrictor fibres to vessels of glands and myoepithelial cells of the duct.
Nervous Control:
On stimulation of the parasympathetic nerves in a cat the following effects are observed- (a) secretion of water; (b) vasodilatation. Parasympathetic nerve fibres act through the medium of acetylcholine and so they are known as cholinergic fibres. Hilton and Lewis have found that after stimulation of the parasympathetic fibres an enzyme (kallikrein) is liberated in the tissue fluid which acts on the proteins and form a polypeptide known as bradykinin which produces vasodilatation.
On stimulation of the sympathetic nerves the following effects are observed:
(a) Secretion of viscous saliva with a higher solid content, and
(b) Vasoconstriction.
Sympathetic nerve fibres act through the medium of adrenaline and adrenaline-like substance and so they are known as adrenergic fibres.
Atropine blocks the action of acetylcholine, and has been used in medicine to inhibit salivary secretion. A diet rich in carbohydrate increases the salivary amylase. Of the endocrine secretions, adrenocorticotrophic hormone lowers the sodium concentration of saliva.
Significance of Double Nerve Supply:
Each glandular cell is supplied by two sets of nerves. Probably one helps in the secretion of fluid and salts, and the other for the secretion of organic constituents.
Some holds that the differences in action of these two sets of nerves are not due to their specific effect on the glandular cells, but are due to their different actions on the blood vessels. The sympathetic carries vasoconstrictor fibres; hence their stimulation will cause vasoconstriction in the gland and produce consequently less amount of saliva which becomes necessarily thick. Parasympathetic fibres will cause vasodilatation thus increasing the amount of saliva, which becomes necessarily thin.
Paralytic Secretion:
Claude Bernard observed that after section of the chorda tympanic nerve in a dog or cat, a scanty secretion of thin turbid saliva is produced which increases until the seventh or eighth day, at which it reaches a peak level, and diminishes about the third week. He called it as paralytic secretion.
His presumption was that section of chorda tympani removed the restraining influence on secretion and as a result there was continuous secretion of saliva. Emmelin in 1952 explained that paralytic secretion was due to increased sensitivity of the gland to adrenaline section of the chorda tympanic nerve.
Experiments:
For studying the mechanism of salivation, it is necessary to adopt certain experimental procedures, by which pure saliva unmixed with food can be collected outside.
This has been done in two ways:
(1) A cannula is inserted into the parotid duct and through this all the saliva secreted by the gland is collected outside.
(2) The opening of the duct is resected out and is shifted upon the outer surface of the cheek. Saliva can be collected outside through the opening (Fig. 9.29).
With such preparations it is seen that when food is given to the dog salivation takes place, but when the corresponding nerves are cut salivation completely ceases. This proves that salivation is a purely reflex phenomenon. There is no direct chemical stimulus involved in it.
On further analysis, it is found that two types of reflexes are involved in salivation:
(1) Conditioned or acquired reflex, and
(2) Unconditioned or inherent or inborn reflex.
It is believed that one type of reflex does not exclude the other; both are called into play together under ordinary condition.
Conditioned Reflex:
The existence of this reflex is proved by the fact that even the sight or smell of food can stimulate salivation, although no food is actually given. Various conditioned stimuli can be established which can produce salivation. Pavlov used to sound a gong just before giving food to the animal. After continuing this procedure for some days, it was seen that only the gong sound was sufficient to cause salivation even when no food was given. The gong sound here acts as the conditioned stimulus.
Unconditioned Reflex:
For this reflex, food should actually be given to the dog.
The sensory stimulus for this reflex may arise from various sources as follows:
i. From the Mouth:
This is the chief place from which the normal unconditioned stimulus for salivation arises. The act of chewing, the sensation of taste, the irritation caused by the presence of food upon the mucous membrane of mouth—all these act as the sensory stimuli which reflexly produce salivation (Fig. 9.30).
Here the effector is the salivary gland, the afferent path is represented in the trunks of the chorda tympani, the pharyngeal branches of the vagus and glossopharyngeal nerves, and the lingual, buccal and the palatine branches of the trigeminal nerve, the efferent path is the secretory fibres of chorda tympanic nerve with another peripheral relay station and its centre is the medulla.
ii. Oesophago-Salivary Reflex:
The sensory stimulus may arise from the oesophagus. When the food passes down the oesophagus, salivation is stimulated to some extent. Pathological conditions of oesophagus, such as ulcer, cancer, or the presence of a foreign body in the oesophagus, stimulates salivation. If the distal end of cut oesophagus is stimulated, salivation occurs. If the vagi are divided, the reflex is abolished.
The purpose of this reflex seems:
(a) To provide enough saliva necessary to wash away the irritating substance, and
(b) Swallowing of saliva will set up peristalsis like movement of oesophagus which is likely to drive on the irritant. Oesophageal movement cannot be initiated by mechanical irritation but only when something is swallowed.
(iii) Gastro-Salivary Reflex:
The stimulus may arise from the stomach. Irritation of stomach stimulates salivation. When food is introduced in the stomach of a sleeping dog (to avoid psychic effects), salivation takes place after about 20 minutes. This is also seen in many irritating conditions of stomach, for instance, gastritis, gastric cancer, etc. Increased salivation, before vomiting, is a typical example.
(iv) From other Viscera:
It is possible that stimulus for salivation may arise in other viscera also. For instance, in pregnancy increased salivation occurs. It is believed that the sensory stimulus arises from the distended uterus.
Mechanical Effects of Salivary Secretion:
As the food is chewed, the contractions of the muscles of mastication help to press out the saliva accumulated in the ducts and acini of the glands. Hence, mastication acts not as a real stimulus but through its mechanical effect.
Observations to Prove that Salivation is a Secretory Process:
Although in the saliva, there are certain products of excretion, such as the thiocyanates, urea, etc., yet, the following observations prove that salivation is mainly a secretory phenomenon:
i. Saliva is Extremely Useful:
Saliva is extremely useful, hence, cannot be an excretory product.
ii. Evidence of Work:
During salivation the glands are found to be actively working. It is only to manufacture a secretion that a gland needs to undergo work. Excretory processes do not involve much work.
The following facts prove that the glands are actively working:
a. During salivation the glands increase in size, become vascular and their temperature increase.
b. The hydrostatic pressure in the salivary duct, during active secretion, may be double the amount of blood pressure in the carotid artery. Had it been a process of filtration, fluid ought to have passed from saliva into blood. But actually the reverse process takes place. This shows that the glands are working against pressure.
c. The osmotic pressure of blood is higher than that of saliva; so that fluid ought to have been drawn out of saliva and passed into the blood stream. But since salivation is just the reverse process, the glands must be working against osmotic pressure.
d. During salivation the amount of oxygen used and CO2 produced by the glands, increase, i.e., the R.Q. value of resting gland is 0.6 to 0.8 increased to 1.0. This increase did not occur in the absence of glucose. Thus source of energy for salivary metabolism is glucose and to some extent fructose. This also is a very important evidence of work.
e. Saliva contains certain substances, which is not present in blood, viz., and ptyalin. Obviously, such things must have been manufactured in the glands. This is a sure proof of secretory activity of the gland.
iii. Histological Changes:
During activity a number of histological changes are seen in the gland. One important change is that, the zymogen and mucinogen granules which are present in the resting glandular cells reduce to a much smaller number during activity. They take up water from the cytoplasm in the process of secretion and are released from the cells due to differences in osmotic pressure.
iv. Electrical Changes:
Change of electric potential takes place in the gland during secretion. The outer surface of the gland becomes electrically positive to the hilus. All these evidences prove that salivation cannot be a process of excretion; it is chiefly a secretory phenomenon.
Reflex Control of Rate of Flow and Composition of Saliva:
The receptor-centre-efferent system has got discriminating power so it can govern the salivary secretion, i.e., rate of flow and composition depend on the nature and intensity of the stimulus (e.g., food).
Spontaneous Secretion:
The continuous secretion of saliva without any known stimulus is termed as spontaneous secretion. Although its mechanism is not known but the acetylcholine may be the factor which is constantly secreted by the parasympathetic postganglionic nerve endings in small amount. Since atropine cannot check and cyanide or other metabolic poisons stop this type of secretion, so it is indicated that this is related and dependent to metabolic functions.
Adaptability of Salivary Reflex:
The saliva secreted from the gland varies in both quantity and quality with the physical and chemical nature of the substances stimulating the secretion. The salivary gland does not secrete as a unit but different sets 6f epithelial cells of the gland contribute different components of secretion and their local productivity depends upon the intensity of excitation coming from the salivary centre.
The afferent nerves are also different groups which carry impulse of specific nature and stimulate the different components of salivary centre which is a compound structure consisting of several parts and these in turn excite reflexly and selectively the different epithelial groups for appropriate types of secretion.
Disturbances of Salivary Secretion:
The salivary secretion may be under certain conditions:
i. When decrease or absent called hyposalivation.
ii. When increase called hypersalivation.
1. Hyposalivation:
i. Temporary:
Emotional state, e.g., anxiety, fear, fever and obstruction of the duct due to calculi (sialolithiasis).
ii. Permanent:
Aptyalism is rare but when occurs is due to congenital hypoplasia or absence of the gland.
2. Hypersalivation:
Also called sialorrhoea occurs during:
i. Pregnancy,
ii. Neoplasm of the mouth, tongue, carious tooth, oesophagus, stomach and pancreas,
iii. Ulceration of oesophagus and stomach, spasm of stomach, and
iv. Neurological disorder, e.g., Parkinsonian disease and schizophrenia.