Nerve supply of salivary glands
Nerve Innervation in salivary glands
In addition to neural or hormonal stimuli the secretory cells of the salivary glands secrete saliva spontaneously. Neural stimulation produce more secretion in most species. The salivary glands receive both efferent innervations of sympathetic and para sympathetic nervous system which mainly act synergistically on the salivary glands.
Sympathetic supply to salivary glands
The sympathetic fibers leaves the spinal cord as pre ganglionic fibers in the ventral roots of the first two or three thoracic nerves and pass through the cervical sympathetic chain to the superior cervical ganglion. The post ganglionic fibers (adrenegic nerves) from the superior cervical ganglion are distributed to blood vessels of the salivary glands and the secretory cells.
Para-sympathetic supply to salivary glands
The preganglionic fibers of the para sympathetic fibers leave the brains stem through glosso-pharyngeal and trigeminal nerves, synapse with the otic ganglion which privides post ganglionic fibers to the parotid gland. The preganglionic parasympathetic fibers to the sub maxillary and sub lingual glands leave the brain stem in the facial nerve and travel along the chorda tympani to join the lingual nerve before their synaptic junctions with sub maxillary ganglion. This ganglion provides cholinergic post ganglionic prasysmpathetic fibers to sub maxillary and sub lingual glands.
Two salivary secretory centers are located in the central nervous system-Brain stem (Medulla), one is concerned with the sub maxillary gland, and the other with the parotid. However stimulation of any one of these two areas causes secretion by both glands.
The medullary salivary nuclei acts as center of the gustatory reflex, It is also regulated by inhibitory or excitatory signals from higher centers of the brain , particularly from certain regions of the diencephalon and cerebral cortex. Afferent impulses from the mouth, pharynx, and olfactory areas are carried by the trigeminal and glossopharyngeal nerves to the salivary centers.
Regulation of salivary secretion
When food enters the mouth, a copious secretion of saliva takes place by reflex stimulation of the salivary glands through the buccal receptors and secretory centers.
Normal foodstuffs causes salivary secretion rich in mucin and enzymes to facilitate easy swallowing, while dry or noxious materials causes watery secretion with little mucin.
The types and volumes of secretion varies with diverse range of stimuli. The chewing of meat cause a flow of saliva from its sub maxillary and sublingual glands but not from parotid gland. While dry bread promoted abundant parotid watery saliva which contains little mucin.
The saliva produced from one gland differs from that of the other. These may be mixed together in differing proportions. A single gland may be composed of different type of cells and their relative contributions to the saliva produced by that gland may vary. The secretion from a gland composed of only one type of secretory cell also varies in composition at differing rates of secretion .There is a general mechanism by which change in the composition of saliva can occur.
The secretion of saliva in response to food in the mouth is an unconditioned reflex,and the stimulus producing it is termed as unconditioned stimulus .It is not necessary for food to enter the mouth to provoke a flow of saliva because the sight,smell or even the thought of food may stimulate salivation .The extent to which this psychic secretion occurs varies in different species and it is particularly marked in dog. The nature of the secretion produced varies,the sight of meat evokes a thick,stringy, mucus secretion from the sub maxillary and sublingual glands, while the sight of dry bread results in an abundant secretion of watery parotid saliva.
Psychic reflexes were termed as conditioned reflexes by Pavlov [1910] because they were dependent on certain conditions for their development known as acquired reflexes.The conditioned reflex is that when a stimulus which in itself does not provoke secretion.
The optic and olfactory nerves function as afferent/sensory pathways for this reflex activity. Psychic salivary secretion is exhibited by goats and pigs, but it is feeble or absent in horse and sheep. Food is a natural stimulus for salivary secretion and salivation during feeding is an inherited reflex. When a dog is conditioned by a neutral stimuli (bell sound at the time of feeding) for certain period of time, only the bell sound without food is sufficient for increased salivary secretion by way of acquired reflex.
Trigeminal and glossopharyngeal are the sensory/afferent to the salivary centers, located in medulla oblongata. The motor/efferent fibers are formed by both sympathetic and parasympathetic nerves of the autonomic nervous system.
Stimulation of sympathetic nerves causes vasoconstriction in the salivary glands results in thick salivary secretion rich in mucin and protein.
Facial and glossopharyngeal nerves are the parasympathetic efferent fibers to salivary glands. They stimulate cholinergic receptors in the salivary acini, causes vasodilation and copious salivary secretion rich in water and HCO3, but low content of protein.
Bradykinina vasodilator polypeptide, formed by parasympathetic stimulation of the salivary glands.
Anticipation of food intake produces increased parasympathetic response results in increased salivary secretion.
Effects of nerve stimulation
Stimulation of the parasympathetic efferent fibres results in a copious secretion of saliva, with high mucoprotein content from the sub maxillary glands, and serous or watery secretion from the parotid glands. The amount and composition of the secretion vary with intensity of stimulation. Stimulation of the parasympathetic efferent fibres shows marked vasodilatation which greatly increases the blood flow to the glands.
Vasodilatation is also brought about by specific vasodilator substance like bradykinin.
The response of stimulation of sympathetic efferent fibers is variable and differences are found in different glands among different species.In the cat, there is a considerable flow the sub maxillary gland and none from the parotid. Sympathetic stimulation immediately followed by para sympathetic stimulation produce saliva containing much higher proportion of mucus than para sympathetic stimulation alone.
Effect of denervation
Sectioning of the nerve supply of the salivary glands leads to complete cessation of the salivary secretion except the glands which exhibit the property of spontaneous secretion. Atrophy of the glands develops after denervation accompanied by histological changes by hypersensitivity to chemical or hormonal agents like epinephrine, nor epinephrine, acetylcholine and pilocarpine. Claude Bernard in 1864 observed increased secretion (paralytic secretion) from the sub maxillary gland after sectioning the chorda tympani by removing the inhibitory action of these gland cells.
