VPB 121: Veterinary Physiology - II (2+1)

NERVE SUPPLY AND FUNCTION

Innervation

• 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

• 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

• 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

• Nerve supply
  • 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 fibres to salivary glands. They stimulate cholinergic receptors in the salivary acini, causes vasodilation and copious salivary secretion rich in water and HCO_3, but low content of protein.
  • Bradykinin a 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.