**2. Physiology of salivation**

The major salivary glands include parotid, submandibular, and sublingual glands; the largest being the parotid gland. These glands secrete saliva, which has a major role in lubrication, digestion, immunity, and maintenance of homeostasis in the human body [2]. The salivary secretion of parotid, submandibular, and sublingual glands is controlled mainly by parasympathetic nervous system, although sympathetic innervation has a minor influence. The parasympathetic fibers originate in the pons and medulla, and they synapse in the otic and submandibular ganglia. Postganglionic fibers originating from the otic ganglion regulate secretory functions of the parotid gland, while the postganglionic fibers from the submandibular ganglion regulate secretory function of the submandibular and sublingual glands. Sympathetic innervation of these glands results in contraction of muscle fibers around the salivary ducts, which enhances the flow of saliva [3].

Salivary secretion is regulated through a reflex arch that produces several actions. The afferent branch contains chemoreceptors in taste buds and mechanoreceptors in the periodontal ligament. Afferent innervations of cranial nerves V, VII, IX, and X carry impulses to salivary nuclei in the medulla oblongata. Efferent impulses are mainly parasympathetic as described above; they come from the chorda tympani nerve (a branch of the cranial nerve VII) and travel to the submandibular, sublingual, and other minor glands via lingual nerve (a branch of the cranial nerve V<sup>3</sup> ). Efferent fibers to the parotid gland are supplied by lesser petrosal nerve (a branch of cranial nerve IX), which travel through the fibers of auriculotemporal nerve (a branch of the cranial nerve V<sup>3</sup> ) and reach the gland [2].

The major salivary glands provide 90% of the nearly 1.5 L of saliva that is produced every day. If salivary secretion is not stimulated, that is in basal state, 70% of total salivary secretion comes from the submandibular and sublingual glands. When stimulated, salivary secretion increases by five times, with the parotid glands delivering the greater amount of the saliva [3]. An example of an exogenous source causing stimulation is chewing [2]. There are two main types of saliva produced by the three major salivary glands; serous saliva is produced mainly by parotid glands by stimulation, which is thin and watery, while viscous saliva is produced by sublingual and submandibular glands throughout the day, which is thicker [4]. Both forms of these secretions can be problematic. Serous saliva results in watery saliva, consistently spilling from the side of the mouth, and viscous saliva may be mucoid and sticky, which makes it harder to clear and causes a sensation of choking, associated with panic. It is important to distinguish between thin, runny saliva and thick, mucous secretions because the treatment options differ. The stimulation of cholinergic receptors produces thin, serous secretions, whereas that of beta-adrenergic receptors produces thick protein and mucus-rich secretions. Therefore, in the case of watery saliva secretions, anticholinergics can be preferred, whereas for thick mucus secretions, the addition of beta blockers may be beneficial [5].
