Vet360 Vet360 Vol 05 Issue 04 - Page 22

NEUROLOGY nerve in the medulla. 5 Preganglionic axons travel in the glossopharyngeal nerve and synapse at the otic ganglion. 5 The postganglionic axons course with the auriculotemporal nerve, a branch of the mandibular nerve, to arrive at the zygomatic and parotid salivary glands. 5 Preganglionic neurons for the parasympathetic innervation of the sublingual and mandibular glands are provided by the parasympathetic nuclei of the facial nerve in the medulla. 5 The preganglionic axons course in the facial nerve, through the tympanic cavity to join the lingual nerve, a branch of the mandibular nerve, at the level of the oval foramen. The oval foramen, located just medial to the temporomandibular joint, is also the foramen through which the mandibular nerve exits the cranial cavity. After the preganglionic axons join the mandibular nerve, they course to the mandibular and sublingual ganglia and synapse with parasympathetic ganglionic neurons. Parasympathetic postganglionic axon coursing with branches of the mandibular nerve ultimately innervate the mandibular and sublingual glands. 5 Parasympathetic stimulation of the salivary glands mediated through acetylcholine is responsible for salivary production and flow. The sympathetic nervous system can modulate salivary secretion and composition through norepinephrine by stimulating the blood vessels and the acinar cells of the salivary glands. 6 In general, sympathetic stimulation to the salivary glands results in vasoconstriction of the blood vessels, which reduces aqueous saliva and exocytosis of the acinar glandular cells. Overall, this results in a more proteinaceous and less voluminous saliva flow. 6 An analysis of Max’s salivation In patients like Max with dysfunction of the trigeminal or the mandibular nerve, abnormal saliva that appears foamy, ropy and stringy accumulates in the oropharynx on the side ipsilateral to the nerve deficit (Figure 2). The saliva buildup is located just caudal to the maxillary fourth premolar, which is where the openings to the oral cavity are for the parotid gland and zygomatic gland. 7 . How can one explain the accumulation of abnormal saliva? Given the normal function of cranial nerve VII (ability to blink the eye) and cranial nerve IX (ability to swallow normally), the preganglionic parasympathetic axons to the salivary glands are likely intact. Therefore, the abnormal composition and flow of saliva produced by the parotid and zygomatic salivary glands, as evidenced by accumulation of the thickened saliva in the caudal oropharynx, is most likely due to dysfunction of the postganglionic axons that course with the branches of mandibular nerve. In cases like Max’s, we hypothesize that the dysfunction of the trigeminal or mandibular nerve also affects the postganglionic parasympathetic axons that course alongside the axons of the trigeminal or mandibular nerve to reach the salivary glands. In other words, the loss of the conduit for the postganglionic axon provided by the trigeminal or mandibular nerve results in dysfunction of the parasympathetic stimulation to the salivary glands, which alters salivary flow and composition. Normally, the parasympathetic nervous system is responsible for the production of voluminous amounts of aqueous saliva. Parasympathetic denervation results in proteinaceous saliva that is more viscous than normal. This is appreciated clinically as an accumulation of thickened, ropy saliva that accumulates in the oropharynx on the side ipsilateral to trigeminal or mandibular nerve dysfunction. The essentiality of salivation, and why it might go wrong Figure 2; View of Max's oral cavity reveals the accumulation of thick, foamy saliva in the left caudal oropharynx. vet360 Issue 04 | SEPTEMBER 2018 | 22 Normal salivation provides an important role in day- to-day life. Not only does it aid in food mastication and digestion but also in lubrication of the oral cavity, maintenance of normal pH of the mouth, and prevention of dental caries. 8 Lack of salivation can cause significant oral discomfort and ulcerations as well as a detrimental decline in oral health. 9 It’s possible that with unilateral trigeminal nerve dysfunction and accumulation of abnormal saliva, there may be an increase in dental tartar on the side of the nerve dysfunction. 7