Vet360 Vet360 Vol 05 Issue 04 - Page 21

NEUROLOGY caudal oropharynx only on the side of the muscle atrophy. You are puzzled and take a second and more thorough look to confirm your findings. Why is there foamy saliva in the back of his mouth, you wonder? Thinking through the causes of muscle atrophy— disuse versus neurogenic atrophy—you think that it must be neurogenic atrophy, given how quickly the owner reported the onset of the atrophy. But then which nerve? Ah, but of course! You remember your vet school neurology training and recall that cranial nerve V, the trigeminal nerve, innervates the muscles of mastication. All those mnemonics you memorized to learn the cranial nerves and their functions are paying off. But then how to account for the abnormal saliva accumulation in Max’s oropharynx? A quick trigeminal nerve review The trigeminal nerve has three main branches—the ophthalmic, maxillary and mandibular nerves. All three branches relay sensory information from the head and face to the brain. 1 Only the mandibular branch is responsible for providing motor function to the muscles of mastication—the masseter, temporalis, lateral and medial pterygoids, rostral portion of the digastricus, and mylohyoideus muscles—as well as some lesser-known muscles such as the tensor tympani muscle (involved in modulating the ossicles in the middle ear) and the tensor veli palatani muscle (involved in opening the pharyngeal orifice of the auditory tube). Clinically, unilateral dysfunction of the mandibular branch of the trigeminal nerve is easy to recognize, often because of the dramatic muscle atrophy of the masseter and temporalis muscles. Pterygoid muscle atrophy is inferred by the enophthalmos, as the pterygoid muscles provide ventral and medial support of the eye’s position in the orbit. The third eyelid will elevate passively with the enophthalmia. The salivary link What probably was not a focus in your veterinary curriculum was the role that the trigeminal nerve serves in the autonomic nervous system. Branches of the trigeminal nerve act as a conduit for the distribution of parasympathetic innervation to target organs such as the lacrimal and salivary glands. Anyone who has suffered from dry eye (keratoconjunctivitis sicca, or KCS), dry mouth (xerostomia) or dry nose (xeromycteria) can understand the life-altering effects such conditions can cause. Since veterinary patients can’t self-report symptoms, most owners don’t recognize altered autonomic function until severe end-stage consequences are present. To appreciate this, one only need examine a dog with untreated KCS. The lack of tear production leads to corneal opacification from corneal oedema, neovascularisation, pigment deposits, squamous metaplasia and hyperkeratinisation of the cornea, which results in visual deficits. 2 People with xerostomia suffer from severe oral discomfort, dental caries, speech problems and difficulty eating and swallowing. 3 In dogs, there are four major salivary glands—the parotid, zygomatic, sublingual and submandibular glands. 4 Although structural diseases of the salivary glands (i.e. sialocoele, sialadenitis, salivary gland neoplasms and sialadenosis) are well-known in veterinary medicine, pure functional disturbances of the salivary glands have yet to be well-described. Quite detailed Neurological Anatomy. Important points in bold for ease of reading...ed Each of the salivary glands is innervated by the sympathetic and parasympathetic nervous system. As a whole, the autonomic nervous system has both afferent (sensory) and efferent (motor) components. Efferents are composed of a two-neuron system— preganglionic and ganglionic neurons. Sometimes these are referred to as first-order and second-order neurons, respectively. The preganglionic neuron is located in the central nervous system. Preganglionic axons synapse with the ganglionic neurons in various ganglia. Postganglionic axons innervate target organs such as glands, the heart, the lungs and many other organs. For sympathetic innervation, preganglionic neurons are located in the intermediate gray matter of the thoracic and cranial lumbar spinal cord. For most of the body, the ganglionic neurons are located in the paired sympathetic trunk, which runs bilaterally along the ventrolateral aspect of the vertebral column. For structures of the head, ganglionic neurons are located in the cranial cervical ganglia near the ventral and caudal aspect of the skull. Long postganglionic axons course to their target organs. Sympathetic postganglionic axons use norepinephrine as their neurotransmitter. For parasympathetic innervation, preganglionic neurons are located in the brain stem, adjacent to the motor neurons for cranial nerves III (oculomotor), VII (facial), XI (glossopharyngeal) and X (vagus), as well as sacral spinal cord segments. The ganglionic neurons are often located in ganglia close to their intended target organs. Short parasympathetic postganglionic axons course to their target organs and use acetylcholine as their neurotransmitter. For the salivary glands, cranial nerves VII (facial) and IX (glossopharyngeal) provide for parasympathetic innervation. 5 Specifically, preganglionic neurons for the parasympathetic innervation of the zygomatic and parotid glands are provided by the parasympathetic nuclei of the glossopharyngeal Issue 04 | SEPTEMBER 2018 | 21