||Andrew G. Lee, MD, Chairman, Department of Ophthalmology, The Methodist Hospital, Houston, TX; Professor of Ophthalmology, Weill Cornell Medicine; Joseph Pecha, Baylor College of Medicine, Class of 2022
||The trigeminal nerve is interesting because it has a mesencephalic nucleus in the midbrain and then a pontine nucleus and then it goes all the way down to the spinal cord. So it's nucleus is quite extensive and extends from the midbrain, from the mesencephalon, all the way down the spinal cord. So it's nucleus extends longitudinally across the entire brainstem. The nerve exits at the pons, and that is the ganglion-the semilunar ganglion. It's called the trigeminal because it's got three parts- "tri", "twins". So you've got three of these identical segments, five subdivision one (V1), five subdivision two (V2), and five subdivision three (V3). The ophthalmic division, the maxillary division, and the mandibular division. The trigeminal nerve is primarily a sensory nerve, so the trigeminal subdivision one travels anteriorly, passes through the cavernous sinus, and then through the superior orbital fissure to the orbit, and then it exits on the skin: lacrimal, frontal, supraorbital, supratrochlear, all are branches of five subdivision one (V1). And inside the orbit, we have nasociliary which passes through the ciliary ganglion without synapse, and to the tip of the nose. That is important clinically because in herpes zoster involvement of the tip of the nose, the Hutchinson's sign, might indicate that you might have ocular involvement because it travels on the nerve, the nasociliary nerve. The V2, exits through a foramen, the foramen rotundum. And the V3 exits through a foramen, the foramen ovale. The sensory nerves of V1 and V2 are to the eye, the periorbital portions, and to the face and the mouth. The V3 carries not just the sensory information, but also motor fibers. So the sensory information of five is a general somatic afferent, the sensory nerve, but the V3 as it passes through foramen ovale and exits also contains not only the general somatic afferents (GSA), but the special visceral efferents (SVE) to the muscles of mastication. So most of the special visceral efferent muscles that are involved in the trigeminal are muscles for biting and chewing and for eating. However, there are some small exemptions, so you should know the names of these muscles including two tensors: the tensor veli palatini and the tensor tympani. It also innervates the anterior portion of the digastric as well as the mylohyoid and the four muscles of mastication-the masseter, the temporalis, and the internal pterygoids, the medial and lateral pterygoids. So these are the motor pieces of the mandibular branch of the trigeminal nerve (mylohyoid, tensor veli palatini, tensor tympani, anterior belly of the digastric), and the four muscles of mastication (masseter, temporalis, medial and lateral pterygoid). The reason that's important is that if you have trigeminal dysfunction that affects this motor branch, there'll be denervation atrophy that is visible on the MRI scan and the patient clinically might have jaw deviation towards the side of the lesion from the muscles of mastication. So the main points about trigeminal: the sensory nucleus is quite large-it runs from mesencephalon to spinal cord which means that sensory trigeminal dysfunction can occur anywhere in the brainstem all the way to spinal cord. Its ganglion is the semilunar ganglion. It's called trigeminal because of three pieces: V1, V2, V3. V1- superior orbital fissure, cavernous sinus; V2 - foramen rotundum; and V3 - foramen ovale. You need to know that because skin cancers and cancers in your face, like adenoid cystic carcinoma, squamous cell or basal cell can go perineural and travel on the nerve and end up inside your head even though they started in your face or on your face, as skin cancer. And you should know that it's mostly a general sensory afferent nerve with some special visceral efferent motor components for mastication.