Originating in the posterior fossa of the brain stem, it follows a long and complex course towards its distribution territory, crossing several regions with a complex anatomy and establishing important relationships with several structures.
The nerve fibers originate in the brainstem and are part of several grey matter nuclei occupying all the brainstem and even the first spinal cervical segments.
Each of these sensitive and motor nuclei represents different processing centers, and there is a true systematization of the information this nervous tract is responsible for conducting.
The sensitive nucleus is the largest, comprising 3 true sub-nuclei, each responsible for each aspect of the general sensitivity. The highest is the mesencephalic nucleus, located in the tegmentum close to the midline and to the grey matter close to the Sylvian aqueduct. The neurons that form this nucleus are in charge of the propioceptive integration in the Vth nerve territory, high level information for correct mastication. The main nucleus is in the pons, it is also situated in the depth of the tegmentum, and is responsible for the tactile integration of the territory of this nerve. Finally, the inferior nucleus occupies the tegmentum of the medulla, extending caudally to the first segments of the cervical spine, and is in charge of thermal and pain information. Its location explains the possible appearance of symptoms in the facial territory in patients with a degenerative/inflammatory disorder of the upper cervical spine. There is one single motor nucleus, located in the pons tegmentum supplying mastication muscles, and is correspondingly called mastication nucleus. The fibers related with all these nuclei gather in the pons and emerge through the lateral sector of its anterior aspect, forming a thick nervous tract with two roots: a thicker and lateral sensitive root and a thinner more medial motor root.
The only intra-axial segment of the Vth ends there and initiates its long course to its distribution territory; it is formed by different sub-segments before dividing itself into its terminal branches (the cisternal and Gasserian or transdural segments).
The point where the roots emerge in the brainstem is called “REZ” (Root Entry Zone), an anatomical landmark of great functional hierarchy.
The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and certain motor functions such as biting and chewing. It is the largest of the cranial nerves. Its name (“trigeminal” = tri- or three, and -geminus or twin, or thrice twinned) derives from the fact that each trigeminal nerve, one on each side of the pons, has three major branches: the ophthalmic nerve (V1), the maxillary nerve (V2), and the mandibular nerve (V3). The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both cutaneous and motor functions.
Sensory information from the face and body is processed by parallel pathways in the central nervous system. The motor division of the trigeminal nerve is derived from the basal plate of the embryonic pons, while the sensory division originates from the cranial neural crest.
see Trigeminal neuralgia.