Walsh & Hoyt: Afferent Trigeminal Fibers in the Brainstem

The afferent sensory trigeminal fibers arising from their cell bodies in the gasserian ganglion enter the lateral pons and course in a dorsomedial direction. Some of them dichotomize into a short ascending and a long descending branch. Most fibers run caudally without bifurcating. The mass of descending fibers constitutes the trigeminal spinal tract, a distinct fiber bundle that is situated just underneath the lateral surface of the medulla oblongata, and slightly deeper in the lower part of the pons, where it is covered by fibers of the middle cerebellar peduncle. The fibers of this tract continue into the two most rostral segments of the spinal cord in the zona terminalis. The fibers that are found in the spinal tract, as well as numerous collaterals, end in a longitudinal cell column medial to the tract, the nucleus of the trigeminal spinal tract. Early investigations on the trigeminal spinal tract and its nucleus indicated that a divisional lamination exists. The ophthalmic component is situated most ventrally in the tract. Kerr studied this lamination and by the Nauta technique was able to show a very discrete segregation of the ventral ophthalmic portion from the more dorsal maxillary and mandibular portions. He showed that the afferent fibers of all divisions proceed in diminishing numbers caudally to C2. Similar laminations of the ophthalmic and other divisions of the trigeminal nerve are present in the spinal nucleus and in the main sensory nucleus.