Spinal Nucleus and Tract
The spinal nucleus and tract extend through the medulla to C4. The nucleus receives pain and temperature afferents from the descending spinal tract, which also carries cutaneous components of CN VII, CN IX, and CN X that serve sensations from the ear and external auditory meatus. The sensory fibers from the ophthalmic division of CN V (V1) terminate in the most ventral portion of the spinal nucleus and tract. Fibers from the maxillary division (V2) end in the midportion of the spinal nucleus (in a ventral–dorsal plane). The fibers from the mandibular division (V3) end in the dorsal parts of the nucleus (the 3 divisions of CN V are discussed in greater detail later in the chapter).
The cutaneous territory of each of the CN V divisions is represented in the spinal nucleus and tract in a rostral–caudal direction. Fibers from the perioral region are thought to terminate most rostrally in the nucleus; fibers from the peripheral face and scalp end in the caudal portion. The zone between them, the midfacial region, is projected onto the central portion of the nucleus. This “onionskin” pattern of cutaneous sensation (see Fig 3-20) has been revealed by clinical studies of patients with damage to the spinal nucleus and tract.
Axons from the main sensory and spinal nuclei, as well as portions of the mesencephalic nucleus, relay sensory information to higher sensory areas of the brain. The axons cross the midline in the pons and ascend to the thalamus along the ventral and dorsal trigeminothalamic tracts. They terminate in the nerve cells of the ventral posteromedial nucleus of the thalamus. These cells, in turn, send axons through the internal capsule to the postcentral gyrus of the cerebral cortex.
The afferent limb of the oculocardiac reflex is mediated by the trigeminal nerve. It is connected to the efferent limb, which is mediated by the parasympathetic neurons of the vagus nerve, via short internuncial fibers to the reticular formation.
Meuwly C, Golanov E, Chowdhury T, Erne P, Schaller B. Trigeminal cardiac reflex: new thinking model about the definition based on a literature review. Medicine(Baltimore). 2015;94(5):e484.
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.