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The trigeminal nerve (the fifth cranial nerve, or simply CN V) is a nerve responsible for sensation in the face and motor functions such as biting and chewing. The largest of the cranial nerves, its name ("trigeminal" = tri, or three and geminus, or twin; thricetwinned) 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, and the mandibular nerve has sensory (or "cutaneous") and motor functions

The three major branches of the trigeminal nerve—the ophthalmic nerve (V1), the maxillary nerve (V2) and the mandibular nerve (V3)—converge on the trigeminal ganglion (also called the semilunar ganglion or gasserian ganglion), located within Meckel's cave and containing the cell bodies of incoming sensorynerve fibers. The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body.

From the trigeminal ganglion a single, large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level. Motor fibers pass through the trigeminal ganglion on their way to peripheral muscles, but their cell bodies are located in the nucleus of the fifth nerve, deep within the pons.

V1/V2 distribution Referring to the ophthalmic and maxillary branches
V2/V3 distribution Referring to the maxillary and mandibular branches
V1V3 distribution Referring to all three branches

The complex processing of paintemperature information in the thalamus and cerebral cortex (as opposed to the relatively simple, straightforward processing of touchposition information) reflects a phylogenetically older, more primitive sensory system. The detailed information received from peripheral touchposition receptors is superimposed on a background of awareness, memory and emotions partially set by peripheral paintemperature receptors.

Although thresholds for touchposition perception are relatively easy to measure, those for paintemperature perception are difficult to define and measure. "Touch" is an objective sensation, but "pain" is an individualized sensation which varies among different people and is conditioned by memory and emotion. Anatomical differences between the pathways for touchposition perception and paintemperature sensation help explain why pain, especially chronic pain, is difficult to manage.
Wallenberg syndrome (lateral medullary syndrome) is a clinical demonstration of the anatomy of the trigeminal nerve, summarizing how it processes sensory information. A stroke usually affects only one side of the body; loss of sensation due to a stroke will be lateralized to the right or the left side of the body. The only exceptions to this rule are certain spinalcord lesions and the medullary syndromes, of which Wallenberg syndrome is the bestknown example. In this syndrome, a stroke causes a loss of paintemperature sensation from one side of the face and the other side of the body.

This is explained by the anatomy of the brainstem. In the medulla, the ascending spinothalamic tract (which carries paintemperature information from the opposite side of the body) is adjacent to the ascending spinal tract of the trigeminal nerve (which carries paintemperature information from the same side of the face). A stroke which cuts off the blood supply to this area (for example, a clot in the posterior inferior cerebellar artery) destroys both tracts simultaneously. The result is a loss of paintemperature (but not touchposition) sensation in a "checkerboard" pattern (ipsilateral face, contralateral body), facilitating diagnosis.

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