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; it is the most complex of the cranial nerves. Its name ("trigeminal" = tri-, or three, and - geminus, or twin: thrice-twinned) derives from the fact that each of the two nerves (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, whereas the mandibular nerve supplies motor as well as sensory (or "cutaneous") functions. Adding to the complexity of this nerve is the fact that autonomic nerve fibers as well as special sensory fibers (taste) are contained within it.

The motor division of the trigeminal nerve derives from the basal plate of the embryonic pons, and the sensory division originates in the cranial neural crest. Sensory information from the face and body is processed by parallel pathways in the central nervous system.

Structure

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 sensory-nerve 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 (portio major) enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root (portio minor) emerges from the pons at the same level. Motor fibers pass through the trigeminal ganglion without synapsing on their way to peripheral muscles, but their cell bodies are located in the nucleus of the fifth nerve, deep within the pons.

The areas of cutaneous distribution (dermatomes) of the three sensory branches of the trigeminal nerve have sharp borders with relatively little overlap (unlike dermatomes in the rest of the body, which have considerable overlap). The injection of a local anesthetic, such as lidocaine, results in the complete loss of sensation from well-defined areas of the face and mouth. For example, teeth on one side of the jaw can be numbed by injecting the mandibular nerve. Occasionally, injury or disease processes may affect two (or all three) branches of the trigeminal nerve; in these cases, the involved branches may be termed:

• V1/V2 distribution – Referring to the ophthalmic and maxillary branches

• V2/V3 distribution – Referring to the maxillary and mandibular branches

• V1-V3 distribution – Referring to all three branches

Nerves on the left side of the jaw slightly outnumber the nerves on the right side of the jaw.

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Regards
Jessica Watson
Editorial Team
Journal of Clinical & Experimental Neuroimmunology