The fifth nerve is primarily a sensory nerve, but it also has certain motor functions (biting, chewing, and swallowing). These are discussed separately.

The sensory function of the trigeminal nerve is to provide the tactile, proprioceptive, and nociceptive afference of the face and mouth. The posterior scalp and the neck are innervated by C2-C3, not by the trigeminal nerve.

The motor function activates the muscles of mastication, the tensor tympani, tensor veli palatini, mylohyoid, and anterior belly of the digastric.

The trigeminal nerve 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 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 sensory and motor functions.

The three branches converge on the trigeminal ganglion (also called the semilunar ganglion or gasserian ganglion), that is located within Meckel's cave, and contains 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 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. Motor fibers of the eye are distributed (together with sensory fibers) in branches of the mandibular nerve.

The areas of cutaneous distribution (dermatomes) of the three branches of the trigeminal nerve have sharp borders with relatively little overlap (unlike dermatomes in the rest of the body, which show considerable overlap). Injection of local anesthetics such as lidocaine results in the complete loss of sensation from well-defined areas of the face and mouth. For example, the teeth on one side of the jaw can be numbed by injecting the mandibular nerve.

The ophthalmic, maxillary and mandibular branches leave the skull through three separate foramina: the superior orbital fissure, the foramen rotundum and the foramen ovale. The mnemonic standing room only can be used to remember that V1 passes through the superior orbital fissure, V2 through the foramen rotundum, and V3 through the foramen ovale.

Motor branches of the trigeminal nerve are distributed in the mandibular nerve. These fibers originate in the motor nucleus of the fifth nerve, which is located near the main trigeminal nucleus in the pons. Motor nerves are functionally quite different from sensory nerves, and their association in the peripheral branches of the mandibular nerve is more a matter of convenience than of necessity.