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Anatomy of the trigeminal nerve
|Cranial nerve (CN) V is a mixed nerve that consists primarily of sensory neurons. It exits the brain on the lateral surface of the pons, entering the trigeminal ganglion within a few millimeters. The trigeminal ganglion corresponds to the dorsal root ganglion of a spinal nerve.
Three major branches emerge from the trigeminal ganglion. Each branch innervates a different dermatome. Each branch exits the cranium through a different site. The first division (V1; ophthalmic nerve) exits the cranium through the superior orbital fissure, entering the orbit to innervate the globe and skin in the area above the eye and forehead.
The second division, V2, maxillary nerve, exits through a round hole, the foramen rotundum, into a space posterior to the orbit, the pterygopalatine fossa. It then re-enters a canal running inferior to the orbit, the infraorbital canal, and exits through a small hole, the infraorbital foramen, to innervate the skin below the eye and above the mouth.
The third division, V3, mandibular nerve, exits the cranium through an oval hole, the foramen ovale. This nerve is interesting because some sensory neurons, specifically those conducting proprioceptive input from jaw muscles, have cell bodies located within the central nervous system rather than in the trigeminal ganglion. The third division also has an additional branchial motor component, which may run in a separate fascial compartment. Most fibres travel directly to their target tissues. Sensory axons innervate skin on the lateral side of the head, the tongue and the mucosal wall of the oral cavity. Motor fibres innervate several muscles, most of which are attached to the mandible. Some sensory axons re-enter a canal in the mandible to innervate the teeth and emerge from the mental foramen to innervate the skin of the lower jaw.
The trigeminal nerve has several important relationships with other cranial nerves, especially those providing visceral motor function. Many primary and secondary parasympathetic (visceral motor) neurones are distributed to ganglia or target tissue through branches of CN V, even though CN V has no intrinsic visceral motor function. In addition, axons conducting the special sensation of taste are distributed to the anterior 2/3 of the tongue through branches of CN V (via the lingual nerve).
Clinical testing of cutaneous sensation is carried out by light stroking of each dermatome and orbital contents.
Clinical testing of motor functions evaluated by chewing movements and jaw asymmetries.
Figure of trigeminal nerve
The ophthalmic nerve divides into three named sensory nerves as it passes through the superior orbital fissure: the frontal, lacrimal and nasociliary nerves.
Ophthalmic nerve: CN V1
The frontal nerve enters the roof of the orbit, where it divides into the supraorbital and supratrochlear nerves.
The lacrimal nerve innervates the lateral portion of the upper eye lid, conjunctiva and lacrimal gland.
The small nasociliary nerve consists of the infratrochlear and external nasal nerves, internal nasal nerve from the anterior nasal cavity, anterior and posterior ethmoidal nerves from the ethmoid air sinuses, and, finally, the short and long ciliary nerves innervating the globe.
Figure of ophthalmic nerve
The maxillary nerve exits the cranium through the foramen rotundum and enters the pterygopalatine fossa. In the fossa, several sensory branches to the teeth and palate are given off. These include the greater and lesser palatine nerves, the nasopalatine nerve, the posterior superior alveolar nerve and communicating branches to the pterygopalatine ganglion.
Maxillary nerve: CN V2
The maxillary nerve continues into the infraorbital canal as the infraorbital nerve.
The zygomatic nerve emerges and branches into its two major terminal branches, the zygomaticofacial and zygomaticotemporal nerves, which innervate the lateral cheek and side of the forehead, respectively.
As it projects anteriorly, the infraorbital nerve gives off the anterior and middle superior alveolar nerves, innervating the upper teeth. It then exits the canal through the infraorbital foramen to innervate the upper lip, cheek and side of the nose.
Figure of maxillary nerve
Mandibular nerve: CN V3
The mandibular nerve exits the cranium through the foramen ovale to enter the infratemporal fossa. Several sensory branches are generated immediately. The meningeal branch re-enters the skull through the foramen spinosum to distribute with the middle meningeal artery.
The buccal nerve innervates the mucosa of the mouth and gums.
The auriculotemporal nerve innervates the external auditory meatus and portions of the external surface of the tympanic membrane.
The lingual nerve provides general sensation to the anterior 2/3 of the tongue.
The inferior alveolar nerve enters the mandibular canal through the mandibular foramen to innervate the lower teeth and gums. Its terminal branch exits the mental foramen as the mental nerve, innervating the chin and lower lip.
Several branchial motor nerves are also given off as the mandibular nerve enters the infratemporal fossa. These include the nerve to the masseter m., 2-3 deep temporal nerves to the temporalis m., the nerve to the medial and lateral pterygoids, and small nerves to the tensor palati and tensor tympani. Some branchial motor axons travel with the inferior alveolar nerve. Just before the nerve enters the mandibular foramen, these axons branch off as the mylohyoid nerve, innervating the mylohyoid m. and anterior belly of the digastric muscle.
Figure of mandibular nerve
The three divisions of the trigeminal nerve come together in an area called the Gasserion ganglion. From there, the trigeminal nerve root continues back towards the side of the brain stem, and inserts into the pons. Within the brain stem, the signals traveling through the trigeminal nerve reach specialised clusters of neurones called the trigeminal nerve nucleus. Information brought to the brain stem by the trigeminal nerve is then processed before being sent up to the brain and cerebral cortex, where a conscious perception of facial sensation is generated.
Figure of gasserion ganglion
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