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The trigeminal nerve is the fifth (CN V) cranial nerve and its primary role is relaying sensory information from the face and head, although it does provide motor control to the muscles of mastication via the mandibular division (TA: nervus trigeminus or nervus cranialis V).
It is both large and complicated and has multiple brainstem nuclei (sensory and motor) as well as many interconnections with other cranial nerves. It swaps parasympathetic fibers and taste fibers somewhat haphazardly and divides into numerous terminal branches.
There are four cranial nerve nuclei: three sensory and one motor.
main sensory nucleus: located in the upper pons, lateral to the motor nucleus, is responsible for touch sensation for all three trigeminal divisions
spinal nucleus: lower pons to upper cervical cord (as far as the third cervical segment), is responsible for pain and temperature; additionally it receives afferent fibers from the glossopharyngeal nerve and vagus nerve.
The motor nucleus is located in the upper pons and gives off the smaller motor root (see below) which bypasses the trigeminal ganglion joining the mandibular division and innervates the muscles of mastication as well as the mylohyoid, anterior belly of digastric, tensor tympani and tensor palatini muscles.
Motor and sensory roots
Although the trigeminal nerve is usually described singularly, it actually emerges/enters from the brainstem as multiple nerve roots 8.
The sensory root is large and single and enters the anterolateral aspect of the pons.
Usually superomedial to, and within 4 mm of the sensory root, one or more motor roots emerge. In ~50% of cases, this is a single nerve root, with two or three roots being less common 8. They are most often located superomedial to the sensory root but can be directly superior, medial or even superolateral 8,9.
These nerve roots can be identified on routine MRI imaging 8.
The motor nerve does not join the trigeminal ganglion, but rather joins the mandibular division as it exits via foramen ovale 9.
Nerve root entry zone and transition zone
The site where the nerve roots exit the brainstem is known as the nerve root entry zone. At, or near, this location oligodendrocytes, that supply insulating myelin to the nerve fibers, give way to Schwann cells. The transition zone between these two areas can measure approximately 2 mm in length and is located within 3 to 4 mm of the nerve root zone 7. This is relevant when assessing for neurovascular compression (causing trigeminal neuralgia) as the transition zone is far more vulnerable than other areas 7.
Intracranial extra-axial course
Having exited the mid pons anteriorly, the cisternal portion of the trigeminal nerve courses through the prepontine cistern (cisternal portion), and crosses the porus trigeminus to enter a prolongation of dura at the apex of the petrous temporal bone known as the Meckel cave (cavernous portion) where its fibers form the trigeminal ganglion (also known as the Gasserian or semilunar ganglion). It then divides into three main branches known as divisions: ophthalmic, maxillary and mandibular.
Ophthalmic division/nerve (V1 or Va)
Courses anteriorly in the lateral wall of the cavernous sinus inferior to the trochlear nerve and is crossed medially by the oculomotor nerve. Just before entering the orbit, the tentorial nerve arises and ascends to supply a large portion of the falx and supratentorial dura. The ophthalmic division then divides into 3 terminal branches before each passes through the superior orbital fissure separately:
lacrimal nerve (superior orbital fissure outside the tendinous ring)
nasociliary nerve (superior orbital fissure through the tendinous ring)
Maxillary division/nerve (V2 or Vb)
Courses anteriorly low in the lateral wall of the cavernous sinus inferior to the ophthalmic division. Just before exiting the skull it runs along the floor of the middle cranial fossa and gives off the middle meningeal nerve which ascends to supply the anterior dura of the middle cranial fossa. It then passes through the foramen rotundum in the greater wing of the sphenoid bone to exit the skull and enter the superior aspect of the pterygopalatine fossa. It gives two branches (pterygopalatine nerves) to the pterygopalatine ganglion, but also receives parasympathetic nerves from the ganglion via the greater petrosal nerve. It then divides into the:
lateral posterior inferior nasal nerve (unnamed foramen)
Mandibular division/nerve (V3 or Vc)
Courses inferiorly through the foramen ovale to enter the infratemporal fossa, hence it does not pass through the cavernous sinus. It consists of a sensory root and a smaller motor root, the latter which bypasses the trigeminal ganglion inferiorly. These roots pass through the foramen ovale separately and then unite just below the foramen. It immediately gives off nervus spinosus and nerve to medial pterygoid from the main trunk. It then descends in the infratemporal fossa passing between the tensor veli palatini and lateral pterygoid muscles before dividing into anterior and posterior divisions:
anterior division (4 branches, all motor except one)
posterior division (3 branches, all sensory except one)
Variations can arise in any of the trigeminal nerve's major branches and its subsequent divisions.
ophthalmic division/nerve (V1)
maxillary division/nerve (V2)
the Maxillary nerve can be bifid 12
mandibular division/nerve (V3)
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