In this section we will discuss the functional anatomy and important anatomical landmarks that will assist the general radiologist to identify the trigeminal nerve throughout its course.
A.
FUNCTIONAL ANATOMY (Figure 1-3)
The functional relevance of the trigeminal nerve is somatosensory supply to the face,
oral and nasal cavities,
the teeth and majority of the scalp and motor supply to the masticatory and some other facial muscles.
It is also responsible for proprioceptive input from the masticatory and extra-ocular muscles.
Figures 1-3 demonstrate the functional anatomy of the trigeminal nerve following its division into the ophthalmic (V1),
maxillary (V2) and mandibular (V3) divisions.
Fig. 1: Diagram demonstrating V1 course.
References: TSdocs, many thanks to Naeem Majid, Saba Fatima, Hina Fatima
Fig. 2: Diagram demonstrating V2 course.
References: TSdocs, many thanks to Naeem Majid, Saba Fatima, Hina Fatima
Fig. 3: Diagram demonstrating V3 course.
References: TSdocs, many thanks to Naeem Majid, Saba Fatima, Hina Fatima
B.
STRUCTURAL/GROSS ANATOMY
B1 Brainstem segment (Figure 4)
The trigeminal nerve arises from one motor nucleus and three sensory nuclei:
> Principal sensory nucleus is situated in the dorsolateral pontine tegmentum and is responsible for tactile sensation from V1-V3.
> Mesencephalic nucleus is located at the lateral region of the periaqueductal grey matter in the midbrain.
It is responsible for proprioception of various muscles and joints including the extra-ocular muscles and the temporo-mandibular joints
> Spinal nucleus extends inferiorly through the medulla and into the upper segments of the cervical spinal cord’s dorsal horns.
Its main function is pain and temperature sensory input from V1-V3.
The second and third order neurons from these nuclei extend to the thalamus and the cerebral cortex respectively.
> Motor nucleus lies anteromedial to the principal sensory nucleus in the upper pons and innervates motor function to the masticatory muscle,
mylohyoid,
tensor tympani and tensor veli palatini.
Fig. 4: T1 sagittal oblique showing right trigeminal nerve emanating from the pons.
B2 Cisternal segment (Figure 5)
The trigeminal nerve exits the ventral aspect of the pons as two trunks on each side; a large sensory root and a smaller,
antero-medial to the former,
motor root.
On its exit from the pons,
the trigeminal nerve traverses the pre-pontine cistern antero-laterally,
just medial to the petrous apex,
at which point it enters the middle cranial fossa and Meckel’s cave via the porus trigeminus opening of the dura.
The point of change from central to peripheral myelin,
just prior to its exit from the pons,
is referred to as the root entry zone (REZ).
Fig. 5: Figure 5.1, 5.2 and 5.3 Axial, sagittal and coronal CISS images: a=fourth ventricle, b=main sensory nucleus, c=main motor nucleus, d=root entry zone, e=porus trigeminus, f=cisternal segment, g=Meckel’s cave
B3 Meckel's cave segment
Meckel’s cave (also called trigeminal cave or cavum trigeminale) is a cerebrospinal fluid containing pouch lined with dura.
It lies immediately lateral to the cavernous sinus and is continuous with the pre-pontine cistern.
Contained within it is the trigeminal ganglion (also referred as Gasserian or semilunar ganglion) from which the three branches of the trigeminal nerve arise from: V1,
V2 and V3.
Apart from the proprioceptive fibres,
the cell bodies of all sensory fibres are situated in this ganglion.
The motor trunk runs inferiorly to the ganglion and takes the course of V3,
bypassing the cavernous sinus,
in order to reach the masticator space.
B4 Cavernous segment
V1 and V2 pass through the lateral wall of the cavernous sinus from a posterior to anterior approach and pass lateral to the cavernous carotid artery and abducens nerve and inferior to the trochlear nerve.
B5 Skull base segment
The three branches of the trigeminal nerve exit the calvarium from different foramina:
> V1 traverses the superior orbital fissure towards the orbit.
(Figure 6)
> V2 reaches the pterygopalatine fossa through the foramen rotundum.
(Figure 7-9)
> V3 exits the intracranial compartment through the foramen ovale to reach the infratemporal fossa.
(Figure 10-11)
Fig. 6: Figure 6.1 – 6.2 Coronal and axial CT: arrows demonstrate location of superior orbital fissures.
Fig. 7: Figure 7.1 – 7.2 Coronal and axial CT: arrows demonstrate location of foramen rotundum.
Fig. 8: Axial CT: arrows demonstrate location of pterygopalatine fossa.
Fig. 9: Figure 9.1-9.2 Axial and coronal T1: arrows demonstrate V2 traversing foramen rotundum.
Fig. 10: Figure 10.1-10.2 Axial and coronal CT: arrows demonstrate location of foramen ovale.
Fig. 11: Figure 11.1-11.2 Axial and coronal T1: arrows demonstrate V3 traversing foramen ovale.
B6 Extra-cranial segment
>V1: The ophthalmic division gives off three further branches: the frontal,
lacrimal and nasociliary nerves which are responsible for sensory input from the globe,
nose,
forehead and scalp.
The supraorbital nerve,
which is a terminal branch of the frontal nerve,
traverses the supraorbital foramen whilst the supratrochlear nerve,
which is a branch of the frontal nerve,
traverses the supratrochlear foramen.
(Figure 12)
>V2: The maxillary division gives off four branches in the pterygopalatine fossa: the zygomatic,
superior alveolar,
meningeal and pterygopalatine nerves.
The continuation of its main root into the infraorbital canal via the infraorbital foramen is the resultant infraorbital nerve.
The inferior orbital fissure which separates the posterior aspect of the lateral wall and floor of the orbits,
contains the zygomatic and pterygopalatine nerves.
(Figure 13)
>V3: The mandibular division has both sensory and motor functions.
The sensory component is grossly responsible for input from the lower third of the face and branches into the inferior alveolar,
lingual and auriculotemporal nerves.
The motor component innervates the tensor veli palatini,
tensor tympani,
anterior belly of digastric muscle and masticator muscles via its branches,
the masticator and mylohyoid nerve. (Figure 14-15)
Fig. 12: Figure 12.1-12.2 Axial CT: white arrows demonstrate supraorbital foramina, black arrows demonstrate supratrochlear foramina.
Fig. 13: Figure 13.1-13.2 Axial & coronal CT: arrows demonstrate infraorbital fissures.
Fig. 14: Figure 14.1-14.3 3D CT reconstructions of (14.1) the base of the skull showing foramen ovale (black arrow), (14.2) the lateral aspect of the skull and face demonstrating the position of the infratemporal fossa (green arrow) where V3 enters after leaving foremen ovale and (14.3) the medial aspect of the right mandible showing the mandibular foramen (short black arrow) and canal (long black arrow) - site of the inferior alveolar nerve (V3).
Fig. 15: 3D CT reconstruction of the mandible showing the mental foramina (white arrows): point of exit for the mental nerves (from the inferior alveolar nerves, V3).