Cranial nerves part 1

CRANIAL NERVESCRANIAL NERVES
PART IPART I
Appearance of Normal Cranial Nerves
(REVIEW)
sherlockradiology.com

Abbreviations
• Olfactory nerve: CN1
• Optic nerve: CN2
• Oculomotor nerve:
CN3
• Trochlear nerve: CN4
• Trigeminal nerve:
CN5
• Abducens nerve: CN6
• Facial nerve: CN7
• Vestibulocochlear
nerve: CN8
• Glossopharyngeal
nerve: CN9
• Vagus nerve: CN1O
• Accessory nerve:
CN11
• Hypoglossal nerve:
CN12

Overview
• Cranial nerve groupings based on area of
brainstem origin
- Diencephalon: CN2
- Mesencephalon (midbrain): CN3 and
CN4
- Pons: CN5, CN6, CN7, and CN8
- Medulla: CN9, CN1O, CN11 and CN12

Imaging Approaches
• CN1, 2, 3, 4 and 6: Include focused orbital sequences
• CN5: Include entire face to inferior mandible if V3
affected
• CN7: Include CPA, temporal bone and parotid space
• CN8: Include CPA-lAC and inner ear
• CN9-12: Include basal cistern, skull base,
nasopharyngeal carotid space
• CN10: Follow carotid space to aortopulmonic window on
left, cervicothoracic junction on right
• CN12: Remember to reach hyoid bone to include distal
loop as it rises into sublingual space

Olfactory nerve: CN1
• First cranial nerve
• Special visceral afferent cranial nerve for
olfaction (sense of smell)
• Olfactory nerve segments
- End receptor in olfactory epithelium in nasal
vault
- Transethmoidal segment through cribriform
plate
- Intracranial olfactory bulb, tract and cortex

CN1: Nasal Epithelium
• Approximately 2 cm² nasal epithelium in roof of each
nasal cavity
- Extends onto nasal septum and lateral wall of nasal
cavity including superior turbinates
• Bipolar olfactory receptor cells (neurosensory cells)
located in nasal pseudostratified columnar epithelium
- Peripheral processes of receptor cells in olfactory
epithelium act as sensory receptors for smell
• Olfactory glands (of Bowman) secrete mucous which
solubilizes inhaled scents (aromatic molecules)

CN1: Transethmoidal Segment
• Central processes of bipolar receptor cells
traverse cribriform plate to synapse with
olfactory bulb
• Hundreds of central processes traverse
cribriform plate as unmyelinated fascicles
(fila olfactoria)
- Fila olfactoria are actual olfactory nerves
- Each side of nasal cavity has - 20 fila
olfactoria

CN1: Intracranial, Olfactory Bulb
and Tract
• Olfactory bulb and tracts are extensions of the
brain, not nerves
- Historically bulb and tract are called "olfactory
nerve"
• Olfactory bulb closely apposed to cribriform plate
at ventral surface of medial frontal lobe
- Rostral enlargement of olfactory tract
- Bipolar cells synapse in olfactory bulb with
secondary neuronal cells (mitral and tufted cells)
- Mitral cell axons project posteriorly in olfactory
tract
- Granule cells modulate mitral cells

CN1: Intracranial, Olfactory Bulb
and Tract
• Olfactory tract divides into medial, intermediate
and lateral stria at anterior perforated substance
- This trifurcation creates olfactory trigone
- Anterior perforated substance is perforated by
multiple small vascular structures
- Olfactory tract is made up of secondary
sensory axons, not primary sensory axons
- Majority of fibers project through lateral
olfactory stria and intermediate stria

CN1: Intracranial, Central
Pathways
• Complex pattern of central connections
• Lateral olfactory striae
- Formed by majority of fibers of olfactory tracts
- Course over insula to prepiriform area
(anterior to uncus) and amygdala
- On way to prepiriform area collaterals are
given to subfrontal or frontal olfactory cortex
- Fibers to subthalamic nuclei with collaterals /
terminal fibers to thalamus and stria medullaris

Pathways
• Medial olfactory striae
- Majority terminate in parolfactory area of Broca (medial
surface in front of the subcallosal gyrus)
- Some fibers end in subcallosal gyrus and in anterior
perforated substance
- Few fibers cross in anterior commissure to opposite
olfactory tract
• Intermediate olfactory striae
- Intermediate olfactory stria terminate in anterior
perforated substance
- Intermediate olfactory area contains anterior olfactory
nucleus and nucleus of diagonal band

Pathways
• Medial forebrain bundle
- Formed by fibers from basal olfactory
region, periamygdaloid area and septal
nuclei
- Some fibers terminate in hypothalamic
nuclei
- Majority of fibers extend to brainstem to
autonomic areas in reticular formation,
saliva tory nuclei and dorsal vagus
nucleus

CN1: Imaging Recommendations
• Coronal sinus CT is best study for isolated
anosmia
- Identifies nasal vault and cribriform plate
lesions
• MR of brain, anterior cranial fossa and
sinonasal region used in complex anosmia
cases
- Identifies intracranial dural and
parenchymal lesions

CN1: Imaging "Sweet Spots"
• Intracranial: Include anterior cranial fossa
floor and medial temporal lobes
• Extracranial: Include nasal vault and
cribriform plate
• Imaging Pitfalls
- Coronal sinus CT insensitive to
intracranial pathology

CN1: Clinical Importance
• CN1 dysfunction produces unilateral anosmia
- Each side of nose must be tested individually
• Esthesioneuroblastoma arises from olfactory
epithelium in nasal vault
• Head trauma may cause anosmia: Cribriform
plate fracture or shear forces; anterior temporal
lobe injury
• Seizure activity in lateral olfactory area may
produce "uncinate fits", imaginary odor,
oroglossal automat isms and impaired
awareness

Cranial Nerve I:
The Olfactory Nerve
• Unlike most cranial nerves, the olfactory nerve consists
of white-matter tracts and is not surrounded by Schwann
cells
• The neurosensory cells for smell reside in the olfactory
epithelium along the roof of the nasal cavity
• The axons of these cells extend through the cribriform
plate of the ethmoid bone into the olfactory bulb at the
anterior end of the olfactory nerve
• The nerve then courses posteriorly through the anterior
cranial fossa in the olfactory groove
• Posterior to the olfactory groove, the cisternal segment
of the nerve runs below and between the gyrus rectus
and the medial orbital gyrus
RadioGraphics 2009; 29:1045–1055

Cranial Nerve I:
The Olfactory Nerve
• These secondary axons in the olfactory nerve
eventually terminate in the inferomedial temporal
lobe, uncus and entorhinal cortex
• To avoid confusing the olfactory nerve with the
gyrus rectus on axial images, it is important to
remember that the olfactory nerve is situated
deep in the olfactory groove, inferior to the gyrus
rectus
• Coronal images are easiest to interpret because
the nerves are seen in cross section

Olfactory nerve.
Axial and coronal 0.8-mm-thick SSFP MR images show the olfactory nerve (white arrow)
within the CSF-filled olfactory groove and the optic nerve (black arrow) ringed by
highsignal-intensity CSF within the dural sheath.
Coronal 1.0-mm-thick SSFP MR image shows the cisternal segment of the olfactory
nerve (arrow), which is located inferior to and between the gyrus rectus (r) and the
medialorbital gyrus (o).

CN2
• Second cranial nerve
• Nerve of sight
• Visual pathway consists of optic nerve,
optic chiasm and retrochiasmal structures

Overview
• Optic nerve not true cranial nerve but
rather extension of the brain
- Represents collection of retinal ganglion
cell axons
- Myelinated by oligodendrocytes not by
Schwann cells as with true cranial nerves
- Enclosed by meninges
- Throughout its course to visual cortex
nerve fibers are arranged in retinotopic
order

Overview
• Optic nerve has four segments
- Intraocular, intraorbital, intracanalicular and
intracranial
• Partial decussation CN2 fibers within optic
chiasm
- Axons from medial portion of each retina cross
to join those from lateral portion of opposite
retina
• Retrochiasmal structures: Optic tract, lateral
geniculate body, optic radiation and visual cortex

Intraocular segment
• 1 mm length
• Region of sclera termed lamina cribrosa
where ganglion cell axons exit globe

Intraorbital segment
• 20-30 mm in length
• Extends posteromedially from back of globe to orbital apex within
intraconal space of orbit
• CN2 longer than actual distance from optic chiasm to globe allowing
for movements of eye
• Covered by same 3 meningeal layers as brain
- Outer dura, middle arachnoid and inner pia
- Subarachnoid space (SAS) between arachnoid and pia contains
cerebrospinal fluid (CSF); continuous with SAS of suprasellar
cistern
- Fluctuations in intracranial pressure transmitted via SAS of optic
nerve-sheath complex
• Central retinal artery
- 1st branch of ophthalmic artery
- Enters optic nerve about 1 em posterior to globe with
accompanying vein to run to retina

Intracanalicular segment
• 4-9 mm segment within bony optic canal
• Ophthalmic artery lies inferior to CN2
• Dura of CN2 fuses with orbit periosteum
(periorbita)

Intracranial segment
• About 10 mm length from optic canal to
chiasm
• Covered by pia and surrounded by CSF
within suprasellar cistern
• Ophthalmic artery runs inferolateral to
nerve

Optic chiasm
• Horizontally oriented; "X-shaped" structure within
suprasellar cistern
• Forms part of floor of 3rd ventricle between optic recess
anteriorly and infundibular recess posteriorly
• Immediately anterior to infundibulum (pituitary stalk),
superior to diaphragma sellae
• Anteriorly chiasm divides into optic nerves
• In chiasm nerve fibers from the medial half of retina
cross to opposite side
• Posteriorly chiasm divides into optic tracts
• Medial fibers of optic tracts cross in chiasm to connect
lateral geniculate bodies of both sides (commissure of
Gudden)

Optic tracts
• Posterior extension of optic chiasm
• Fibers pass posterolaterally curving
around cerebral peduncle and divide into
medial and lateral bands
- Lateral band (majority of fibers) ends in
lateral geniculate body of the thalamus
- Medial band goes by medial geniculate
body to pretectal nuclei deep to superior
colliculi

Optic radiation and visual cortex
• Efferent axons from lateral geniculate body form
optic radiations (geniculocalcarine tracts)
• Fan out from lateral geniculate body and run as
broad fiber tract to calcarine fissure
- Initially pass laterally behind posterior limb
internal capsule and basal ganglia
- Extend posteriorly around lateral ventricle
passing through posterior temporal and parietal
lobes
- Terminate in calcarine cortex (primary visual
cortex) on medial surface of occipital lobes

Imaging Recommendations
• CT best for skull base and optic canal
bony anatomy
• MR for CN2, optic chiasm and
retrochiasmal structures
- Axial and coronal thin-section T2, T1
and T1 C+
• Imaging Pitfalls: orbital CT may see subtle
calcified optic sheath meningioma when
MR may not

Clinical Importance
• Lesion location
- Optic nerve pathology: Monocular visual loss
- Optic chiasm pathology: Bitemporal heteronymous
hemianopsia (loss of bilateral temporal visual fields)
- Retrochiasmal pathology: Homonymous hemianopsia
(vision loss in contralateral eye)
• Increased intracranial pressure transmitted along (SAS)
of optic nerve-sheath complex
- Manifests clinically as papilledema
- Imaging shows flattening of posterior sclera, tortuosity
and elongation of intraorbital optic nerves and dilatation
of perioptic SAS (subarachnoid space)

Cranial Nerve II:
The Optic Nerve
• Like the olfactory nerve, the optic nerve is a white-matter tract
without surrounding Schwann cells.
• It includes four anatomic segments: retinal, orbital, canalicular,
and cisternal.
• The retinal segment leaves the ocular globe through the lamina
cribrosa sclerae (the optic foramen of the sclera).
• The orbital segment, which is surrounded by a dural sheath
containing CSF, travels through the center of the fat-filled orbit.
• The canalicular segment is the portion that lies in the optic canal,
below the ophthalmic artery. This segment of the nerve is frequently
overlooked on radiologic images, so it should be specifically sought
when imaging for vision loss.
• Finally, the cisternal segment of the nerve can be visualized in the
suprasellar cistern, where the nerve leads to the optic chiasm. The
anterior cerebral artery passes over the superolateral aspect of the
cisternal segment of the nerve.

Cranial Nerve II:
The Optic Nerve
• Key anatomic landmarks in the suprasellar cistern
include the infundibulum (stalk) of the pituitary gland, the
anterior cerebral artery, and, posterior to the chiasm, the
mamillary bodies.
• The optic nerve terminates at the optic chiasm, where
the two nerves meet, decussate, and form the optic
tracts.
• The optic tracts travel around the cerebral peduncles,
after which most axons enter the lateral geniculate body
of the thalamus, loop around the inferior horns of the
lateral ventricles (Meyer loop), and enter the visual
cortex in the occipital lobe.

Optic nerve.
Axial oblique 0.8-mm-thick SSFP MR image shows three of four segments of the optic
nerve: the retinal (black arrow), orbital (black arrowheads), and canalicular (white
arrowhead) segments. The infundibulum of the pituitary gland (white arrow) also is
seen. The fourth (cisternal) segment of the optic nerve would be visible on more
superior images.
The cisternal segment of the optic nerve (white arrow) leads to the chiasm, which
resembles the Greek letter χ in this plane. The optic tract (white arrowheads) leads
backward from the chiasm to the thalamus. Important anatomic landmarks include the
mamillary bodies (black arrowhead) and the anterior cerebral artery (black arrow).

Residual tumor near the optic chiasm in an 18-year-old woman
after resection of a pituitary adenoma.
Axial oblique 0.8-mmthick SSFP MR image shows a thin layer of
cerebrospinal fluid (arrow) between the residual tumor (t) and
the left optic nerve and chiasm, a finding suggestive of
resectability.
The residual tumor was removed successfully with an expanded
endonasal approach.

Overview
• Third cranial nerve
• Motor nerve to extraocular muscles except
lateral rectus (CN6) and superior oblique
muscles (CN4); parasympathetic to
pupillary sphincter and ciliary muscle
• Mixed cranial nerve (motor and
parasympathetic)
• Four anatomic segments: Intra-axial,
cisternal, cavernous and extracranial

Intra-Axial Segment
Oculomotor nuclear complex
• Paired paramedian nuclear complex
- Located in midbrain anterior (ventral) to
cerebral aqueduct at level of superior colliculus
- Partially embedded in periaqueductal gray
matter
- Bounded laterally and inferiorly by medial
longitudinal fasciculus
• Consists of five individual motor subnuclei that
supply individual extraocular muscles

Intra-Axial Segment
Edinger-Westphal parasympathetic nuclei
• Located dorsal to oculomotor nuclear complex in
poorly myelinated periaqueductal gray matter
• Preganglionic parasympathetic fibers exit
nucleus,course ventrally with motor CN3 fibers
• Innervation of internal eye muscles (sphincter
pupillae and ciliary muscles)

Intra-Axial Segment
• Oculomotor fascicles course anteriorly through
medial longitudinal fasciculus, red nucleus,
substantia nigra and medial cerebral peduncle
- Exit midbrain into interpeduncular cistern
• Parasympathetic Perlia nuclei
- Located between the Edinger-Westphal nuclei
- Thought to be involved in ocular convergence

Cisternal Segment
• Courses anterolaterally through interpeduncular
and prepontine cisterns
• Passes between posterior cerebral (PCA) and
superior cerebellar arteries (SCA)
• Courses inferior to posterior communicating
artery and medial to free edge of tentorium
cerebelli
• Crosses the petroclinoid ligament and
penetrates dura to enter roof of cavernous sinus

Cavernous Segment
• Enters roof of cavernous sinus surrounded
by narrow oculomotor CSF cistern
• Courses anteriorly through lateral dural
wall of cavernous sinus
• CN3 remains most cephalad of all
cranial nerves within cavernous sinus
• CN3 superolateral to cavernous internal
carotid artery

Extracranial Segment
• CN3 enters orbit through superior orbital fissure and
passes through annulus tendineus (annulus of Zinn)
• Divides into superior and inferior branches
• - Superior branch supplies levator palpebrae superioris
and superior rectus muscles
- Inferior branch supplies inferior rectus, medial rectus
and inferior oblique muscles
• Preganglionic parasympathetic fibers follow inferior
branch to ciliary ganglion of orbit
- Postganglionic parasympathetic fibers continue as
short ciliary nerves to enter globe with optic nerve
- In globe short ciliary nerves to ciliary body and iris
- Control papillary sphincter function and
accommodation via ciliary muscle

• Bone CT best for skull base, bony foramina
• MR for intra-axial, cisternal, cavernous segments
- Thin-section high-resolution T2 MR sequences in axial
and coronal planes
. Depicts cisternal CN3 surrounded by CSF with high
contrast and high spatial resolution
• Imaging "Sweet Spots“: CN3 nuclear complex and intra-
axial segment not directly visualized
- Find periaqueductal gray matter to localize
• Imaging Pitfalls: Negative MR and MRA does not
completely exclude posterior communicating artery
aneurysm
- Cerebral angiography still represents gold standard to
exclude this diagnosis

Clinical Importance and Findings
• Uncal herniation pushes CN3 on petroclinoid ligament
• During trauma downward shift of brainstem upon impact can
stretch CN3 over petroclinoid ligament
• CN3 susceptible to compression by PCA aneurysms
• CN3 neuropathy divided into simple if isolated and complex if with
other CN involvement (CN4 & CN6)
- Simple CN3 with pupillary involvement
• Must exclude PCA aneurysm as cause
• Explanation: Parasympathetic fibers are peripherally distributed
- Simple CN3 with pupillary sparing
.Presumed microvascular infarction involves vessels supplying
core of nerve with relative sparing of peripheral pupillary fibers
• Oculomotor ophthalmoplegia: Strabismus, ptosis, pupillary
dilatation, downward abducted globe and paralysis of
accommodation

Cranial Nerve III:
The Oculomotor Nerve
• The oculomotor nerve originates from nuclei deep to the
superior colliculus, ventral to the cerebral aqueduct, and
inferior to the pineal gland.
• The nerve then travels across the midbrain from
posterior to anterior.
• The oculomotor nerve root emerges into the
interpeduncular cistern, and this root entry zone in the
cistern is a good way to identify the oculomotor nerve on
axial SSFP MR images.
• In the prepontine cistern, the nerve travels between the
superior cerebellar and posterior cerebral arteries, which
makes it easy to identify on coronal SSFP images.

Cranial Nerve III:
The Oculomotor Nerve
• The cavernous segment of the oculomotor nerve
runs along the lateral wall of the cavernous sinus
and is the most superior of the nerves in this
sinus.
• The oculomotor nerve then enters the orbit
through the superior orbital fissure, before
splitting into superior and inferior divisions lateral
to the optic nerve.
• Knowledge of this anatomy may be helpful for
identifying the precise location of a nerve
abnormality.

Oculomotor nerve.
Axial oblique 0.8-mm-thick SSFP MR image shows the nerve (small arrows) where it emerges
from the interpeduncular cistern (large arrow), which lies medial to the cerebellar peduncle (p).
Coronal 0.8-mm-thick SSFP MR image shows the oculomotor nerve (white arrow) in cross section
between the posterior cerebral artery (white arrowhead) and the superior cerebellar artery (black
arrowhead), which are distal branches of the basilar artery (black arrow).

Oculomotor nerve compression in na 82-year-old woman
with ptosis of the right eye.
Axial 0.8-mm-thick SSFP MR image shows displacement
and compression of the right oculomotor nerve in the
root entry zone (long arrow) by the distal basilar artery
(short arrow).
The left oculomotor nerve (arrowhead), in comparison,
appears normal.

Overview
• Fourth cranial nerve
• Motor nerve to superior oblique muscle
• CN4 is a pure motor nerve
• Four segments: Intra-axial, cisternal,
cavernous and extracranial

Intra-Axial Segment
Trochlear nuclei
• Paired nuclei located in paramedian
midbrain
- Anterior to cerebral aqueduct
- Dorsal to medial longitudinal fasciculus
- Caudal to oculomotor nuclear complex
at level of inferior colliculus

Intra-Axial Segment
• Trochlear nerve fascicles course posteriorly and
inferiorly around cerebral aqueduct
- Fibers then decussate within superior
medullary velum
- Key concept: each superior oblique muscle is
innervated by contralateral trochlear nucleus
• CN4 exists dorsal midbrain just inferior to inferior
colliculus
- Key concept: CN4 is the only cranial nerve to
exit dorsal brainstem

Cisternal Segment
• CN4 courses anterolaterally in ambient cistern
- Runs underneath the margin of the tentorium
- Passes between free edge of tentorium
cerebelli and midbrain just superolateral to pons
• Passes between posterior cerebral artery above
and superior cerebellar artery below
- Oculomotor nerve travels this gap as well
- CN4 is just inferolateral to oculomotor nerve
• Penetrates dura to enter lateral wall of
cavernous sinus just inferior to oculomotor nerve

Cavernous Segment
• Courses anteriorly through lateral dural
wall of cavernous sinus
• Intracavernous relationships of CN4
- Remains inferior to CN3
- Superior to ophthalmic division of
trigeminal nerve (CNV1)
- Lateral to cavernous internal carotid
artery

Extracranial Segment
• CN4 enters orbit through superior orbital
fissure together with CN3 and CN6
• Crosses over CN3 and courses medially
• Passes above annulus of Zinn (CN3 and
CN6 go through annulus)
• Supplies motor innervation to superior
oblique muscle

• CT best for skull base, bony foramina
• MR best for brainstem, cisternal,
cavernous and intra-orbital imaging
• Intra-orbital segment not visualized by any
imaging modality or sequence

Imaging "Sweet Spots"
• CN4 nucleus and intra-axial segment not directly
visualized
- Nuclei position inferred by identifying periaqueductal
gray matter and cerebral aqueduct at level of inferior
colliculi on high-resolution MR
• MR for intra-axial, cisternal and cavernous segments
- Thin-section high-resolution T2 and T1 C+ MR in axial
and coronal planes
• Coronal imaging margins: Fourth ventricle to anterior
globe
• Axial imaging margins: Orbital roof-diencephalon to
maxillary sinus roof-medulla

Imaging Pitfalls
• Difficult to visualize CN4 despite best MR
imaging efforts
• During image interrogation by radiologist,
view known landmarks along its course
- Midbrain → tentorial margin →
cavernous sinus → superior orbital fissure
→ extraconal orbit

Normal Measurements
• CN4 is smallest cranial nerve
• CN4 has longest intracranial course
(- 7,5 cm)

Clinical Importance
CN4 neuropathy divided into simple and complex
• Simple CN4 neuropathy (isolated)
- Most common form; usually secondary to trauma
- Cisternal segment injury by free edge of tentorium
cerebelli or from posterior cerebral or superior cerebellar
artery aneurysm
- Contusion of superior medullary velum
• Complex CN4 neuropathy (associated with other CN
injury, CN3 ± CN6)
- Brainstem stoke or tumor
- Cavernous sinus thrombosis, tumor
- Orbital tumor

Clinical Findings
• Paralysis of superior oblique muscle results in
extorsion (outward rotation) of affected eye
• Extorsion is secondary to unopposed action of
inferioroblique muscle
• Patient complaints: Diplopia, weakness of
downward gaze, neck pain from head tilting
• Physical exam: Compensatory head tilt usually
away from affected side

Cranial Nerve IV:
The Trochlear Nerve
• The trochlear nerve is the only nerve with a root
entry zone arising from the dorsal (posterior)
brainstem.
• After exiting the pons, the trochlear nerve curves
forward over the superior cerebellar peduncle,
then runs alongside the oculomotor nerve
between the posterior cerebral and superior
cerebellar arteries.
• The trochlear nerve then pierces the dura to
enter the cisterna basalis between the free and
attached borders of the cerebellar tentorium.

Cranial Nerve IV:
The Trochlear Nerve
• After completing its cisternal course, the
trochlear nerve runs through the lateral
cavernous sinus just below the oculomotor
nerve and enters the orbit through the
superior orbital fissure to innervate the
superior oblique muscle.
• The nerve is named for the trochlea, the
fibrous pulley through which the tendon of
the superior oblique muscle passes.

Cranial Nerve IV:
The Trochlear Nerve
• The cisternal segment of this tiny nerve is most
easily identifiable posterolateral to the
brainstem.
• Along part of its intracranial course, the trochlear
nerve lies between dural layers, where it is
difficult to visualize on radiologic images.
• Particular attention should be given to the
anterior aspect of the tentorium in patients in
whom the presence of isolated trochlear nerve
palsy is suspected.

Trochlear nerve.
Axial 0.8-mm-thick SSFP MR image shows both trochlear nerves
(arrows) where they emerge from the dorsal midbrain to cross
the ambient cisterns.
The characteristic course of the trochlear nerves allows their
differentiation from the nearby superior cerebellar artery
(arrowheads).

Overview
• Trigeminal nerve: CN5, CNV
• Ophthalmic division, trigeminal nerve: CNV1
• Maxillary division, trigeminal nerve: CNV2
• Mandibular division, trigeminal nerve: CNV3
• Fifth cranial nerve, nervus trigeminus
• Great sensory cranial nerve of head and face;
motor nerve for muscles of mastication
• Mixed nerve (both sensory, motor components)
• Four segments: Intra-axial, cisternal,
interdural and extracranial

Intra-Axial Segment
Four nuclei (3 sensory, 1 motor)
Located in brainstem, upper cervical cord
Mesencephalic nucleus CNS
• Slender column of cells projecting cephalad from pons to
level of inferior colliculus
• Found anterior to upper fourth ventricle/aqueduct near
lateral margin of central gray
• Afferent fibers for facial proprioception (teeth, hard
palate and temporomandibular joint)
• Sickle-shaped mesencephalic tract descends to motor
nucleus, conveys impulses that control mastication and
bite force

Intra-Axial Segment
• Main sensory nucleus CNS
- Nucleus lies lateral to entering trigeminal root
- Provides facial tactile sensation
• Motor nucleus CNS
- Ovoid column of cells anteromedial to principal
sensory nucleus
- Supplies muscles of mastication (medial and lateral
pterygoids, masseter, temporalis), tensor palatine/tensor
tympani, mylohyoid and anterior belly of digastric
• Spinal nucleus CNS
- Extends from principal sensory root in pons into upper
cervical cord (between C2 to C4 level)
- Conveys facial pain, temperature

Cisternal (Preganglionic) Segment
• Two roots: Smaller motor, larger sensory
• Emerges from lateral pons at root entry zone
(REZ)
• Courses anterosuperiorly through prepontine
cistern
• Enters middle cranial fossa by passing beneath
tentorium at apex of petrous temporal bone
• Passes through an opening in dura matter called
porus trigeminus to enter Meckel cave

Interdural Segment
• Meckel cave formed by meningeal layer of dura
lined by arachnoid
- Cave is filled with cerebrospinal fluid (CSF)
(90%) and continuous with prepontine
subarachnoid space
• Pia covers CNS in trigeminal cave
• Preganglionic CNS ends at trigeminal ganglion
(TG)
- TG located in inferior aspect of Meckel cave
- TG synonyms: Gasserian or semilunar
ganglion

Divisions (Post-Ganglionic) of CNS
Ophthalmic nerve (CNV1)
• Courses in lateral cavernous sinus wall
below CN4
- Exits skull through superior orbital
fissure
- Enters orbit, divides into lacrimal, frontal
and nasociliary nerves
• Sensory innervation scalp, forehead,
nose, globe

Maxillary nerve (CNV2)
• Courses in cavernous sinus lateral wall
below CNV 1
• Exits skull through foramen rotundum
• Traverses roof of pterygopalatine fossa
• Continues as infraorbital nerve in floor of
orbit
• Exits orbit through infraorbital foramen
- Sensory to cheek and upper teeth

Mandibular nerve (CNV3)
• Does not pass through cavernous sinus
• Exits directly from Meckel cave, passing inferiorly
through foramen ovaIe into masticator space
• Carries both motor and sensory fibers
- Motor root bypasses TG, joins V3 as it exits through
foramen ovale
- Divides into masticator (muscles of mastication) and
mylohyoid nerves (mylohyoid and anterior belly of
digastric muscles)
- Masticator nerve take off just below skull base
- Mylohyoid nerve take off at mandibular foramen
• Main sensory branches include inferior alveolar, lingual
and auriculotemporal nerves

• CT best for skull base and bony foramina
• MR for intra-axial, cisternal and intradural
segments
- Thin-section T2 in axial and coronal planes
• T1 C+ fat-saturated MR of entire extracranial
course
• Imaging Pitfalls: Trigeminal ganglion is small
crescent of tissue found in the anteroinferior
Meckel cave
- Trigeminal ganglion lacks blood-nerve barrier
therefore normally enhances with contrast

Clinical Importance
• Sensory complaints: Pain, burning, numbness in
face
• Motor (V3 only): Weakness in chewing
- Proximal V3 injury causes motor atrophy of
masticator muscles within 6 weeks to 3 months
- Distal V3 injury (above mylohyoid nerve
takeoff) affects only anterior belly of digastric &
mylohyoid
• Tic douloureux (trigeminal neuralgia)
- Sharp, excruciating pain in V2-3 distributions
- Look for vascular compression at REZ (on
MR)

Cranial Nerve V:
The Trigeminal Nerve
• The trigeminal nerve is the largest cranial nerve.
• It is composed of a large sensory root that
runs medial to a smaller motor root.
• The roots emerge from the lateral midpons and
travel anteriorly through the prepontine cistern
and the porus trigeminus to the Meckel
(trigeminal) cave, a CSF-containing pouch in the
middle cranial fossa.
• Because the trigeminal nerve is large and its
course proceeds straight forward from the lateral
pons, it is easy to recognize on most MR
images.

Cranial Nerve V:
The Trigeminal Nerve
• In the Meckel cave, the nerve forms a meshlike web that
can be visualized only with high-resolution imaging.
• Along the anterior aspect of the cavity, the trigeminal
nerve forms the trigeminal (gasserian) ganglion before
splitting into three subdivisions.
• The ophthalmic (V1) and maxillary (V2) divisions of the
nerve move medially into the cavernous sinus and exit
the skull through the superior orbital fissure and foramen
rotundum, respectively.
• The mandibular division (V3), which includes the motor
branches, exits the skull inferiorly through the foramen
ovale.

Trigeminal nerve.
Axial 0.8-mm-thick SSFP MR image shows the sensory
(arrowhead) and motor (large arrow) roots of the trigeminal
nerve
where they cross the prepontine cistern and enter the Meckel
cave (small arrows).

Trigeminal nerve.
Coronal 0.8-mm-thick SSFP MR image at the level of the Meckel
cave shows the complex web of trigeminal nerve branches
(arrows), which coalesce anteriorly to form the gasserian
ganglion. The temporal horn of the lateral ventricle (arrowhead)
is also shown.

Overview
• Sixth cranial nerve
• Motor nerve to lateral rectus muscle only
• CN6 is a pure motor nerve
• Five segments can be defined: Intra-axial,
cisternal, interdural, cavernous and
extracranial (intra-orbital)

Intra-Axial Segment
Abducens nucleus
• Paired CN6 nuclei located in pontine tegmentum near
midline
- Found just anterior to fourth ventricle
• Facial colliculus: Axons of facial nerve (CN7) loop
around abducens nucleus creating this bulge in floor of
fourth ventricle
Abducens nerve axons course anteroinferiorly through
pontine tegmentum
Emerges from anterior brainstem near midline through
groove between pons and pyramid of medulla oblongata
(bulbopontine sulcus)

Cisternal Segment
• CN6 ascends anterosuperiorly in
prepontine cistern toward site where it
penetrates dura
• May be posterior or anterior to anterior
inferior cerebellar artery
• Penetrates dura of basisphenoid to enter
Dorello canal

Interdural Segment
• Dorello canal represents channel within basilar venous plexus
(petroclival venous confluence)
- Channel is located between two layers of dura
- Basilar venous plexus is continuous with cavernous sinus
anteriorly
- Basilar venous plexus drains into inferior petrosal sinus
• Extends from point where CN6 pierces inner (cerebral) layer dura
mater to its entrance into cavernous sinus
• Within Dorello canal, abducens nerve is surrounded by layer of
arachnoid mater & occasionally dura mater
• After penetrating dura, CN6 passes superiorly through basilar
venous plexus
- It then arches over petrous apex below petrosphenoidalligament
into upper posterior region of cavernous sinus
- Bony sulcus of CN6 as it passes over top of petrous apex usually
present

Cavernous Segment
• CN6 courses anteriorly within cavernous sinus
- Abducens nerve is only cranial nerve to lie
within cavernous sinus
- Cranial nerves 3, 4, VI and V2 are all
embedded within lateral wall of cavernous sinus
• Within cavernous sinus CN6 runs along
inferolateral aspect of cavernous internal carotid
artery

Extracranial (Intra-Orbital) Segment
• CN6 enters orbit through superior orbital
fissure together with CN3 and CN4
• Passes through annulus of Zinn
• Supplies motor innervation to lateral
rectus muscle

• MR for intra-axial, cisternal, interdural &
cavernous segments
- Thin-section high-resolution T2 and contrast-
enhanced Tl in axial and coronal planes
• Depicts small structures including cranial nerves
surrounded by CSF with high contrast & high
spatial resolution
• Bone CT best for skull base and its bony
foramina
• Dorello canal, cavernous sinus and orbital CN6
not visualized on routine MR imaging

Imaging "Sweet Spots"
• Axial and coronal MR sequences should include
brainstem, fourth ventricle, cavernous sinus and orbit
• CN6 nucleus and intra-axial segment not directly
visualized
- Position of CN6 inferred by identifying facial colliculus
in floor of fourth ventricle on high-resolution thin-section
T2 MR
• Cisternal segment routinely visualized on high-resolution
T2
• CN6 entrance into Dorello canal may be visualized due
to invagination of cerebrospinal fluid into proximal canal

Imaging Pitfalls
• Use of fat-saturation on post-contrast T1
MR sequences can amplify blooming
(susceptibility) artifact around a well
aerated sphenoid sinus
- Cavernous sinus & orbital apex subtle
lesions may be obscured by this artifact
- Remove fat-saturation and repeat Tl
post-contrast MR if this artifact obscures
key areas of interest

Clinical Importance
• In abducens neuropathy, affected eye will not
abduct (rotate laterally)
• CN6 neuropathy divided into simple if isolated &
complex if associated with other CN involvement
(CN3, 4 and 7)
- Simple CN6 neuropathy most common ocular
motor nerve palsy
- Usually presents as complex cranial
neuropathy:
Pontine lesions affect CN6 with CN7
Cavernous sinus, superior orbital fissure
lesions affect CN6 with CN3, 4 and CNV1

Cranial Nerve VI:
The Abducens Nerve
• The abducens nerve emerges from nuclei anterior to the
fourth ventricle, then courses anteriorly through the pons
to the pontomedullary junction and into the prepontine
cistern.
• After crossing the prepontine cistern in a posterior-to-
anterior direction, the abducens nerve runs vertically
along the posterior aspect of the clivus, within a fibrous
sheath called the Dorello canal.
• The nerve then continues over the medial petrous apex
and through the medial cavernous sinus, entering the
orbit through the superior orbital fissure to innervate the
lateral rectus muscle.

Cranial Nerve VI:
The Abducens Nerve
• It is important to note that the abducens nerve
runs almost the entire length of the clivus.
• Radiologists should be vigilant for clivus and
petrous apex abnormalities in the setting of
abducens nerve palsy.
• Although the abducens nerve lies near the
anterior inferior cerebellar artery and has a
similar caliber, the two structures course in
orthogonal directions and are thus easily
distinguished.

Abducens nerve.
Axial 0.8-mm-thick SSFP MR image at the level of the
pontomedullary junction shows both abducens nerves (arrows)
where they traverse the prepontine cistern.
The bottom of the pons (p) and the top of the medulla (m) are
visible in this section, and the cerebellopontine angle (CPA) and
basilar artery (arrowhead) are important anatomic landmarks.

Abducens nerve.
Axial 0.8-mm-thick SSFP MR image shows the abducens nerve
where it enters the Dorello canal (arrow) along the posterior
aspect of the clivus.
Vascular landmarks include the basilar artery (black arrowhead)
and the anterior inferior cerebellar artery (white arrowhead).

References
• Diagnostic and Surgical Imaging Anatomy.
Brain, Head & Neck, Spine / H. Ric
Harnsberger. [et al.] ; managing editor, Andre
Macdonald. n 1st ed. I:174-I:259.
• RadioGraphics 2009; 29:1045–1055 • Sujay
Sheth, BA • Barton F. Branstetter IV, MD •
Edward J. Escott, MD. Appearance of Normal
Cranial Nerves on Steady-State Free
Precession MR Images.

Cranial nerves part 1

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