Cranial nerves

Introduction

There are a total of 12 cranial nerves, two of which originates from the cerebrum while the rest originates in the brainstem. The cranial nerves are numbered by their location on the brainstem (superior o inferior, then medial to lateral) and the order of their exit from the cranium (anterior to posterior).

Cranial nerve	Name	Origin	Function Type
CN I	Olfactory nerve	Cerebrum	Sensory
CN II	Optic nerve	Cerebrum	Sensory
CN III	Oculomotor nerve	Midbrain-pontine junction	Motor
CN IV	Trochlear nerve	Midbrain	Motor
CN V	Trigeminal nerve: - Opthalmic - Maxillary - Mandibular	Pons	Both
CN VI	Abducens nerve	Pontine-medulla junction	Motor
CN VII	Facial nerve	Pontine-medulla junction	Both
CN VIII	Vestibulocochlear nerve	Pontine-medulla junction	Sensory
CN IX	Glossopharyngeal nerve	Medulla	Both
CN X	Vagus nerve	Medulla	Both
CN XI	Accessory nerve	Medulla	Motor
CN XII	Hypoglossal nerve	Medulla	Motor

Tip

Cranial nerves with “2” exits the skull via canal with the same name.
Optic nerve (CN 2) -> Optic canal
Hypoglossal nerve (CN 12)-> Hypoglossal canal

Cranial Nerves and Medical Imaging

Each cranial nerve is described below briefly and shown in different image modalities.

Olfactory nerve

Consists of white matter tracts not surrounded by Schwann cells. Enters the anterior cranial fossa through the cribriform plates and terminate in the olfactory bulb. The nerve courses between the gyrus rectus and medial orbital gyrus. The secondary axons terminate in the inferomedial temporal lobe, uncus, and entorhinal cortex.

MRI

A Steady-state free precession (SSFP) axial image showing the olfactory nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

CT

Coronal plane image of cribriform plate (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Optic nerve

Emerges from the posterior pole of ocular globe and divided into four segments:

• intraocular
• intraorbital
• intracanalicular
• prechiasmatic

The optic nerve from each side passes through its respective optic canal and joins to form the optic chiasm. Then optic tract courses along the cerebral peduncles and synapses at the lateral geniculate nuclei after which optic radiation reaches the primary visual cortex in the occiptal lobe (Brodmann area 17).

MRI

A Steady-state free precession (SSFP) axial image showing the optic nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

CT

Optic canal (asterisks) and superior orbital fissure (arrow)

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Also see: Annotated MRI of Optic pathway from Radioepaedia

Oculomotor nerve

Somatic and Parasympathetic functions:

• Somatic motor fibers (innervates inferior superior, middle rectus, inferior oblique, and levator palpebrae superior muscles) originate from nuclear complex located in midbrain at level of superior colliculi, ventral to the cerebral aqueduct and the periaqueductal gray matter.
• Parasympathetic fibers arise in Edinger-Westphal nucleus situated dorsally to the oculomotor nuclear complex.

The nerve emerges from interpeduncular cistern running through the perimensencephalic cistern superiorly to the posterior cerebral artery (PCA) and inferiorly to the superior cerebellar artery (SCA). It then enters the cavernous isnus where iti s the most superior nerve to reach the orbit through superior orbital fissure. Passes through superior orbital fissure.

MRI

A Steady-state free precession (SSFP) axial image showing the oculomotor nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Trochlear nerve

Originates from the posterior part of midbrain, then runs anteriorly and inferiorly within the subarachnoid space before piercing the dura mater adjacent to the posterior clinoid process of the sphenoid bone. Then it moves along the lateral wall of cavernous sinus before entering the orbit of eye via superior orbital fissure to innervate the superior oblique muscle.

MRI

A Steady-state free precession (SSFP) axial image showing the trochlear nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Note

Trochlear nerve is the only nerve to exit from the posterior midbrain.

Trigeminal nerve

The nerve emerges straight forward from the lateral pons and pass in the Meckel’s cave where the trigeminal ganglion is located, then splits into three subvisions:

• Opthalmic nerve (CN V1) Passes through lateral wall of cavernous sinus and exits the skull via superior orbital fissure.
• Maxillary nerve (CN V2) Passes through lateral wall of cavernous sinus and exits the skull via foramen rotundum and crosses the pterygopalatine fossa and then enters the orbits via the inferior orbital fissure, pases within the infraorbital groove, and reaches the face through infraorbital foramen.
• Mandibular nerve (CN V3) exits the skull via formane ovale and passes in the infratemporal fossa where it divides into other nerves.

Terminology

• Meckel’s cave: a cerebrospinal fluid-containing dural pouch in the middle cranial fossa that contains the trigeminal nerve between prepontine cistern and cavernous sinus and situated in posterolateral aspect of cavernous sinus on either side on sphenoid

MRI

A Steady-state free precession (SSFP) axial image showing the trigeminal nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

CT

Axial CT of foramen rotundum (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Also see:

• Annotated coronal MRI of trigeminal nerve. Radioepaedia case contributed by Frank Gaillard

Abducens nerve

Originates at the pontomedullary junction, crosses prepontine cistern in a dorsal to ventral direction, descends near posterior of clivus, passes Dorello canal and enter cavernous sinus, then passes superior orbital fissure to innervate lateral rectus muscle.

MRI

A Steady-state free precession (SSFP) axial image showing the abducens nerves (circles).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Facial nerve

Consists of two portions:

• Proper VII nerve
• Intermediate nerve

Motor nucleus is located at the lower part of pons and motor fibers exit from the lateral portion of pontomedullary sulcus. It then traverses the cerebelopontine cistern and enters the temporal bone through internal acoustic meatus. After passing through the facial canal of the petrous bone, it spits off into three segments:

• labyrinthe
• tympanic
• mastoid

The nerve exits the skull base at stylomastoid foramen and reaches the parotid gland. The mastoid portion seperates into nerve of stapedius and chorda tympani. Afferent sensory fibers of first 2/3 of tongue via chorda tympani to geniculate ganglion Parasympathetic fibers:

Termonology

• Geniculate ganglion

MRI

A Steady-state free precession (SSFP) axial image showing the facial nerves (dotted arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Vestibulocochlear nerve

Passes thorugh internal acoustic meastus.

See Facial nerve for MRI image.

Tip

Seven Up Coke Down refers to poisiton of facial and vestibulocochlear nerve.

Glossopharyngeal nerve

Mixed sensory, motor and secretory nerve. Emerges from the lateral medulla into lateral cerebellomedullary cistern where it is closely associated with flocculus of cerebellum.

Exits the skull via jugular foramen -> enters carotid space -> turns lateral to carotid artery and the stylopharyngeal muscle and continues lateral to the Palatine tonsil area

Branches:

• Lingual at posterior sublingual space to innervate posterior third of tongue (taste and sensory)
• Pharyngeal (sensation from posterior oropharynx and soft palate)
• Sinus nerve (parasympathetic supply to carotid body and sinus)
• Stylopharyngeus (Stylopharyngeus muscle)
• Tympanic branch (Jacobson nerve), sensory information from. Middle ear

MRI

A Steady-state free precession (SSFP) axial image showing the glossopharyngeal and vagus nerves (circles).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Vagus nerve

Emerges from the lateral medulla, enters the lateral cerebellomedullary cistern and then exits the skull via jugular foramen. The nerve then forms the superior ganglion and inferior vagal ganglion. It descends vertically in the retrostyloid space together with carotid artery to join the upper mediastinum where it gives off the recurrent laryngeal nerve. The nerve enters the abdominal cavity through the esophageal hiatus.

MRI: See Glossopharyngeal nerve.

Accessory nerve

Two components, spinal and cranial:

• Spinal: Forms from C1-C5/6 nerve roots then ascends towards the foramen magnum, exits the skull through jugular foramen, after which the spinal part then innervates sternonucleidomastoid muscles and trapezius.
• Cranial: Originates from medulla oblongata and shortly after exiting the skull, combines with the vagus nerve at inferior ganglion. Considered part of the vagus nerve.

MRI

A Steady-state free precession (SSFP) axial image showing the spinal accessory nerves (circles).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Hypoglossal nerve

Emerges from medulla oblongata as series of rootlets extending from ventrolateral sulcus of the medulla into the lateral cerebellomedullary cistern near to vertebral artery and PICA, then exits the skull through canal. runs medial to the glossopharyngeal, vagus, and accessory nerves, deeply to the digastric muscle, to innervate a large part of extrinsic and intrinsic tongue muscles

MRI

A Steady-state free precession (SSFP) axial image showing the hypoglossal nerves (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Passageways of the Skull

Optic Canal & Superior orbital fissure

Axial CT

Axial CT of superior orbital fissure (arrows) and optic canal (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Foramen Rotundum

Axial CT

Axial CT of foramen rotundum (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Foramen Ovale

Axial CT

Axial CT of foramen ovale (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Facial canal

Axial CT

Axial CT of facial canal (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Internal auditory meatus

Axial CT

Axial CT of internal auditory meatus (arrows).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Jugular foramen

Axial CT

Axial CT of jugular foramen (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Hypoglossal canal

Axial CT

Axial CT of hypoglossal canal (asterisks).

Image from “Imaging of Cranial Nerves: A Pictorial Overview.”, Romano et al.

Sources

1. Romano, Nicola, Margherita Federici, and Antonio Castaldi. “Imaging of Cranial Nerves: A Pictorial Overview.” Insights into Imaging 10, no. 1 (2019): 33. https://doi.org/10.1186/s13244-019-0719-5.
2. Malhotra, Ajay, Long Tu, Vivek B. Kalra, et al. “Neuroimaging of Meckel’s Cave in Normal and Disease Conditions.” Insights into Imaging 9, no. 4 (2018): 499–510. https://doi.org/10.1007/s13244-018-0604-7.