Extraocular Nerves

Oculomotor nerve

• Largest extraocular nerve
• Nucleus in midbrain at level of the superior colliculus
• 2 types of nuclei

• 5 complexes of somatic efferent nuclei containing cells of neurones that directly supply EOMs

• The levator has a centrally located, fused nucleus (the central caudal subnucleus) and is bilaterally innervated. Hence a lesion of the levator nucleus causes a bilateral ptosis

• These motor nuclei are connected to the fourth and sixth nerve nuclei via the medial longitudinal fasciculus

• General visceral efferent nuclei (Edinger-Westphal) nuclei containing preganglionic parasympathetic neurone cell bodies

• The parasympathetic nucleus lies posterior to the motor nucleus

Course

• The nerve emerges from midbrain at the level of the superior colliculus (at the border of the midbrain and the pons) in the interpeduncular fossa

• Passes between the posterior cerebral artery and superior cerebellar artery
• Travels parallel to posterior communicating artery
• Enters dural roof of cavernous sinus
• Travels in upper lateral wall of cavernous sinus

• Enter orbits through superior orbital fissure within the tendinous ring. Divides into superior and inferior branches prior to entering orbit

• Smaller superior branch supplies superior rectus and pierces it to supply levator.
• Larger inferior division gives small branches to medial rectus and inferior rectus

• Thick inferior branch passing alongside inferior rectus reaches inferior oblique and contains preganglionic parasympathetic fibres

• These pass to the ciliary ganglion and synapse

• Second order neurones then travel to iris (sphincter pupillae), choroid, CB muscle in short ciliary nerves

• Branches to EOMs enter the muscles approx 1/3rd along muscle length
• Parasympathetic fibres travel most superficially within the nerve

Trochlear nerve

• Longest intracranial course
• Pure motor nerve: has fewest nerve fibres of all the cranial nerves

• Nucleus lies at the level of the inferior colliculus (midbrain, caudal to third nerve nucleus)

• Decussates before exiting brainstem
• Therefore supplies contralateral superior oblique
• Nerve exits the posterior aspect of the midbrain 
• The only motor nerve to do this
• Wraps around crus of midbrain
• Passes between PCA and superior cerebellar artery
• Initially below CNIII
• Enters lateral wall of cavernous sinus
• On exiting the sinus it is above and lateral to CNIII
• Enters orbit outside of tendinous ring
• Travels beside the origin of the levator towards superior oblique.

• Patients demonstrate a contralateral head tilt to make up for torsion of unopposed inferior oblique

• Trochlea:
• Composed of fibrocartilage (not bone)
• Attached to the frontal bone
• Contains a synovial sheath that surrounds the tendon of superior oblique
• Medial to supraorbital nerve
• Divides the preaponeurotic fat into the medial and lateral fat pads

Abducens nerve

• Longest intradural course
• Pure motor nerve
• Nucleus in floor of fourth ventricle (pons)

• Emerges from midpons beneath facial colliculus at junction between medulla and pons (cerebello-pontine angle)

• Arises from a nucleus close to the facial nerve

• Anterior inferior cerebellar artery crosses at origin and fixes the nerve to the brainstem

• Passes upwards in pontine cistern

• Changes to horizontal orientation at upper border of petrous temporal bone where it penetrates the dura

• Passes within cavernous sinus (therefore more vulnerable as not protected by dura in lateral wall)

• Lies lateral to ICA: atheroma can compress nerve here
• Enters orbit within tendinous ring
• Short course to enter lateral rectus 1/3rd along its medial surface

Note: lateral rectus has two origins: one from tendinous ring, other from greater wing of sphenoid.

Trigeminal nerve

• Largest cranial nerve
• Nucleus extends throughout brainstem but the main sensory nucleus is in the pons

• Mesencephalic nucleus extends up into midbrain: sensory info from muscles of mastication, facial expression and EOMs.

• Spinal nucleus extends down to medulla: tactile, nociceptive, thermal info from face (responsible for mainly pain and temperature)

• Main sensory nucleus responsible for touch and pressure

• Emerges from brainstem in posterior cranial fossa as large sensory root and smaller motor root

• Trigeminal ganglion (aka semilunar ganglion)
• Crescentic shape
• Lies in Meckel’s cave near the apex of the petrous part of the temporal bone
• Within the middle cranial fossa
• Closely related to the motor root
• But is mainly a sensory ganglion (the motor root passes beneath the ganglion)
• 3 branches emerge from flattened anterolateral surface of ganglion
• Ophthalmic lies in lateral wall of cavernous sinus
• See Sensory Nerves chapter for more details
• Maxillary: purely sensory
• Lies in lateral wall of cavernous sinus

• Passes through foramen rotundum into pterygopalatine fossa where it supplies pterygopalatine ganglion

• Becomes infraorbital nerve as enters inferior orbital fissure
• Passes through infraorbital groove, canal and foramen
• Gives zygomatic branch: zygomaticofacial and zygomaticotemporal branches

• Note: within the orbit they pass beneath periorbita/periosteum so not technically intraorbital

• Infraorbital supplies: maxillary sinuses, incisor, premolar and canine teeth, skin of cheek, lower eyelid, cheek, upper lip)