Overview
The trochlear nerve is the fourth and smallest cranial nerve (CN IV) and carries somatic efferent fibres which innervate the superior oblique muscles (controls the abduction and intorsion of the eye).
Origin
The trochlear nucleus is located at the level of the inferior colliculus in the dorsoventral midbrain, ventral to the periaqueductal grey matter.
Segments & Course
It has the longest intracranial course as it is the only nerve to exit dorsally from the brainstem.
Nucleus and Intraparenchymal segment
- From the trochlear nucleus, its fibres course dorsally between the inferior colliculus and periaqueductal grey matter
- It then decussats dorsal to the periaqueductal grey matter, before exiting the brainstem immediately below the inferior colliculus.
Cisternal segment
- The nerve courses around the cerebral peduncles in the ambient cistern, then enters the infratentorial part of the quadrigeminal cistern.
- Along with the oculomotor nerve (CN III), it courses anteriorly between the superior cerebellar artery below and posterior cerebral artery above before piercing the dura between the free and attached edge of tentorium cerebelli.
Cavernous Sinus segment
- Within the cavernous sinus, the trochlear nerve is located initially below the oculomotor nerve in the lateral wall of the sinus, although by the time it reaches the superior orbital fissure, it lies above it (outside the tendinous ring).
Orbital portion
- Enters the orbit outside the tendinous ring, between the superior ophthalmic vein and the superolateral quadrant of the ring.
- Within the orbit, it arches up and medially above superior rectus and levator palpebrae superioris muscles to innervate the superior oblique muscle.
Blood supply
- The midbrain is supplied by the posterior cerebral artery, superior cerebellar artery and basilar artery.
Relations
- Inferior to the oculomotor nerve within the middle cranial fossa & cavernous sinus
- Superior to the trigeminal and abducens within the middle cranial fossa
- Superior to the oculomotor nerve within the superior orbital fissure
- Lateral to the cavernous segment of internal carotid artery within the cavernous sinus
Development
The trochlear and abducens nerves are derivatives of the somatic efferent column of the basal plates