Trochlear nerve

Dr Craig Hacking and A.Prof Frank Gaillard et al.

The trochlear nerve is the fourth cranial nerve and is the motor nerve of the superior oblique muscle of the eye.  It can be divided into four parts:

  1. nucleus and an intraparenchymal portion
  2. cisternal portion
  3. cavernous sinus portion
  4. orbital portion

Gross anatomy

Nucleus and intraparenchymal portion

The trochlear nucleus is located in the dorsal midbrain, ventral to the periaqueductal grey matter. Its fibres course dorsally and decussate dorsal to the periaqueductal grey matter before exiting the brainstem immediately below the inferior colliculus. It is the only cranial nerve to exit the brainstem posteriorly.

Cisternal portion

The nerve rounds the cerebral peduncles in the ambient cistern. Eventually, along with the oculomotor nerve (CN III), it runs between the superior cerebellar artery and posterior cerebral artery and pierces the dura between the free and attached edge of tentorium cerebelli.

Cavernous sinus portion

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). It is the "Tarts" in this infamous mnemonic.

Orbital portion

It enters the orbit outside and superolateral to the tendinous ring, between the superior ophthalmic vein and the superolateral quadrant of the ring. Once in the orbit, it arches up and medially above superior rectus and levator palpebrae superioris to innervate the superior oblique.

Related pathology

Anatomy: Brain
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Article information

rID: 2213
Section: Anatomy
Synonyms or Alternate Spellings:
  • Trochlear nerve (CN IV)
  • Trochlear nerve (IV)

Cases and figures

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    Figure 1: midbrain anatomy
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    Figure 2: brain stem nuclei
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    Figure 3: cavernous sinus
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    Figure 4: orbital apex diagram
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    Figure 5: CN IV orbital portion
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