Trochlear nerve

Last revised by Assoc Prof Craig Hacking on 02 Aug 2021

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

The trochlear nucleus is located in the dorsoventral midbrain, ventral to the periaqueductal grey matter. Its fibers 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.

The nerve rounds the cerebral peduncles in the ambient cistern. Eventually, 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.

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.

It enters the orbit outside 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.

Although the nerve is very thin, on high-resolution T2 weighted imaging (e.g. FIESTA/CISS) it can be visualized as a very thin structure. Dedicated higher-resolution sequences can also be performed if greater detail is required 5

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Cases and figures

  • Figure 1: midbrain anatomy
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  • Figure 2: brain stem nuclei
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  • Figure 3: brainstem motor nuclei - Gray's anatomy illustration
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  • Figure 4: cavernous sinus
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  • Figure 5: orbital apex diagram
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  • Figure 6: CN IV orbital portion
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  • Figure 7: cranial nerve origins (illustration)
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  • Figure 8: cranial nerves in the posterior fossa (Gray's illustration)
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  • Figure 9: nerves of the orbit (Gray's illustration)
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