Trochlear nerve root (CN IV)

Radix nervi trochlearis

NervousOrgansHead

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What it is
The only cranial nerve to leave the back of the brainstem, just below the inferior colliculus; its fibres cross before exiting. Shown as a short stub on both sides; the nerve continues in the peripheral nervous system.
Function
Motor to the superior oblique muscle, which turns the eye down and in.
Size
Stub about 1 cm; root 1.2 mm thick
Clinical note
The thinnest cranial nerve (~1 mm); its long course makes it vulnerable in head injury.

Nearby nervous structures Cerebral aqueduct, Facial nerve root (CN VII), Fourth ventricle, Midbrain, Oculomotor nerve root (CN III), Pons, Tentorium cerebelli, Trigeminal nerve root (CN V), Vestibulocochlear nerve (CN VIII), Vestibulocochlear nerve root (CN VIII)