Point 19










Point: 19. Trochlear nucleus - Cranial Nerve IV

This tiny nucleus contains cells that directly innervate the CONTRA. superior oblique muscle.

The trochlear nucleus sits on top of the M.L.F. in the caudal midbrain (level #9) and at the junction of the pons and midbrain (level #8). Neurons in this nucleus send axons dorsally around the cerebral aqueduct to exit from the dorsal surface of the midbrain. These fibers decussate just caudal to the inferior colliculus, course ventrally adjacent to the cerebral peduncle, enter the superior orbital fissure and innervate the superior oblique muscle.

Lesion of the trochlear nucleus or nerve prior the decussation results atrophy of the CONTRA. superior oblique muscle. Patient will TILT and slightly flex their head TOWARDS or IPSI. to the lesion in order to ameliorate the double vision. A lesion of the trochlear nerve AFTER the decussation results in atrophy of the IPSI. superior oblique (ipsi. to the lesion) and the patient will TILT and slightly flex their head AWAY (CONTRA.) from the lesion (compare with lesion of C.N. VI).

Other Note:
Lesions result in difficulty going down stairs. Trochlear nerve is only C.N. to be completely CROSSED and to exit DORSALLY from the brainstem.