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