Trochlear Nerve

Cranial nerve IV is one of the set of three oculomotor nerves (III, IV, and VI) and innervates one of the six extraocular muscles of the eye, the superior oblique muscle. The trochlear nerve is a purely motor nerve with only a GSE component. The nerve arises from motor neurons in the trochlear nucleus, which lies in a medial position in the dorsal part of the caudal half of the midbrain tegmentum. The trochlear nerve is unique among cranial nerves in that it decussates to the contralateral side (through the superior medullary velum that forms the roof of the fourth ventricle), and its point of exit is through the dorsal surface of the brain. The trochlear nerve thus innervates the superior oblique muscle of the contralateral eye.

The superior oblique muscle arises from the dor-somedial surface of the orbit and ends in a tendon that bends through a connective tissue pulley and then passes laterally to insert on the dorsal part of the eye bulb lateral to its center. Contraction of the superior oblique muscle causes intorsion (rotation of the eyeball around a horizontal, anteroposterior axis through the pupil such that the top moves medially), depression, and abduction. Deviation of the eye due to isolated damage to the trochlear nucleus or nerve is not readily apparent due to the actions of the other extraocular muscles, which mask most of the deficit. Supranuclear innervation of the trochlear nucleus is via the gaze centers in the brain stem, and the gaze centers, the three oculomotor nuclei, and vestibular nuclei are interconnected via the medial longitudinal fasciculus.

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