Cranial Nerve IV (Trochlear Nerve), CNIV

Synonyms

CN IV; Cranial nerve IV; Trochlear nerve

Definition

CN IV begins in the trochlear nerve nucleus located in the caudal mesencephalon, both ventral to the cerebral aqueduct and caudal to the oculomotor nucleus complex. CN IV fascicles derive from both nuclei, travel dorsally around the cerebral aqueduct, and decussate in the anterior medullary velum just caudal to the inferior colliculi. Thus, the trochlear nerve emerges contralaterally between the inferior colliculi and superior cerebellar peduncles on the dorsal brainstem. The trochlear nerve is unique in that it is the only cranial nerve exiting from the dorsal brainstem as well as providing contralateral innervation to its target. Further, as will be detailed below, the fourth nerve has the longest intracranial pathway and thus is more vulnerable to trauma.

Once in the subarachnoid space, the CN IV travels ventrally around the cerebral peduncles to project toward the cavernous sinus and orbit. CN IV is positioned lateral to...