Trochlear nerve (CN IV)

The trochlear nerve (Latin: nervus trochlearis), the fourth cranial nerve (CN IV), is a motor nerve that innervates exclusively one extraocular muscle of the eye - the superior oblique muscle. The trochlear nerve is the thinnest of all cranial nerves and has the longest intracranial pathway.

Trochlear nerve origin and course

The trochlear nerve carries general somatic efferent fibers that arise from neurons located in the trochlear nucleus. The trochlear nucleus is found in the midbrain at the level of inferior colliculus. Before exiting the brainstem, the fibers arising from the trochlear nucleus intercross on the superior medullary velum.

The trochlear nerve emerges on the dorsal surface of the brainstem lateral to the frenulum of the superior medullary velum, making it the only cranial nerve which emerges from the posterior aspect of the brainstem. Further, each trochlear nerve ventrally “wraps around” the cerebral peduncles of the midbrain and continues in ventral direction inside the lateral wall of the cavernous sinus.

Within the wall of the cavernous sinus, the trochlear nerve is located between the oculomotor nerve (CN III) and the trigeminal nerve (CN V). Upon leaving the sinus, the trochlear nerve crosses the oculomotor nerve and enters the orbit through the superior orbital fissure. In the orbit, the trochlear nerve reaches the superior oblique muscle and innervates it.