The trochlear nerve (fourth cranial nerve, CN IV, latin: nervus trochlearis) is a motor nerve that carries general somatic efferent fibers to innervate exclusively the superior oblique muscle of the eye.
The motor neurons providing general somatic efferent fibers for the trochlear nerve originate in the trochlear nucleus, which is located in the midbrain. 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 (note, that the trochlear nerve is the only cranial nerve which emerges from the dorsal surface of the brainstem!). Then each trochlear nerve ventrally “wraps around” the cerebral peduncles 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 ophthalmic nerve (CN V1). 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.
The trochlear nerve is the thinnest of all cranial nerves and has the longest intracranial pathway.