trochlear nerve trochlear nerve the trochlear nerve (/ˈtrɒklɪər/), (lit. pulley-like nerve) also known as the fourth cranial nerve, cranial nerve iv, or cn iv, is a cranial nerve that innervates a single muscle - the superior oblique muscle of the eye (which operates through the pulley-like trochlea). unlike most other cranial nerves, the trochlear nerve is exclusively a motor nerve (somatic efferent nerve). trochlear nervethe trochlear nerve entering the orbit, seen from above, supplies the superior oblique musclethe trochlear nerve (cn iv) seen with other cranial nerves. it is the only cranial nerve to emerge from behind the brainstem, and curves around it to reach the frontdetailsinnervatessuperior oblique muscleidentifierslatinnervus trochlearismeshd014321neuronames466ta98a14.2.01.011ta26191fma50865anatomical terms of neuroanatomy cranial nerves cn 0 – terminal cn i – olfactory cn ii – optic cn iii – oculomotor cn iv – trochlear cn v – trigeminal cn vi – abducens cn vii – facial cn viii – vestibulocochlear cn ix – glossopharyngeal cn x – vagus cn xi – accessory cn xii – hypoglossal overviewtable the trochlear nerve is unique among the cranial nerves in several respects: it is the smallest nerve in terms of the number of axons it contains. it has the greatest intracranial length. it is the only cranial nerve that exits from the dorsal (rear) aspect of the brainstem. it innervates a muscle, the superior oblique muscle, on the opposite side (contralateral) from its nucleus. the trochlear nerve decussates within the brainstem before emerging on the contralateral side of the brainstem (at the level of the inferior colliculus). an injury to the trochlear nucleus in the brainstem will result in an contralateral superior oblique muscle palsy, whereas an injury to the trochlear nerve (after it has emerged from the brainstem) results in an ipsilateral superior oblique muscle palsy. the superior oblique muscle which the trochlear nerve innervates ends in a tendon that passes through a fibrous loop, the trochlea, located anteriorly on the medial aspect of the orbit. trochlea means “pulley” in latin; the fourth nerve is thus also named after this structure. the words trochlea and trochlear (/ˈtrɒkliə/, /ˈtrɒkliər/) come from ancient greek τροχιλέα trokhiléa, “pulley; block-and-tackle equipment”. structure the trochlear nerve provides motor innervation to the superior oblique muscle of the eye, a skeletal muscle; the trochlear nerve thus carries axons of general somatic efferent type. course