CRANIAL NERVS ( III, IV, and VI ) Made by : DANI MAMO

Eye Movements (CN III, IV, and VI) Three cranial nerves innervate the muscles of the eyes: the oculomotor nerve (CN III) the trochlear nerve (CN IV) the abducens nerve (CN VI)

Oculomotor nerve (CN III) The nuclear area of the oculomotor nerve lies in the periaqueductal gray matter of the midbrain, ventral to the aqueduct, at the level of the superior colliculi

It has two major components: (1) a medially situated parasympathetic nucleus, the so-called Edinger Westphal nucleus (accessory autonomic nucleus) which innervates the intraocular muscles (the sphincter pupillae muscle and the ciliary muscle) (2) a larger and more laterally situated nuclear complex for four of the six extraocular muscles (the superior, inferior, and medial rectus muscles and the inferior oblique muscle). There is also a small nuclear area for the levator palpebrae muscle

The motor radicular fibers that emerge from these nuclear areas travel ventrally together with the parasympathetic fibers; some of them cross the midline others do not (all of the fibers for the superior rectus muscle cross the midline). The combined motor and parasympathetic fibers traverse the red nucleus and finally exit the brainstem in the interpeduncular fossa as the oculomotor nerve.

The oculomotor nerve first runs posteriorly between the superior cerebellar and posterior cerebral arteries, in close apposition to the tentorial edge, then penetrates the dura mater, traverses the cavernous sinus, and enters the orbit through the superior orbital fissure. The parasympathetic portion of the nerve branches off at this point and travels to the ciliary ganglion, where the preganglionic fibers terminate and the ganglion cells give off short postganglionic fibers to innervate the intraocular muscles.

The somatic motor fibers of the oculomotor nerve divide into two branches: 1- a superior branch supplying the levator palpebrae and superior rectus muscles 2- an inferior branch supplying the medial and inferior rectus and the inferior oblique muscle

Trochlear nerve (CN IV) The nucleus of the fourth cranial nerve lies ventral to the periaqueductal gray matter immediately below the oculomotor nuclear complex at the level of the inferior colliculi. Its radicular fibers run around the central gray matter and cross to the opposite side within the superior medullary velum.

The trochlear nerve then exits the dorsal surface of the brainstem, emerging from the midbrain tectum into the quadrigeminal cistern. Its further course takes it laterally around the cerebral peduncle toward the ventral surface of the brainstem, so that it reaches the orbit through the superior orbital fissure together with the oculomotor nerve. It then passes to the superior oblique muscle, which it innervates

function The trochlear nerve carries axons of type GSE, general somatic efferent, which innervate skeletal muscle of the superior oblique muscle. The superior oblique muscle 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 named after this structure.

Abducens Nerve (CN VI) The abducens nerve or abducent nerve (the sixth cranial nerve) is a somatic efferent nerve that, in humans, controls the movement of a single muscle, the lateral rectus muscle of the eye.

Structure The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).

The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus. At the tip of the petrous temporal bone it makes a sharp turn forward to enter the cavernous sinus.It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

Nucleus The abducens nucleus is located in the pons, on the floor of the fourth ventricle, at the level of the facial colliculus. Axons from the facial nerve loop around the abducens nucleus, creating a slight bulge (the facial colliculus) that is visible on the dorsal surface of the floor of the fourth ventricle. The abducens nucleus is close to the midline, like the other motor nuclei that control eye movements (the oculomotor and trochlear nuclei).

Motor axons leaving the abducens nucleus run ventrally through the pons. They pass lateral to the corticospinal tract (which runs longitudinally through the pons at this level) before exiting the brainstem at the pontomedullary junction.

Development The human abducens nerve is derived from the basal plate of the embryonic pons. Function The abducens nerve carries axons of type GSE, general somatic efferent, which innervate skeletal muscle of the lateral rectus.