The Occulomotor, Trochlear & Abducent Cranial Nerves
Dr. NimirDr. Safaa
Objectives
• Discuss the central connections of the Occulomotor(III)
nerve.• Discuss the central connections of the
trochlear(IV) nerve.• Discuss the central connections of the
abducent(III,IV,VI) nerve.
• Oculomotor Nerve (Cranial Nerve III)
• Oculomotor nerve is entirely motor in function.
• Has two motor nuclei: • Main motor nucleus.• Accessory
parasympathetic nucleus.• Main nucleus is anterior
to cerebral aqueduct at level of superior colliculus.
• It supplies all extrinsic muscles of eye except superior oblique and lateral rectus.
• Main nucleus receives:• Corticonuclear fibers
from both cerebral hemispheres.
• Tectobulbar fibers from superior colliculus (information from visual cortex).
• It is connected to nuclei of fourth, sixth, and eighth cranial nerves by medial longitudinal fasciculus.
• Accessory parasympathetic nucleus (Edinger-Westphal nucleus) is posterior to main nucleus.
• Preganglionic fibers accompany other oculomotor fibers to orbit & synapse in ciliary ganglion.
• Postganglionic fibers pass through short ciliary nerves to constrictor pupillae of iris and ciliary muscles.
• Accessory nucleus receives corticonuclear fibers for accommodation reflex and from pretectal nucleus for the direct and consensual light reflexes.
• Course of Oculomotor Nerve
• Oculomotor fibers passes through red nuleus and emerge between cerebral peduncles & pass forward between posterior cerebral and superior cerebellar arteries.
• It then continues into the middle cranial fossa in lateral wall of cavernous sinus. Here, it divides into a superior and an inferior branches, which enter the orbital cavity through superior orbital fissure.
• Oculomotor nerve supplies levator palpebrae superioris, superior rectus, medial rectus, inferior rectus, and inferior oblique.
• It also supplies, through parasympathetic fibers, constrictor pupillae of iris and ciliary muscles.
• It is responsible for lifting upper eyelid, turning eye upward, downward, and medially.
• Constricting pupil and accommodating the eye.
Trochlear Nerve • Trochlear nerve is entirely
motor in function.• Trochlear Nerve Nucleus:• The trochlear nucleus is
situated anterior to cerebral aqueduct.
• It lies inferior to oculomotor nucleus at level of inferior colliculus. The nerve fibers, after leaving nucleus, pass posteriorly to reach posterior surface of midbrain.
• The trochlear nucleus receives
• Corticonuclear fibers from both cerebral hemispheres.
• It receives:• Tectobulbar fibers, which
connect it to visual cortex through superior colliculus.
• Fibers from medial longitudinal fasciculus, by which it is connected to nuclei of third, sixth, and eighth cranial nerves.
• Course of the Trochlear Nerve
• Trochlear , most slender cranial nerves and the only one to leave posterior surface of brainstem.
• Emerges from midbrain and immediately decussates with that of opposite side.
• Trochlear nerve passes forward through middle cranial fossa in lateral wall of cavernous sinus and enters orbit through superior orbital fissure.
• Trochlear nerve supplies superior oblique muscle of eyeball.
• Trochlear nerve assists in turning eye downward and laterally.
• Abducent Nerve (Cranial Nerve VI):
• It is entirely motor in function.• Abducent has small motor
nucleus situated beneath floor of upper part of fourth ventricle, close to midline and beneath facial colliculus.
• The nucleus receives:• Afferent corticonuclear fibers
from both cerebral hemispheres. • Tectobulbar tract from superior
colliculus, by which it is connected to visual cortex.
• Fibers from medial longitudinal fasciculus, by which it is connected to the nuclei of third, fourth, and eighth cranial nerves.
• Course of the Abducent Nerve
• Abducent fibers pass anteriorly through pons and emerge in groove between lower border of pons and medulla oblongata.
• It passes forward through cavernous sinus, lying below and lateral to internal carotid artery.
• The nerve then enters orbit through superior orbital fissure.
• Abducent supplies lateral rectus muscle and, therefore, is responsible for turning eye laterally.