The oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) cranial nerves are responsible for transmitting motor functions to all extraocular muscles of the eye.
In addition to the extraocular muscles, the oculomotor nerve (CN III) innervates two intrinsic eye muscles.
Sympathetic fibers from the ophthalmic division of the trigeminal nerve (CN V) are responsible for supplying:
Dilator pupillae muscle.
Smooth muscle component of the levator palpebrae superioris (LPS).
Anatomy of Cranial Nerves III, IV, and VI
General Anatomy
Cranial Nerves:
CN III: Oculomotor nerve.
CN IV: Trochlear nerve.
CN VI: Abducens nerve.
Together with CN XII, these nerves belong to the general somatic efferent (GSE) group of nuclei.
Individual Nerves
Oculomotor Nerve (CN III)
Nucleus Location: Situated in the periaqueductal grey matter at the level of the superior colliculus.
Components:
GSE component is located ventrolaterally.
GVE component (Edinger-Westphal Nucleus - EWN) is dorsomedially.
Intraneural Course:
The axons proceed ventrally, emerging between the cerebral peduncles.
Muscle Innervation:
Innervates three of the four rectus muscles, the inferior oblique muscle, and the levator palpebrae superioris.
Sympathetic fibers are “hitch-hiked” to the levator palpebrae superioris.
Nucleus Features:
Edinger-Westphal Nucleus supplies parasympathetic fibers to the eye, facilitating pupil constriction and lens accommodation.
Damage to CN III results in a dilated pupil.
Intracranial Course:
Emerges between the cerebral peduncles into the interpeduncular cistern.
Proceeds forward, laterally to posterior communicating artery (circle of Willis), and enters the cavernous sinus above CN IV.
Divides into superior and inferior branches at the anterior pole of the sinus.
Extracranial Course:
Superior Branch:
Supplies superior rectus and levator palpebrae superioris.
Inferior Branch:
Divides into three branches for medial rectus, inferior rectus, and inferior oblique; longer branch goes to inferior oblique containing EWN fibers.
Innervating fibers synapse in the ciliary ganglion; postsynaptic fibers reach sphincter pupillae and ciliary muscles via short ciliary nerves.
Trochlear Nerve (CN IV)
Nucleus Location: In the periaqueductal grey matter at the level of the inferior colliculus.
Intraneural Course:
Unique in that it exits the brainstem dorsally and crosses on its way out, resembling a half-tied shoelace in the roof of the fourth ventricle.
Intracranial Course:
Winds around cerebral peduncles, emerges between PCA and SCA, and courses forwards in the lateral wall of the cavernous sinus below CN III.
Extracranial Course:
Exits through SOF, passing medially above the levator palpebrae.
Enters the orbital surface of the superior oblique muscles.
Abducens Nerve (CN VI)
Nucleus Location: Located in the lower pons deep to the floor of the fourth ventricle.
Intraneural Course:
Fibers course ventrally to emerge at the pontomedullary junction, one of four cranial nerves that do so, positioned rostrally to the pyramids.
Intracranial Course:
Passes forwards in subarachnoid space between pons and clivus, leaves the posterior cranial fossa to enter the middle cranial fossa, lying in the cavernous sinus lateral to the internal carotid artery (ICA).
Extracranial Course:
Exits through SOF within the cone of extraocular muscles and enters the ocular surface of the lateral rectus around 1/3 of its length.
Visual Pathways Overview
The eyeball is protected by eyelids and the lacrimal apparatus, maintaining corneal moisture through the tear film on the conjunctiva and cornea.
The Visual Pathway
General Organization
The visual pathway is organized topographically to the occipital lobe.
Ganglion Cell Functions:
Nasal ganglion cells send axons that cross the midline at the optic chiasm, while temporal ganglion cell axons remain ipsilateral.
Synaptic Pathways:
Ganglion cell axons terminate largely in the Lateral Geniculate Body (LGN), organized into six layers.
Optic Radiations from LGN pass to the primary visual cortex in the calcarine cortex for conscious visual perception.
Association visual cortex connects for processing form, color, and motion.
Connections to the temporal lobe aid in high-resolution object recognition (e.g. faces).
Connections to the parietal cortex analyze motion and positional relationships of visual scene objects.
Pathway Details
Begins with the stimulation of photoreceptors (first-order neuron) in the retina, generating vision.
Axons leave the eye via the optic nerve (second-order neuron).
Partial crossing occurs at the Optic Chiasm allowing integration of visual fields:
The retina is split into two halves (temporal and nasal).
Nasal retina crosses while temporal retina remains ipsilateral.
The optic tract wraps around the midbrain to reach the lateral geniculate body, where synapsing occurs.
Post-synaptic fibers fan out as optic radiations to the visual cortex.
Retinal Layers
Structures in the eyeball contributing to visual function include:
Lens.
Cornea.
Iris.
Ciliary body.
Nerve fiber layer.
Ganglion cell layer.
Inner plexiform layer (functional layer).
Outer plexiform layer.
Choroid.
Photoreceptor layer.
Sclera (outermost layer).
Fovea (central darker area in the retina, densely populated with cones).
Visual Fields
The images perceived by the retina form two halves: temporal and nasal halves, with vision being inverted through the lens.
For instance, the temporal retina of the right eye perceives the left visual field, while the nasal retina perceives the right visual field.
Both eyes receive data from both halves of the visual field, creating a unified visual experience.
Clinical Notes on Ametropias
Ametropias involve improper focusing of light rays on the retina, causing vision disturbances:
Myopia (Nearsightedness): 80% of ametropias where focusing occurs in front of the retina.
Hyperopia (Farsightedness): Age-related focusing occurring behind the retina.
Astigmatism: Focus at multiple points due to irregular corneal shape, affecting 25% to 40% of people.
Presbyopia: Progressive loss of lens elasticity, requiring corrective lenses for near vision.
Visual Field Lesions
Hemianopia: Blindness affecting one half of the visual field. Specific lesions include:
Lesion 1: Blindness in the left eye, appearing as a blacked-out area.
Lesion 2: Left nasal hemianopia causes loss of the temporal visual field.
Lesion 3: Bitemporal hemianopia, where nasal retinas are affected, leading to loss of peripheral vision. A common cause is a pituitary tumor.
Lesion 4: Right homonymous hemianopia, with loss of the right hemifield in both eyes.
Lesion 5: Right homonymous hemianopia with macular sparing, where central vision is still preserved.