Cranial nerves II,III, IV,VI and Visual Pathway

The optic nerve consists of more than 1 million axons that originate in the ganglion cell layer of the retina and extend towards the occipital cortexType: Special sensory.Function: Vision.

Optic Nerve

Optic Nerve

• Axons of retinal ganglion cells converge at the optic disc and pass into optic nerve.

• Then the nerve passes posteromedially in the orbit, exits through the optic canal to enter the middle cranial fossa to joins the optic chiasma.

Optic NerveLesion results in: visual field defects and loss of visual acuity, a defect of vision is called anopsia.

• The chiasma measures approximately 12 mm wide, 8 mm long in the anteroposterior direction, and 4 mm thick.

• The exact location of the

chiasma with respect to the sella is variable. Most of the time it is directly superior

Optic Chiasma

Optic Chiasma• Fibers from the nasal (medial)

half of retina decussate within the chiasma and join uncrossed fibers from the temporal (lateral) half of the retina to form the optic tract.

• The decussation of nerve fibers in the chiasma results in the right optic tract conveying impulses from the LEFT visual field and vice versa.

• The partial crossing of optic nerve fibers in the optic chiasma is a requirement for binocular vision.

Damage to the optic tract or more central parts of the pathway tend to cause comparable visual deficits in both eyes

Optic Tract

• Each optic tract contains ipsilateral temporal and contralateral nasal fibers from the optic nerves .

Optic Tracts• Fibers in the optic

tracts: Mainly terminate

in the lateral geniculate bodies of the thalamus

A few fibers terminate in pretectal area and superior colliculus. These fibers are related to light reflexes

Lateral geniculate body

Optic radiations• The optic radiations

connect the lateral geniculate body with the cortex of the occipital lobe

• The fibers of the optic radiations leave the lateral geniculate body and run around the temporal horn of the lateral ventricle, approaching the anterior tip of the temporal lobe (the so-called loop of Meyer)

Optic Radiation• Optic radiation or

thalamocortical or geniculocalcarine fibers originating in the lateral geniculate nucleus.

• Passes through the retrolenticular part of the internal capsule to the visual cortex above and below the calcarine sulcus.

Visual Cortex• The primary visual cortex (area 17 of Brodmann's)

occupies the upper and lower lips of the calcarine sulcus on the medial surface of the cerebral hemisphere.

• The visual cortex, the thinnest area of the human cerebral cortex

The visual association cortex is extensive, including the whole of the occipital lobe, the adjacent posterior part of the parietal lobe. This cortex is involved in recognition of objects and perception of color, depth, motion, and other aspects of vision.

Visual Cortex

Visual Pathway

1. Cones2. Bipolar neurons3. Ganglion cell’s axon

forms the optic nerve

4. Optic nerve to the Optic Chiasm

5. Optic tract6. Lateral geniculate

nuclei of the thalamus

7. Optic Radiations8. Primary visual areas

of the occipital lobes

Visual Pathway Photoreceptors: Rods & Cones of retina 3 neuron pathway

1st order neurons: Bipolar cells of retina.

2nd order neurons: Ganglion cells of

retina. Their axons form the optic nerve

3rd order neurons: Neurons in the lateral

geniculate body. Their axons terminate in primary visual cortex.

Visual Field Deficits

Cut at level :1. A lesion of the right optic nerve causes a total loss of vision (blindness) in the right eyeCut at level :2. A lesion of the optic chiasma causes a loss of vision in the temporal half of both visual fields: bitemporal hemianopsia.Cut at level: 3 & 4. A lesion of the right optic tract & right optic radiation just after the LGN causes a loss of vision in the left hemifield: contralateral homonymous hemianopsia.

A lesion of both visual cortices causes a complete blindness.

Occulomotor nerveMotor for most of extraocular muscles.Also carries preganglionic parasympathetic fibers for pupillary constrictor and ciliary muscle. Has two nuclei:1- Main occulomotor nucleus; Lies in the mid brain, at the level of superior colliculus2- Accessory nucleus (Edinger-Westphal nucleus); Lies dorsal to the main motor nucleus, Its cells are Preganglionic Parasympathetic Neurons. It receives; Corticonuclear fibers for the accommodation reflex, and from the pretectal nucleus for the direct and consensual pupillary light reflexes.

Axons from the oculomotor nucleus curve ventrally through the tegmentum and the red nucleus.

The nerve emerges on the anterior surface of the midbrain in the interpeduncular fossa.

Then it passes forward between posterior cerebral and superior cerebellar arteries.

In the middle cranial fossa it runs in the lateral wall of the cavernous sinus, then it divides into superior and inferior divisions which enter the orbit through the superior orbital fissure.

Axons from the Edinger-Westphal nucleus accompany the oculomotor nerve fibers to the orbit, where they terminate in the ciliary ganglion.

Postganglionic Fibers pass through the short ciliary nerves to the eyeball, where they supply:

Constrictor pupillae muscle of the iris and ciliary muscle.

• CN III usually divides into superior and inferior divisions after passing through the annulus of Zinn in the orbit

Occulomotor nerve supplies: Motor to:

• Superior division: Superior rectus . Levator pelpabrae superioris.

• Inferior division: Inferior rectus. Medial rectus. Inferior oblique.

Occulomotor nerve supplies:

Parasympathetic fibers to :

1- Constrictor pupillae and

2- Ciliary muscles.It is responsible for; Elevation of upper eyelid. Turning the eye upward, downwards and medially,Constricting the pupil. Accommodating reflex of the eyes.

Occulomotor nerve lesion

• Lesion results in:– Lateral squint.(MR)– Ptosis.(LPS)– Diplopia.– Pupillary dilatation.(SP)– Loss of accommodation.

(CM)– Impaired downward

(IR)& extorsion (IO)movement of the eye ball on the damaged side.

Trochlear Nerve Type: motor Small motor nucleus

located in the periaqueductal grey matter at the level of inferior colliculus.

Fibers curve backwards and decussate

The nerve emerges immediately caudal to the inferior colliculus, on the dorsal surface of brain stem.

It passes forward through middle cranial fossa in the lateral wall of the cavernous sinus.

The nerve then enters the orbit through the superior orbital fissure.

It supplies; Superior oblique muscle, (only one muscle).Its function; Rotates the eye ball downwards and laterally.(Extorsion)

Trochlear Nerve lesion

Lesion results in diplopia &

Inability to rotate the eye infero-laterally.

So, the eye deviates; upward and slightly inward.

This person has difficulty in walking downstairs.

Abducent NerveOnly one motor

nucleus.Lies in caudal pons in

the floor of the 4th ventricle.

It forms the facial colliculus with the fibers of facial nerve looping around the nucleus.

It emerges from the ventral aspect, at the junction of the pons and the pyramid of the medulla.

Abducent Nerve It passes through

cavernous sinus, lying below and lateral to the internal carotid artery

Then it enters the orbit through the superior orbital fissure.

It supplies; the lateral rectus muscle which rotates the eye ball laterally ; (abduction).

Abducent nerve lesion

Lesion results in: Inability to direct the

affected eye laterally. (medial squint).

A nuclear lesion may also involve the nearby facial nucleus or axons of the facial nerve, causing paralysis of all the ipsilateral facial muscles.

Arterial system

Internal carotid artery

Ophthalmic artery

Central retinal AShort post ciliary ALong post ciliary A

Anterior ciliary A

Lacrimal ASuperior muscular A

Inferior muscular APosterior ethmoidal AAnterior ethmoidal A

Supraorbital ASupratrochlear A

Venous system

Central retinal VSuperior vortex VSuperior episcleral plexus

Inferior vortex VInferior episcleral plexus

Pterygoid plexus

Superior ophthalmic V

Cavernous sinus

Jugular v