Optic Nerve (MT3)

What creates the optic nerve?

Nerve fiber layer

Axons of ganglion cells

Where is the nerve fiber layer the thickest?

Around the optic disc

How does the ON appear?

Light yellow/pink color

Not a lot of pigment, collagen is white and contributes to light look

Physiological cup

Depression in the center of the disc, usually lighter than the disc

Depth is stable throughout life, C/D ratio remains stable in health

ISNT rule

Thickness of neural rim in decreasing order

Inferior, Superior, Nasal, Temporal

Neural rim

Space between the disc and the cup

Neural rim in glaucoma

Thins neural rim → Retina is no longer in proper plane with the ON

Lateral geniculate nucleus

Where most nerve fibers terminate

Portions of the ON

Intraocular - Smallest portion

Intraorbital - Inside of orbit

Intracanalicular - Pass through foramen

Intracranial - Past foramen

Intraocular portion

Pre-laminar (anterior lamina cribrosa), laminar (passing through lamina cribrosa), post-laminar (posterior lamina cribrosa)

Lamina cribrosa

Fills the posterior scleral foramen

Collagen extensions of sclera to support axons, cellularized with astrocytes to bundle + package axons as they pass through

Pre-laminar portion

ON head

Astrocytes surround axons

Post-laminar region

First mm of ON behind the eye

Axon bundles surrounded by CT, fewer astrocytes and more oligodendricytes (myelinate axons)

Axons before post-lamina are…

Unmyelinated

Astrocytes

Glue, contribute to the blood brain barrier with tight junctions

Separate other ocular structures and their nutrients from the ON

Where does the sclera divide the ON from the choroid?

Temporally

Why does the sclera divide the ON from the choroid?

ON comes in at an angle because the optic foramen is medial

Sclera fills in space created by angle

Intermediary tissue of Kuhnt

Astrocytes dividing neural retina from ON

Border tissue of Jacoby

Astrocytes dividing choroid from ON

Found nasally only

Glial covering

Continues the length of ON

Marginal tissue of Elschnig

Sclera dividing choroid from ON

Found temporally only, between the astrocytes surrounding the ON and stroma of choroid

Internal limiting membrane of Elschnig

Innermost covering of disc, directly continuous with ILM of retina

Made by astrocytes and vitreous, contains central meniscus of Kuhnt

Central meniscus of Kuhnt

Highest density of astrocytes

Intraorbital portion

Extends to the apex of the orbit

Surrounded by adipose tissue for protection

Intracanalicular portion

ON as it travels through the optic foramen

Intracranial portion

From the optic foramen to the chiasm

Maculopapillary bundle

Nerve fibers running from macula to the ON

Comprises the temporal third of the ON

Superior/inferior arcuate fibers

Nerve fibers arching around fovea to reduce light scatter

Superior/inferior radiating bundle

Nerve fibers running from nasal retina to ON

Horizontal raphe

Horizontal line that dives superior from inferior nerve fibers

Why are nerve fibers specifically arranged?

Brain will recognize where image is coming from, topographic map

Approaching the optic chiasm, the fiber distribution….

Progressively changes

Internalization of macular fibers, split into respective superior/inferior and nasal/temporal

Ipsilateral

Stays on the same side

Contralateral

Crosses over

Where is the optic chiasm

Right above the pituitary gland

Superior temporal fibers

Enter chiasm superior + temporal aspect → Pass through nasally → Exit ipsilateral optic tract nasally

Optic chiasm general principle

Temporal fibers stay ipsilateral

Nasal fibers cross and end in contralateral tract

Superior nasal fibers

Enter chiasm superior + nasally → Pass through temporally → Bend slightly into ipsilateral tract → Exit contralateral optic tract nasally

Posterior knees of Wilbrand

Superior nasal fibers that loop into the ipsilateral optic tract before exiting contralateral optic tract

Inferior temporal fibers

Enter chiasm inferior + temporal aspect → Pass through the chiasm temporally → Exit ipsilateral optic tract temporally

Inferior nasal fibers

Enter chiasm inferior + nasal aspect → Cross into anterior part of contralateral optic tract and loop down → Exit contralateral optic tract temporally

Anterior knee of Wilbrand

Inferior nasal fibers that loop into the contralateral optic tract before exiting contralateral optic tract

Do the “knees” actually exist?

No, nerves take more of an H shape

Nasal fibers still cross, temporal fibers stay ipsilateral

Optic tract

Contains ipsilateral temporal fibers and contralateral nasal fibers

Most fibers continue to LGN, some leave prior

Lateral Geniculate Nucleus

Receive info from both eyes

Each LGN projects information to occipital cortex

Optic nerve sheath

Dura mater, external layer, fuses with Tenon’s capsule (directly continuous for barrier function)

Dense vascularized CT, innervated with sensory nerves, protects ON

Arachnoid mater

Vascularized trabeculae spanning between dura mater and pia mater

Pia mater

Thin and vascular, wraps directly around axons, nutrient supply to ON

Astrocytes present

Subarachnoid space

Directly continuous with subarachnoid space of brain, filled with CSF

Fasicles

Bundles of axons

Anterior to lamina cribrosa: Surrounded by astrocytes

Posterior to lamina cribrosa: Additional layer of CT sheath, nerve fibers become myelinated, astrocytes and oligodendrocytes

Papilledema

Swelling of ON head, caused by increase in intracranial pressure

Appears as elevated disc, blurred margins, loss of cup

Urgent situation