Optic Nerve & Visual Pathway

the ON is composed of:

GC axons that make a 90 degree turn at the optic disc and make up the ON

4 paths of GC

1. synapse in the LGN and travel to the primary visual cortex (V1)

2. superior colliculus to help with saccades

3. pretectal nucleus to help with pupil movement

4. hypothalamus to help with circadian rhythm

does the ON have regenerative abilities

no (little to none)

what surrounds the intraorbital (post-laminar) ON

pia, arachnoid, and dura membranes continuous with the meninges of the brain (inner to outer layers: PAD)

what is the subarachnoid space of the ON between and what does it contain

between the pia and arachnoid

contains CSF

what 2 meningeal layers fuse together

dura and arachnoid

the dura and arachnoid fuse and become continuous with what ocular structures

sclera and periorbita

what meningeal layer continues with the intracranial portion of the ON

only the pia

the dura and arachnoid do not continue intracranially with the ON

why does papilledema cause blurred disc margins

an increase in ICP causes CSF from the subarachnoid space to leak over into the superficial optic disc

CSF spreads over the disc margins to the surrounding RNFL

what cells provide myelination to ON axons posterior to the lamina

oligodendrocytes

role of oligodendrocytes in the ON

myelinate the ON axons posterior to the lamina cribrosa

role of astrocytes in the ON

replace muller cells in the ILM of the ON

provide structural support to ON axons

schwann cells (do/do not) provide myelination for the ON

do not

schwann cells are not found within the ON

why do 90% of patients with optic neuritis report pain on eye movement

the ON sheath is attached to the sheaths that surround the SR and MR

blood supply of retinal NFL of ON

SPCAs and CRA

blood supply of intraocular ON (pre-lamina and lamina)

circle of zinn (anastomoses of SPCAs)

other SPCA branches

blood supply of intraorbital ON (post-lamina)

CRA branches

pial mater arterial plexus

blood supply of intracranial ON

ophthalmic, anterior, cerebral, anterior communication, and IC arteries

ON (is/is not) able to autoregulate its blood supply

is

like the retina

the choroid cannot autoregulate

optic disc location

most anterior portion of ON

nasal retina 15 degrees from fixation (fovea)

subtends angle of 5-7 degrees

the optic disc is larger (vertically/horizontally)

vertically

why does the ON act as a blind spot

the ON has no photoreceptors

only has RNFL and a glial membrane made of astrocytes

what layers is the ON composed of

RNFL

glial membrane made of astrocytes (ILM)

name of ILM made of astrocytes in the ON

ILM of Elschnig

length and 4 sections of ON

50-60 meters long

intraocular, intraorbital, intracanalicular, intracranial

length and borders of intraocular ON

1 mm long

from the optic disc to the lamina cribrosa, made of the prelaminar ON and the laminar ON

composition of prelaminar ON

axons bundled in sheaths of astrocytes for structural support

the prelaminar ON (has/does not have) myelin

does not have

intermediary tissue of kuhnt

ring of glial tissue that separates ON fibers from the surrounding retinal tissue in the prelaminar ON

border tissue of jacoby

posterior continuation of glial tissue that makes up the tissue of kuhnt

separates ON fibers from choroid

border tissue of elschnig

scleral collagen fibers that surround the glial fibers of the tissue of jacoby

summarize border tissues of Kuhnt, Jacoby, and Elschnig

kuhnt: glial tissue separates ON from retina

jacoby: more posterior glial tissue separates ON from choroid

elschnig: scleral tissue that surrounds glial tissue of jacoby

role of tight junctions within the glial tissue of the prelaminar ON

protects ON fibers from fluid leaking from fenestrated choroidal vessels

section of the ON that exits the globe

lamina cribrosa

what does the lamina cribrosa consist of

network of collagen and elastic fibers that bridge across the posterior scleral foramen and support the ON

length and borders of intraorbital ON

posterior to the lamina until it exits the orbit via the optic canal

30 mm (longest portion)

what aspects of the ON anatomy allow for eye movements to happen without damaging the ON

the intraorbital ON is long and S-shaped

its axons are separated by CT septae that protect them from damage

role of CT septae in the intraorbital ON

allow the eye to move without damaging the ON

protect the ON axons

where does the intraorbital ON exit the orbit

optic canal of the lesser wing of the sphenoid

when do ON axons become myelinated, and what cells are they myelinated by

become myelinated posterior to the lamina cribrosa (in the intraorbital portion of the ON)

myelinated by oligodendrocytes

what surrounds the ON axons in the intraocular portion of the ON

CT septae, myelin, meninges that cover the rest of the brain

borders of intracanalicular ON

6-10 mm

part of ON running through the optic canal

borders of the intracranial ON

10-16 mm

part of ON from optic canal to optic chiasm

what 2 things start at the lamina cribrosa

meninges & myelination

M&M

where is the CSF made

ventricles

what within the ventricles makes CSF

choroid plexus

where is the optic canal located

lesser wing of the sphenoid

the ON exits the orbit through which bone

lesser wing of the sphenoid

what nerve provides blood supply to the optic disc

SPCA

what is the longest section of the ON

intraorbital section (30 mm)

where do most optic nerve fibers go

90% go to synapse in the LGN and travel to the primary visual cortex (V1)

damage to what 2 things can cause a central scotoma

macula

papillomacular bundle via ON disease

path of papillomacular bundle fibers

go from the macula to the temporal optic disc

path of nasal ON fibers

go from RGCs nasal to the ON into the nasal ON

path of arcuate fibers

go from RGCs temporal to the fovea, arc over the papillomacular bundle, and insert into the inferior and superior ON

thickest to thinnest ON sections

ISNT

inferior, superior, nasal, temporal

which fibers cross at the optic chiasm

superior and inferior nasal fibers that carry info from the temporal visual field

3 different paths nasal fibers can take when crossing the chiasm

anterior knees of wilbrand

posterior knees of wilbrand

optic chiasm

path of anterior knees of wilbrand

inferior nasal fibers cross at chiasm and loop anteriorly to the contralateral ON before entering the optic tract

the inferior fibers cross and then loop forward towards the opposite ON and then exit

path of posterior knees of wilbrand

superior nasal fibers loop posteriorly into the ipsilateral ON before crossing the optic chiasm

what is the optic chiasm located within

circle of willis

which fibers go into the anterior vs posterior knees of wilbrand and how does this affect fiber orientation in optic tract

inferior nasal fibers go to the anterior knees of wilbrand

superior nasal fibers go to the posterior knees of wilbrand

allows the inferior fibers to loop up and out to the temporal wall of the optic tract, while the superior fibers go down and in to the medial wall of the optic tract

in the optic tract, all superior fibers (contra temporal and ipsi nasal) are on the medial side, and all inferior fibers are on the temporal side

what is located inferior to the optic chiasm

pituitary gland

which arteries travel along the lateral edges of the optic chiasm

ICA and posterior communicating arteries

borders and contents of the optic tract

from the optic chiasm to the LGN

has crossed and uncrossed fibers from each eye (makes it so each eye sees its nasal field and the other eyes temporal field, translating to half a person’s visual field of either the right or left side in each eye)

the optic tract travels along the lateral surface of the:

cerebral peduncles

the optic tract is parallel to what artery

posterior cerebral artery

in the right optic tract, where are the superior, inferior, and macular fibers

superior fibers: nasal and temporal are on the medial side

• superior nasal fibers of the left eye and superior temporal fibers of the right eye

inferior fibers: nasal and temporal are on the lateral side

• inferior nasal fibers of the left eye and inferior temporal fibers of the right eye

macular fibers: middle of the tract

where do fibers within the optic tract synapse

lateral geniculate nucleus of the thalamus

layers of LGN

magnocellular layers: layers 1 & 2

parvocellular layers: layers 3-6

koniocellular layers: between the layers

types of cells in each LGN layer

magno: large magno cells (1 & 2)

parvo: medium parvo cells (3-6)

konio: small cells

do the LGN layers go bottom up or top down

bottom up (layers 1 & 2 are located below layers 3-6)

which layers of the LGN do crossed and uncrossed fibers synapse in

U235

layers 2, 3, and 5 are uncrossed

layers 1, 4, and 6 are crossed

how are the optic tract fibers organized when they synapse in the LGN

maintain the same topography as the optic tract

remember: Lower are Lateral

• superior fibers synapse in the medial LGN

• inferior (lower) fibers synapse in the lateral LGN

• macular fibers form a wedge of synapses at the dorsal LGN

where is the location of the first synapse within the post-ON visual pathway

LGN

how does the LGN fine-tune vision

gets fibers from subcortical areas and V1 that help fine-tune vision before it is again relayed to V1

optic radiations

fibers that leave the LGN and travel to the primary visual cortex (V1)

path of inferior optic radiations

• inferior lateral fibers of the LGN travel through the temporal lobe

• form Meyer’s loop by looping around the lateral ventricle into the parietal lobe

• terminate in V1 in the occipital lobe

Lower (inferior) fibers travel Laterally in the optic tract and form meyer’s Loop before ending in the Lingual gyrus

order of lobes of the brain that inferior optic radiations travel through

temporal lobe (LGN) > parietal lobe (Meyer’s loop) > occipital lobe (V1)

Meyer’s loop

loop formed by inferior optic radiations as they course around the lateral ventricle of the temporal lobe into the parietal lobe

path of superior optic radiations

• superior medial fibers of the optic tract course posterior through the parietal lobe

• terminate in the posterior portion of the occipital lobe of V1

location of primary visual cortex (V1, Brodmann’s area 17, striate cortex)

medial surface of the occipital lobe (in the middle of the back of the brain)

calcarine fissure

divides the back of the occipital lobe/visual cortex into superior and inferior regions

cuneus gyrus

superior part of V1 in the calcarine fissure division

superior retinal fibers terminate here

lingual gyrus

inferior part of V1 of the calcarine fissure division

inferior retinal fibers terminate here

Lower fibers travel Laterally, do a meyer’s Loop, and terminate in the Lingual gyrus

what gyri do the inferior and superior retinal fibers terminate in

superior: cuneus gyrus

inferior: lingual gyrus

what makes up 50% of V1 fibers

macular fibers

where do macular fibers project to and terminate in

project to outer surface of the apex of the occipital lobe

superior macular fibers terminate in the cuneus gyrus

inferior macular fibers terminate in the lingual gyrus

fibers of V1 are (myelinated/unmyelinated)

myelinated

fibers from what 3 places project to the primary visual cortex

LGN, superior colliculus, frontal eye fields

how is the primary visual cortex organized

6 horizontal layers and multiple vertical columns of neurons

in what V1 layer do the optic radiations synapse with the LGN cortex

layer 4

once optic radiations synapse in layer 4 of the LGN, where do they go

project to higher cortical areas of the visual cortex, V2-V5, for further processing of information

where does layer 5 send axons

the superior colliculus for control of saccadic EMs

what does layer 6 do in the visual cortex

sends feedback info to the LGN

what are vertical columns in V1 called

ocular dominance columns

how are ocular dominance columns organized

each column contains fibers from only 1 eye

the columns are further organized by stimulus orientation (only respond to stimulus in front of a particular eye that is oriented in a particular way)

where does binocular visual processing begin

primary visual cortex

blood supply of the optic chiasm

circle of willis and ICA

blood supply of the optic tracts

anterior choroidal and middle cerebral artery