The Optic Pathway Flashcards
Where do the axons of retinal ganglion cells end up?
With excitatory synapses with the lateral geniculate nucleus
Which cell forms the origin of the optic pathway?
Retinal ganglion cells
Briefly describe order of the optic pathway
From retinal ganglion cell axons from back of eye travel down the optic nerve, through a junction called the optic chiasm into the optic tract and to the nucleus of the thalamus called the LGN
How much % of retinal ganglion cell axons end up in the LGN?
90%
How long is the optic nerve all together?
30-50mm long
Name the 4 consecutive regions of the optic nerve
- Intra ocular
- Intra orbital
- Intra canalicular
- Intra cranial
How long is the intra canalicular consecutive region
5mm
How long is the intra orbital consecutive region
20-30mm
How long is the intra ocular consecutive region
1mm
How long is the intra cranial consecutive region
5-15mm
Which is the shortest consecutive region
Intra ocular
Which is the longest consecutive region
Intra orbital
Where is the intra ocular consecutive region found
Inside the eye
Where is the intra cranial consecutive region found
Inside the skull
Where does the intra cranial consecutive region head towards
Optic chiasm
What does the intra canalicular consecutive region go through
The hole at the back of the orbit through the canal
What is the intra ocular consecutive region known as
Optic nerve head
What leaves through the optic nerve head
Axons of retinal ganglion cells
Which region of the retina is the optic nerve head found
Nasal retina
Which main artery and vein do arteries coming in and veins going out branch off from?
Central retinal artery and central retinal vein
Which region of the optic nerve head of the veins and arteries of the central retinal artery and vein enter and exit from
Nasal side
Why do the veins and arteries exit and enter nasally on the optic nerve head?
As macula and fovea are at the temporal side of the retina where blood vessels skirt this region
What part of the optic nerve head is visible with the ophthalmoscope
Optic disc
What’s the normal range of the variable disc area?
0.90-5.5mm squared mean 2.7mm squared
How many % of normal subjects do an indentation or cup occur
75%
In which case would the obvious indentation or cup not be the same in both eyes of the same individual be
Pathological
What is surrounding the indentation or cup?
Neuroretinal rim
Why should you look for bilateral asymmetry in cup and disc sizes
Indicates something’s gone wrong eg pathological change
What is pallor
Pale optic disc due to water building up behind optic nerve head = oedema
How many ganglion cell axons run in the nerve fibre layer on the inner retinal surface
1 million
Where in the retina is the nerve fibre layer found
Inner retinal surface
Which retinal quadrants do ganglion cells take direct course to the nerve head?
Nasal
Superior
Inferior
From which region of the retina do ganglion cells take an arcuate course
Temporal
What do the temporal retinal ganglion axons arc around
Fovea centralis
From where to where do the arcuate ganglion axons go to
Temporal retina to the macula
What is the papillomacular bundle
Axons from temporal ganglion cells nearer the nerve head
What do ganglion cell axons pass through when they leave the eye
Lamina cribrosa
What is the lamina cribrosa
Modified region of sclera and compact perforated connective tissue plates layed down in sheets
What does the lamina cribrosa contain
Numerous pores in a sieve like arrangement
What does the lamina cribrosa do
Holds axons together and provides them with mechanical support prior to entering the the intra orbital part of the nerve
Why do ganglion cell axons need support from the lamina cribrosa
Because they are delicate floppy and flimsy
What does the lamina cribrosa also supply other than support to the ganglion cell axons
Useful reference point on the optic nerve head
How many segments does the lamina cribrosa divide into
Three
What are the names of the distinct segments that lamina cribrosa divides into
- Pre laminar
- Laminar
- Post laminar
Regions
Why is the nerve fibre layer big and elevated at the neuro retinal rim?
Due to accumulating retinal axons traveling down the optic nerve fibre layer and exiting the eye
What does the pre laminar segment of the ONH consist of
The nerve fibre layer & supporting astrocytes which go all the way down to the brain
What type of cell is an astrocyte
Glial cell
What else apart from the lamina cribrosa support the axons
Astrocytes
What does the laminar segment of the ONH consist of
Supporting connective tissue & pores
What does the post laminar segment of the ONH consist of
Wide diameter axons myelinated by oligodendrocytes and wider nerves which have meninges and lots of astrocytes all the way down the ONH to the brain
Why are the pores wider at the post laminar segment that the laminar at the ONH
Because the axons at the post laminar region acquire a myeline sheath so has lots of oligodendrocytes down the myeline causing wider diameter & the edge surrounding the nerve requires the meninges which increases it’s diameter
Describe the appearance of an astrocyte
A cell body with lots of fine processes and feet which come out with a star like arrangement
Which part of the nervous system are glial cells resident
All parts
What do the pre laminar segment of the ONH form borders with
Vitreous = inner limiting membrane of elsching (special astrocytes which connects to the inner limiting membrane which separates the retina with vitreous)
&
Outer retina and choroid tissues of kuhnt & jacoby (astrocytes which seal part of the choroid with the sclera axons if ganglion cells)
Where in the pathway are astrocytes found
Everywhere
What do astrocytes do for retinal axons
Provides mechanical support and separates them into bundles
What is the name of the bundles of retinal axons formed by astrocytes
Fascicles
What do astrocytes wrap around in the optic pathway
Capillaries
What barrier is formed when astrocytes wrap around capillaries in the optic pathway
Blood nerve barrier called carivascular endfeet
What do astrocytes do in response to injury of the optic pathway
Proliferate (divide) and form scar tissue
What is the lamina cribrosa composed of
Elastin
Laminin and
Collagen
What do the pores of the lamina cribrosa transmit
Axon bundles or fascicles to pass through
Describe the pore diameters of the lamina cribrosa
Larger in inferior and superior regions with corresponding reduced supporting connective tissue
Smaller in nasal and especially temporal region
What is the susceptibility to risk to the axons in the different pure sizes of the lamina cribrosa
Larger size pore = less mechanical support to axons
So temporal region containing the papillomacular bundle is least at risk (hence periphery visual loss with glaucoma)
What rule is followed of decreasing axon damage susceptibility of the pores of the lamina cribrosa
ISNT
Inferior is larger than superior is larger than nasal is larger than temporal
What causes cupping of the ONH
Raised iop due to failure of aqueous drainage
Where does the pressure try to escape through in the eye with raised iop
The optic nerve head
Which area of nerves are initially most vulnerable in the lamina cribrosa due to raised iop
Inferior which has least support of axons
Which area of visual field is lost due to damage to the inferior lamina cribrosa
Superior visual field loss
Which type of glaucoma is associated with raised iop
Both open and closed angle
From the pre laminar to the post laminar region of the ONH, by how much does the optic nerve increase
Double from 1.5-3mm
What are oligodendrocytes
Few branching cells which repeatedly wrap around axons to form myelin sheaths
What is the composition of the three layer meningeal sheath of the post laminar nerve
Pia
Arachnoid
Dura
What is continuous with the pia mater peripherally
Axon bundles supported by connective tissue septa formed by collagen fibres and astrocytic processes
Where is the dura mater found
In the outer layer, continuous anteriorly with the sclera
What is the dura mater comprised of
Thick and tough collagenous layer (lots of collagen fibres)
What does the dura mater provide
Support for the whole of the optic nerve
What does the arachnoid mater comprise of
Numerous trabeculae of collagen surrounded by flattened meningothelial cells
Where is the arachnoid mater located in relation to the dura and pia maters
Inbetween
Where is the pia mater located
Covers the inside of the optic nerve (forms a delicate inner layer)
What is embedded within the pia septa
Numerous capillaries
What does the pial septa do
Enter and support axons within the nerve
What is the space between the arachnoid and pial membranes and what does it contain
The subarachnoid space
Cerebo spinal fluid
As well as cerebo spinal fluid, what else does the subarachnoid space contain
Larger blood vessels
What are the name of the cells which make the arachnoid mater
Meningothelial cells
What do the meningothelial cells have the ability to become
Cancerous called meningioma
The post laminar nerve contains
Bundles/fascicles of axons with connective tissue/septa in between
What does the septa comprise of
Connective tissue of collagen fibres and processes that are keeping axons into bundles
What is the subarachnoid space in the optic nerve containing the CSF continuous with
The ventricles containing CSF inside the brain
What is the build up of CSF inside the ventricles of the brain called
Hydrocephalus
What can hydrocephalus cause if it percolates down the space behind the brain
Oedema/optic disc pallor
How many segmental blood supplies are there to the ONH
Three
Which two arteries supply blood to the three segments
Central retinal artery &
Short posterior ciliary artery
The sensory nerve fibre layer and pre laminar segments receive their blood supply from…
Branches of the central retinal artery and choroidal supply
The segment of the lamina cribrosa receive their blood supply from…
Circle of zinn & haller from the short posterior ciliary artery
The segment of the post laminar receive their blood supply from…
Branches of the short posterior ciliary artery running in and from the pia mater
How much of slack does the intra orbital portion contain
5mm
What is the intra orbital portion
Area between the optic nerve head to the optic canal (apex of orbit)
Why is there slack within the intra orbital portion
Prevents trauma during eye movements eg axons from snapping when moving the eye
What is above the optic chiasm
The hypothalamus and thalamus
What is between the hypothalamus and thalamus
3rd ventricle (a slit)
What does the 3rd ventricle contain
Cerebo spinal fluid
What is just below the hypothalamus
Pituitary gland
Where does the pituitary gland sit in relation to the optic chiasm
1cm below
What things sit laterally (next to) the optic chiasm
Blood vessels
Internal carotid artery
Cavernous sinus
How big is the cavernous sinus
Size of a grape/finger tip
What is the cavernous sinus
Network of small arteries and
mainly veins
Which sequence does the blood supply to the optic pathway have
Anterior - posterior
What is the first sequence of blood supply to the optic pathway
Intra orbital nerve
What is the second sequence of blood supply to the optic pathway
Intra canalicular nerve
What is the third sequence of blood supply to the optic pathway
Intra cranial nerve and chiasm
What is the fourth sequence of blood supply to the optic pathway
Optic tract (leads to LGN)
What is the blood supply to the intra orbital nerve
Perforating arteries from branches of ophthalmic artery
What is the blood supply to the intra canalicular nerve
Short branches from anterior cerebral and anterior communicating arteries
What is the blood supply to the optic tract
Perforating thalamic branches of posterior cerebral artery
What is the blood supply to the intra cranial nerve and chiasm
Short branches from posterior cerebral and posterior communicating artery
What is the partial decussation (crossing over) of retinal axons in the chiasm
Nasal crossed (goes to opposite LGN) Temporal uncrossed
Which side of the visual field does the right half of each eye look at
Left visual field
Which side of the brain is the right hemi field linked to
Left
Which side of the brain is the left hemi field linked to
Right
What is the result of compression of retinal axons
Cannot conduct impulses anymore from the retina resulting in visual field loss
What does a compression of the chiasm and tract at the midline cause
Damage to nasal axons from both eyes
What is the cause of damage to nasal axons on both eyes
Pituitary tumour (adenoma) Raised pressure in 3rd ventricle/hydrocephalus
Which direction does a pituitary tumour grow
Upwards towards the chiasm
How does hydrocephalus affect the chiasm
Pressure from CSF can crush axons crossing the midline
What does compressions from the side of the chiasm damage
Damage temporal axons
Same eye
What causes damage to the temporal axons of the chiasm
Internal carotid artery out pocketing (aneurysm)
&
Cavernous (compression of nasal axons) sinus expansion (thrombosis)
What is caused from damages to nasal axons from both eyes
Bi temporal hemi anopia
What does damages of temporal axons of the same eye cause
Uni nasal hemi anopia
How does cavernous out pocketing cause damage to the chiasm
Can move to midline and crush the axons that run through that region
Compression of the chiasm and tract can also be caused from…
Anywhere eg from
Meningiomas (arachnoid tumours)
Or from
Loss of blood supply (blockage or haemorrhage)
Where do most of excitatory ganglion axons synapse/terminate
LGN cells of the thalamus
The optic tract is after the…
Cross region
The LGN is found at the edge of the…
Thalamus
