1. Visual Pathways 1 Flashcards
Out of the 2 RGCs, which one crosses at the chiasm and which one stays on the same side?
The axons of nasal RGCs cross as the chiasma and the axons of temporal RGCs stay on the same side.
Out of the 2 RGCs, describe where the cells in each look at in terms of receptive feids? For example on the left side- before vs after crossing chiasma?
On the left side: the temporal cells look right visual field and nasal cells look at the left visual field. Hence, once the RGCs cross the chiasma the left eye is looking at the right visual field.
Right optic tract get information from RGCs that are looking at which visual field?
Left hemi- field.
Majority of RGCs go to?
Where do the rest of RGCs go to?
Majority go to LGN
Rest go to the Tectum and supra- chiasmatic nucleus.
Role of Supra- Chiasmatic nucleus?
Light- mediated onset of body rhythms in the morning.
Role of pretectum?
Control pupillary light reflex.
Superior colliculus, Supra- chiasmatic nucleus (SCN) and Pretectum are reflexive and are involved in secondary visual pathways, explain why?
Because they are not involved in conscious vision.
Which pathway is involved in conscious vision?
LGN- Referred to as primary visual pathway.
Which cranial nerve is the optic nerve?
CN II
What is the reference point of visual fields? This point is also known as?
Fovea- retinal coordinates are determined by this point.
Also known ad: Fixation point
What is the lower retina in terms of visual fields?
Lower retina= upper field
What is the temporal retina in terms of visual field?
Temporal retina= nasal field
Optic disc is temporal or nasal to the fovea?
Nasal
Vertebtrae retina is referred to as reversed retina, why?
Because light has to go through all superficial layers of the retina before hitting the RPE (photoreceptors).
Horizontal cells integrate information from?
Adjacent photoreceptors
AND generate centre-surround receptive fields in cones.
Role of bipolar cells ?
Provide the main pathway from photoreceptors to retinal ganglion cells
Bipolar cells vs amacrine cells, which cells have axons?
Bipolar cells have axons, amacrine cells have no axons.
RPE is the location where what synapses?
RGCs define what?
Bipolar and amacrine cells synapse. RGCs define visual properties that are then sent to the brain.
The brains only contact to the eye is through?
Through axons of RGCs
Optic nerve fibres emerge on the surface of the retina and proceed to?
The optic nerve head
What is found in the inner nuclear layer of the retina?
Cell bodies and amacrine cells
Where is the outer segment of photoreceptors found in the retina?
In the retinal pigment epithelium (RPE).
Role of retinal pigment epithelium (RPE) in the retina?
Important for metabolism and disc shredding. It is opaque hence found behind. And it is where Bipolar and Amacrine cells synapse.
Why is the retina reversed?
Retinal pigment epithelium is opaque, if it was in the front you would not be able to see anything.
Retina has what kind of blood supply?
Dual blood supply, each layer of the retina is supplied differently.
What gives the optic nerve stability?
It has some underlying structures that give stability.
Optic nerve fibres from axons and RGCs go through the holes in the lamina cribrosa, describe the organisation of fibres that go through the holes
The fibres that go into a given hole are from cells that are adjacent to each other.
The fibres that emerge of the optic nerve head are fatter, why? And why are they seen as white?
As fibres are moving to the optic nerve head they are de- myelinated and hence become thicker and hence seen as white.
Role of myelin sheath?
It is an insulated later.
Role: speeds up propagation of nerve impulses to the brain.
In the fovea the inner layers of the retina are excavated, why?
Even though the inner layers of the retina are transparent they do still scatter light. When these layers are absent and you have the highest concentration of photoreceptors, this improves vision. As the fovea is responsible for sharp central vision.
Retinal layers end near the border of the optic nerve they are separated from nerve by ring of glial tissues known
Intermediary tissue of Kuhnt.
Choroid and sclera are separated from optic nerve by ring of white fibrous tissue containing glia, fibres and some pigments known as
Marginal or border tissue.
Optic nerve head has some underlying structures that give it stability- this is stability against pressure, what are this structures?
The intermediary and marginal tissues
Does Brunch’s membrane and choroidal vascular layer enter the optic nerve head?
Bruch’s membrane extends all the way to the optic nerve, but choroidal vascular layers end some distance from it.
What is the role of lamina cibrosa?
A sieve- like structure through which RGC axons come through to the optic nerve and become myelinated.
Optic disc is not completely white, why?
Some underlying RGCs.
What covers the optic nerve?
Dural sheath- this are the meninges membranes.
Is the eye part of the brain or a separate structure?
Eye is part of the brain and it emerges from the brain during early development.
Describe the optic nerve head seen in patients with glaucomatous?
Due to increased IOP causes cells to die and results in fewer fibres entering the optic nerve. Hence the cup is seen excavated- due to tissue loss.
Why does the cup appear to be more white in patients with glaucomatous?
Due to the unobstructed view of myelinated fibres that leave the optic nerve.
In early development the lens is supplied by what structure?
Hyaloid arteries
Is Hyaloid arteries seen in adults/ after development?
Some patients may still have this artery and it is normal, it does not impact vision.
What is scleral canal?
Opening in the sclera through which the optic nerve will pass.
Are all the nerves in the optic nerve myelinated?
Yes
What is the beginning point of all the nerves found in the optic nerve head?
Lamina Cribrosa
Optic nerve is the extension of?
Brain tissue
Nerve fibres are compressed of which 3 fibers?
- Visual Fibres
- Pupillary Fibres
- Centrifugal Fibres
Visual Fibres and Pupillary Fibres are from?
Axons of RGCs
Visual fibres on to the path to ?
LGN
Pupillary fibres go to?
Pretectum
Optic nerve is divided into bundles, divided by connective tissues called?
Septa
Is septa constant in all places?
If not what is the role of this?
No, in places bundles of tissues are divided by glial tissue just before the optic chiasm.
This plays an important role in the nasal and temporal crossing of fibres at the chiasm.
Large glial septum runs through middle of nerve dividing fibres into?
Ventromedial and dorsolateral portions
Optic nerve fibres are surround by?
3 meninges- pia mater, arachnoid and dura mater.
What is meant by subdural?
Space under the dura and above arachnoid.
What is meant by subarachnoid?
Space below the arachnoid and above the pia.
What disease that affects the myelination of the optic nerve
MS (multiple sclerosis)
Early signs of MS? (What does the patient notice)
Patients complain vision in 1 eye is darker than the other eye- luminance value changes.
Compare Pia matter to dura matter:
Pia matter is the inner most layer this area is highly vascularized. Dura mater is much less vascularized since it provides structural support and is the other most layer.
Location of optic chiasm
Optic Chiasm is located at the junction of the anterior wall and floor of the third ventricle. It lies above the pituitary fossa.
Optic Chiasm is covered in?
Covered in pia mater and suspended in CSF.
Right optic tract mediates information from which field?
Left- hemi feild
Optic chiasm is known as the location where?
Where nasal fibres cross
Left and right hemi-fields are defined by?
Defined by the vertical meridian
Optic tract 1st synapse at?
LGN- that gives rise to radiations
LGN project to?
Back brain- visual cortex
