BS - Optic Nerve - Week 7 Flashcards
How long is the optic nerve?
5-6cm
What percentage of the information conveyed by the optic nerve goes where?
LGN - 90%
Superior colliculus - 10%
Consider the optic nerve. What does the pia mater cover?
Covers individual optic nerve axons, as well as the optic nerve.
The sclera partially infiltrates the optic nerve. Consider the portion that does not infiltrate. What does this form?
The dura mater of the brain.
Define the prelaminar optic nerve, and how optic nerve axons are arranged here. What cells are found here?
The optic nerve anterior to the lamina cribrosa.
2000 axons are arranged per channel here. Astrocytes surround the channels.
Define the retrolaminar optic nerve, and what happens here to the optic nerves.
What cells are present here, and in what arrangement?
Posterior to the lamina cribrosa. Optic nerve axons are myelinated here.
Oligodendrocytes and astrocytes are found here in bundles.
What purpose do astrocytes have at the pre/retrolaminar regions of the optic nerve?
They separate bundles of nerve fibres, they dont bind axons together.
They have repair and protection functions.
What forms the blood retina barrier?
Astrocytes
How are pores on the lamina cribrosa arranged.
Larger pores are found at the superior and inferior regions of the optic nerve.
Specifically at what point do the axons become myelinated?
Posterior limit of the lamina cribrosa.
What is the typical diameter of the optic disc?
1.5-2mm
Is the optic disc completely flat?
No, the edges are slightly raised.
What appearance does the optic cup have?
White/pale appearance.
Lamina cribrosa can sometimes be seen.
When checking the optic disc for health, what is an important indicator?
The proportion of the cup-disc ratio.
What is a typical cup:disc ratio?
<0.4 is normal, 0.3 is the median
Define the horizontal raphe.
The midpoint on the retina where the nasal radial and temporal radial nerve fibres meet.
Explain why higher diameter pores are found in certain regions of the lamina cribrosa.
More are found in the superior/inferior regions because the nerve fibres travel radially towards the optic disc, and so they meet chiefly at the superior and inferior end.
Describe the nerve fibre arrangement distal to the optic chiasm.
Macula fibres occupy at least half of the nerve cross section, with ~12.5% each for superior temporal/nasal, and inferior temporal/nasal fibres.
Describe the nerve fibre arrangement proximal to the optic chiasm.
Left eye
Macula fibres are located in the middle of the cross section.
Top right and left are occupied by superior temporal and nasal fibres respectively.
Bottom right and left are occupied by the inferior temporal and nasal fibres respectively.
How do temporal fibres exit the optic chiasm?
Ipsilateral side.
How do superior and inferior nasal fibres exit the optic chiasm?
Inferior - some loop at the anterior knee of Wilbrand
Superior - some loop at the posterior knee of Wilbrand
What supplies blood to the inner retina?
Central retinal artery
What structures does the posterior ciliary artery supply blood to?
Choroid and retrolaminar optic nerve
What is the Circle of Zinn-Haller.
Rich blood vessel network formed by the posterior ciliary artery, supplying the choroid and retrolaminar optic nerve.
What supplies blood to the nerve surface (2)?
Branches from the central retinal artery, and small branches from the choroid.
What supplies blood to the prelamina and lamina cribrosa (2)?
Circle of Zinn-Haller and small branches from the central retinal artery.
What supplies blood to the retrolamina (2)?
Branches from pial blood supply, and small branches of the central retinal artery.
What happens to the neuroretinal rim and axons in glaucoma?
Axonal loss
Neuroretinal rim loss
Define the scleral crescent, and what occurs here, and why its there.
Crescent-shaped area on the neuroretinal rim where there is no overlying retina. Pigment from the RPE beneath can be seen.
Is phsiologic, high myopia.
Define the ISNT rule.
Describes the order of neuron fibre density from highest to lowest:
Inferior>superior>nasal>temporal
What happens to the cup:disc ratio in glaucoma?
Increases.
Where does axonal loss present first initally in glaucoma?
Inferior region.
What happens to the appearance of the lamina cribrosa with glaucoma? What appearance do the pores have, and which direction to the central retinal vessels shift?
Lamina visibility increases.
Pores are more like slits than circles.
Vessels shift nasally.
Name the three dietary carotenoids that contribute to macular pigments.
Lutein
Zeaxanthin
Meso-zeaxanthin
What purpose do macular pigments have (3)?
- Pre-receptoral absorption of blue light
- Reduces chromatic abberation and light scatter
- Antioxidant capacity
Where are macular pigments concentrated?
Central retina along the axons of cone receptors.
Why do nerve fibres segregate the way they do post-optic chiasm?
Possibly to protect the macular fibres in the event of trauma.