Vision Flashcards
What are some important arteries that supply the eye?
- ICA branches into ophthalmic artery branches into central retinal artery (pierces optic nerve to supply inner 2/3 of retina - end artery)
What is the lamina cribrosa?
Sieve-like hole in sclera to allow exit of optic nerve
What are the parts of the optic disc we can see when we look at the retina? What are they made up of?
Rim (made up of axons)
Cup (lamina cribrosa/bare sclera)
What is the clinical importance of the central retinal artery?
It is an end artery, can become blocked from a clot that has travelled from ICA and this damages retina if clot is not cleared
Signs - pale retina, cherry-red spot (fovea)
What are the features of the macula when looking at the retina?
- darker
- BVs avoid it
- temporal side of optic disc
How can you tell the difference between arteries and veins in the retina?
Arteries appear thinner than veins
Describe the features of the branching of the central retinal artery.
Divides into four branches which keep subdividing into capillary networks that are most dense in macula and absent from the foveal avascular zone (which can be seen in an OCTA)
What happens to the foveal avascular zone in diabetes?
It gets bigger due to capillary dropout
How is a fluorescein angiogram of the retina described?
Hyper-fluorescent/hypo-fluorescent
How is optic disc described in fundoscopy?
Colour of rim
- orange (normal)
- pale
Contour
- sharp/defined (normal)
- undefined (disc is swollen)
Cupping
What does the retina look like in raised intracranial pressure?
Pale, red streaks, indistinct optic disc
-optic nerve surrounded by dura mater + arachnoid sheaths - central retinal vein within is compressed by pressure transmitted along subarachnoid space - congestion - papilloedema
What do the ciliary arteries branch off and supply?
Branch off ophthalmic artery to supply uvea (choroid, ciliary body, iris)
What are the conceptual layers of the retina?
- Inner neural layer
- Outer pigmented layer
Where would the eye be injected?
Aim of ora serrata as there are no important structures underneath and retina stops here
Describe the retinal pigment epithelium.
choroid - Bruch’s membrane - RPE - potential space - photoreceptors
- multiple microvilli extend out + surround outer segments of photoreceptors
- potential space is where retinal detachment occurs
- normally, GAGs act as glue and water is actively transported out, creating -ve pressure
- tight junctions (outer retina-blood border)
Gives eye immune privilege + highly selective transport of glucose, retinal from Vit A and specialised phospholipids - photoreceptors shed outer segments -> creates free radicals
- choroid has high blood flow but little O2 is extracted
- light energy
- RPE live in highly oxidative environment (lipofuscin can accumulate over life)
- RPE absorbs light
- RPE phagocytoses shed outer tips of photoreceptors
- secretes VEGF which keeps capillaries in choroid fenestrated
Describe the visual cycle for rods.
Photon of light absorbed in photoreceptors by supply of 11-cis retinal (rhodopsin= 11-cis retinal + opsin)
- change in conformation
- changes to all-trans retinal + free opsin
- re-isomerisation in RPE
How does the visual cycle allow us to see?
Opsin closes sodium channels in photoreceptor outer segments
- intracellular Na+ falls
- transmembrane potential falls
- hyperpolarisation transmitted along visual pathway to our consciousness
What is the visual pathway through the eye?
photoreceptor - bipolar cell - ganglion cells - optic nerve
What are the different parts of the optic nerve and its pathway through the brain?
-optic nerve - orbital portion - optic canal (in sphenoid bone) - intracranial portion - optic chiasm (meets optic nerve from other side) - optic tracts (emerge from posterolateral corners of chiasm) - most fibres synapse in LGB - optic radiation - primary visual cortex
What are the important relations of the optic chiasm?
Laterally - carotid arteries
Inferiorly - pituitary gland
Which fibres cross at the optic chiasm?
Fibres from nasal side cross (serve temporal side of vision)
Fibres from temporal side do not cross (serve nasal side of vision)
Describe the pupillary light reflex.
AP in optic nerve (CNII) - optic chiasm - optic tract - small number fibres leaves before LGB to divert to pretectal nucleus in midbrain - Edinger Westphal nucleus (midbrain) of both sides - CNIII - ciliary ganglion - constrictor pupillae in iris
Therefore, both eyes constrict to light shone in one eye
