Vision Flashcards
Describe the process of phototransduction
In more detail:
Rhodopsin > bathrodopsin> lumirhodopsin> metarodhopsin I > metarodhopsin 2 > metarodhopsin 3
metarodhopsin2 activates transducin
When rhodopsin undergoes chemical changes in phototransduction, which step is the only one that is reversible?
metarodhopsin1 to metarodhopsin2
In the phototransduction cascade, how does transdusin work?
It is a G protein.
Alpha subunit dissociates and activates PDE.
In turn PDE can catalyse the break down of hundreds of CGMP molecules
How can the phototransduction cascade be inhibited?
1) darkness
2) rhodopsin binds to arrestin or is phosphorylated
3) PDE can recombine with its gamma subunit
4) alpha subunit of transducin can rejoin with its beta-gamma complex
5) low calcium stimulates guanylyl cyclase, which helps recover cGMP
Which photoreceptor is faster at phototransduction?
Which is better at adapting to light
Which is more sensitive
Cones.
Cones are also better at adapting to ambient light.
Cones are comparatively less sensitive compared to rods
Why do cones have a better flicker fusion frequency compared to rods?
They have neurally mediated negative feedback which allows them to turn on and off rapidly. This involves:
1) horizontal cells work antagonistically with cones, providing negative feedback
2) cones release GABA which is inhibitory helping with a faster off response.
After phototransduction, how are cone chromophores recycled?
All trans retinal is reisomerised in the retina to form 11-cis retinal
Faster than cones
After phototransduction, how are rod chromophores recycled?
All trans retinal converted to all trans retinol by retinol dehydrogenase.
All trans retinol is then transported into the RPE via IRBP.
Then converted into 11-cis retinol in the dark
Then converted to 11-cis retinal- cralbp.
11 cis retinal-cralbp then transported back into the retina where it dissociates from cralbp and attaches to opsin.
Slower than cone
Where is vitamin A found?
Carrots, fish and dairy
Where is vitamin A stored
Liver
How is retinol produced from vit A
It is stored in the liver as a retinyl ester and hydrolysed into retinol. It is transported in the serum to the rpe via retinol binding protein
What are the symptoms of vit a deficiency
Poor corneal healing, Xerosis, nyctalopia (night blindness)
In the visual pathway, what are the first and second order neurons?
First = photoreceptors
Second= bipolar cells
What are Parvocellular cells, what are their function and where do they synpase.
Type of ganglion cell, aka p ganglion cell in the retina aka midget cell.
They are responsible for: spatial resolution and colour.
They connect the cone cells to the X cells of the LGN in layers 3-6
What are magnocellular cells, what are their function and where do they signapse.
Type of ganglion cell (M cell) aka parasol cells
They are responsible for motion detection and course vision.
They connect rod cells to the Y cells of the LGN in layers 1 and 2
Which layers of the LGN receive ipsilateral input?
235
Which layers of the LGN receive contralateral input?
146
The optic radiations connect the LGN to…
Primary visual cortex- aka Brodmann area 17
Name the 3 remenants of the fetal hyloid vasculature
-Mittendorfs dot
-Bergmeister papilla
- vascular loops
What is an optic pit? What symptoms can it cause?
Herniation of the retina into the meninges.
Can cause serous retinal detachment.
Causes centrocecal scotoma
What is an optic disc Coloboma and what are the causes
Underdeveloped disk which can result in altidunal field defects.
Causes include:
CHARGE syndrome
Idiopathic
Sjogrens
Trisomy 18 and 22
When do optic nerves become myelinated?
When exiting the lamina cribrosa
In the optic nerve, what is the function of microglia
Macrophages
In the optic nerve, what is the function of astrocytes:
Potassium regulation
In the optic nerve, what is the function of oligodendrocyte:
Myelination
What are the dimensions of the intraocular portion of the optic nerve
3mm by 1.5mm
In the intra ocular aspect of the optic nerve, where do the macular fibres occupy
Centrally the fibres are in the centre of the optic nerve. Distally, they lie laterally of the optic nerve. Nasal fibres lie medially
What is the blood supply to the intraocular aspects of the optic nerve?
Circle of sin which is derived from the short posterior ciliary arteries.
Surface is supplied by branches of the CRA
What’s is the blood supply to the intraorbital section of the optic nerve?
Plial plexus
Collateral branches of the central retinal artery
Where does the intracranial aspect of the optic nerve lie and what is its blood supply
Travels, posterior,superior and medial surrounded by meninges towards the chiasm. Lateral to it is the internal carotids. /
Supplied by plial plexus from superior hypophyseal artery from internal carotid and ophthalmic artery
Where does the intracanalicular aspect of the optic nerve lie?
What is the blood supply?
In the lesser wing of the sphenoid.
Plial plexus, recurrent branch of ophthalmic nerve
What els runs with the intracanalicular aspect of the optic nerve?
Postganglionic sympathetic fibres.
Dura also fuses with periorbita
Inferior, nasal fibres cross the optic chiasm…
Anteriorly and inferiorily, superior fibres, cross posteriorly and superiorly.
This is why craniopharyngioma patients get inferior bitemoporal hemianopia
