L07 - Vision Flashcards
What is aqueous humour?
- It is secreted into the posterior chamber behind the iris
- It returns and is reabsorbed in the anterior chamber
- It provides nutrients to the cornea (since the cornea doesn’t have a blood supply)
What is glaucoma?
- Visual impairment due to an increase in pressure, which in turn is due to a decrease in removal (by reabsorption) of aqueous humour
- Decrease in removal caused by:
1 - Angle-closure (iris adheres to the cornea, blocking reabsorption)
2 - Open-angle (sclerosis of veins draining the eye)
What treatments are available for glaucoma?
1 - Surgery
2 - Beta blockers (to reduce production of aqueous humour)
3 - Prostaglandin analogues to increase blood drainage of the eye
How are cells within the retina organised?
1 - Photoreceptors at the back of the retina
2 - Bipolar cells synapse with photoreceptors, connecting them to:
3 - Ganglion cells, which transmit the signal by merging into the optic nerve
4 - Horizontal cells synapse at 2 different photoreceptor-bipolar cell synapses
5 - Amacrine cells synapse with 2 amacrine cell-optic nerve synapses
Which cells of the retina generate action potentials?
Ganglion cells only
Where are rods found?
What is the density of rods in this location?
What photopigments do they contain?
- In intracellular membrane disks
- They are found in relatively high densities
- Rhodopsin
Where are cones found?
What is the density of rods in this location?
What photopigments do they contain?
- In infoldings of surface membranes
- They are found in relatively low densities
- Red, green and blue photopigments
What 2 functional differences are there between rods and cones?
- Rods are very sensitive to light, and therefore contribute to night vision, whereas cones are not very sensitive to light, and contribute to daytime vision
- Rods are not sensitive to colour, whereas cones are
What causes the blind spot?
It is caused by the the area of the eye where the optic nerve leaves the eye
Which cell type predominates in the fovea?
Which cell type predominates in the peripheral retina?
- Fovea: cone cells
- Peripheral retina: rod cells
Why are rod cells highly sensitive to light?
What is the compensation for this?
- High sensitivity to light due to high photopigment (rhodopsin) density
- However, low visual acuity as many rod cells connect to only one ganglion cell
Why do cone cells have a low sensitivity to light?
What is the tradeoff for this?
- Low sensitivity to light due to low photopigment density
- However, high visual acuity as one cone cell connects to one ganglion cell
How does rhodopsin in rod cells respond to light?
- In dark: rhodopsin is inactive, but cGMP-gated cation channels on rod cell surface membrane are open → Na+ influx (= dark current) → cell depolarises to -30mV → glutamate release at synapse with bipolar cell
- In light: photons hit retinal → H atom flips (11-cis => all trans) → opsin activated → GTP binds to G-protein transducin → activates PDE → PDE-mediated breakdown of cGMP to GMP → channel closure → hyperpolarisation
What are the types of bipolar and ganglion cells and what is their state of depolarisation in light?
- ON bipolar & ganglion cells = depolarised in light
- Dark: glutamate → hyperpolarisation (not active)
- Light: ↓ glutamate → depolarisation
- OFF bipolar & ganglion cells = hyperpolarised in light
- Dark: glutamate → depolarisation
- Light: ↓ glutamate → hyperpolarisation
What are receptive fields?
- The region of the retina that influences a bipolar or ganglion cell
- Central part - direct connections from photoreceptor
- Surround part - indirect connections via horizontal/amacrine cells
