Vision Flashcards
(29 cards)
Where is aqueous humour stored?
In the anterior and posterior chambers.
What is the function of the aqueous humour?
To provide nutrients to the cornea (since the cornea doesn’t have a blood supply).
Describe the course of flow of aqueous humour.
- It is produced by the ciliary body in the anterior chamber.
- It is then secreted into the posterior chamber behind the iris.
- It returns and is reabsorbed in the anterior chamber.
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.
List the types of glaucoma.
Describe the pathophysiology of each type.
1 - Angle-closure (iris adheres to the cornea, blocking reabsorption).
2 - Open-angle (sclerosis of veins draining the eye).
List 3 treatments for glaucoma.
1 - Surgery.
2 - Beta blockers (to reduce production of aqueous humour).
3 - Prostaglandin analogues to increase blood drainage of the eye.
List the cells of the retina.
Describe the organisation of these cells in the retina.
1 - Photoreceptors at the most superficial surface.
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.
What is the function of horizontal and amacrine cells?
They modulate the transmission of information by providing alternate pathways.
List the types of photoreceptors.
1 - Rods.
2 - Cones.
Where are rods found?
What is the density of rods in this location?
- In intracellular membrane disks.
- They are found in relatively high densities.
Where are cones found?
What is the density of rods in this location?
- In infoldings of surface membranes.
- They are found in relatively low densities.
How many types of rods exist?
List the photopigment(s) involved in each type.
- 1.
- Rhodopsin.
How many types of cones exist?
List the photopigment(s) involved in each type.
- 3:
- Red, green and blue photopigments.
List 2 functional differences 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.
What is the fovea?
The central part of the retina.
Which cell type predominates in the fovea?
Cone cells.
Which cell type predominates in the peripheral retina?
Rod cells.
Why are rod cells highly sensitive to light?
What is the compensation for this?
- Because many rods connect to one ganglion cell, so more light is transmitted per ganglion cell.
- The compensation is that rods produce low visual acuity.
Why do cone cells have a low sensitivity to light?
What is the tradeoff for this?
- Because only one cone cell only connects to one ganglion cell, so less light is transmitted per ganglion cell.
- The tradeoff is that cones produce high visual acuity.
List the components of rhodopsin.
1 - Retinal (the chromophore - part of the molecule that responds to light).
2 - An opsin (a GPCR), known as transducin in photoreceptors.
Describe the mechanism by which rhodopsin in rod cells responds to light.
- When there is no light, a cGMP-gated ion channel causes Na+ and K+ influx, causing depolarisation of -30mV.
- This results in glutamate release at the synapse.
- When there is light, the retinal is activated, and changes conformation to an all-trans structure.
- This causes the opsin (transducin) to activate phosphodiesterase, which catalyses the breakdown of cGMP to GMP.
- The loss of Na+ and K+ influx causes hyperpolarisation and therefore stops glutamate release.
List the types of bipolar and ganglion cells.
What is their state of depolarisation in the presence of light?
What is the implication of this with regards to the effect of glutamate on depolarisation?
1 - On cells (depolarised in light).
2 - Off cells (hyperpolarised in light).
- Since glutamate is being released by photoreceptors in the dark, this must mean that:
- Glutamate hyperpolarises on cells.
- Glutamate depolarises off cells.
