Chapter 4 - Vision Part 1 (The Retina) Flashcards
Which part of the eye has the highest concentration of neurons?
Fovea
How many layers are there in the retina, and list them.
Ganglion cell layer –> Bipolar, amacrine and horizontal cell layer –> Photoreceptor layer.
Describe how a blind spot is created.
The optic nerve exits the retina through the optic disc, which has no retinal tissue.
Is the photoreceptor nearer to the anterior or posterior part of the retina?
In the middle but towards the posterior part.
Why do we not experience blind spots? (3)
1) Blind spot is a place with no retinal tissue, which means no photoreceptors. No photoreceptors mean you don’t perceive a lack of perception (you cannot see that you can’t see smth)
2) Our brain may “fill-in” the information that is unavailable to the senses, by extrapolating fro neighbouring locations.
3) The blind spots from both eyes don’t overlap.
How are photons absorbed by photoreceptors?
- Photons are absorbed by photoreceptors.
- Bipolar cells then synapse onto ganglion cells, which serves as the output cells of the retina.
- they send axons into the brain through the optic nerve, and hence optic nerve contains axons of retinal ganglion cells. - Horizontal and amacrine cells have side connections and they are important modulators of the activity of photoreceptors, bipolar and ganglion cells.
What is counterintuitive about the design of the retina, and how is that resolved?
CI: Light need to pass through ganglion layer and BAH layer in order to get to the photoreceptors.
Resolved: Fovea has a morphological specialization. Most of the tissue is pushed to the side, allowing light to go straight to the photoreceptors in the area.
Does the parafovea or the peripheral retina have high density of rods?
Parafovea.
Peripheral retina has a little bit of everything
What are the differences between rods and cones? (3)
Hint: DSL: Density, sensitivity, location
1) C: At the fovea, the density is very high, but very quickly the density becomes very low.
At the fovea, rods are virtually absent. But right around the fovea (parafoveal area) there is the highest density of rods.
Cones have lower sensitivity, which means that during the day, when the light shines strongly, their activity reflects changes in luminosity. However, at night, where it’s much darker, cones are completely quiet.
Explains why during the day, when you want to look at something, you place the object straight on your fovea.
2) Rods are hypersensitive. During the night, where luminosity is very low, rods are able to modulate their activity based on small changes in luminosity. However, during the day, they are all saturated, so small changes in luminosity will not affect their activity.
Explains why at night, if the light is very dim, if you want to look at something, you do not look at it directly, but rather you look at a location just adjacent to the object.
This way, the light coming from the object falls on the parafoveal region, which contains the highest density of rods.
3) If we zoom in on the fovea, cones are tightly packed.
If we look at a part of the retina in the periphery, that is, a region that receives light from far away from where you’re looking at directly, there will be both kinds of photoreceptors here - rods and cones.
Cones here will be very large, and rods here will be smaller.
Define phototransduction.
conversion of light energy to electrochemical energy
*note: remember, in this process, light energy MUST be absorbed somewhere in order for conversion to occur. don’t be STUPID.
Where are rhodopsin molecules found in?
Outer segment of photoreceptor cells.
How is light energy absorbed by rhodopsin molecules?
- Rhodopsin molecules are photopigment proteins that sit on top of the membrane.
- Rhodopsin molecules contain this small molecule called retinal, which sits in the middle of these rhodopsin molecules.
- Initially, these retinal’s 11th and 12th carbon atoms are in the cis formation, meaning their H atoms are facing the same direction –> 11-cis retinal.
- A photon hits the rhodopsin molecule, and the retinal might absorb the light energy from this photon.
- The absorption changes the retinal’s structure from 11-cis retinal to 11-trans retinal, meaning that the H atoms are facing the opposite direction now.
- In order to accommodate this change in retinal structure, the whole rhodopsin molecule has to change its structure too.
Briefly describe how the absorption of photons leads to vision.
- When it is dark, ion channels are open. This allows the flow of positive ions into the cell, making it depolarized. This allows photoreceptors to release more neurotransmitters.
- Photoreceptors more active in the dark.
- They do not generate action potentials. The amount of neurotransmitters released depends on how depolarized the cells are. - Transducin attaches itself to rhodopsin but only when light absorption has occurred. When that happens, transducin attaches itself to rhodopsin, and trades the GDP it was holding for a GTP, both of which are sources of energy. This causes the cleavage of transducin.
- Cleavage occurs, and the alpha part of the transducin molecule attaches itself to this molecule called phosphodiesterase. This activates the phosphodiesterase and causes the cGMP floating around to become GMP.
- However, in order for the ion channels to be open, cGMP must be attached to it. The cGMP previously attached to these channels were converted to GMP too, hence prompting the closure of ion channels. Positive ions stop coming in, hence depolarizing the cell.
- However, there are still K+ channels that are open, hence hyperdepolarizing the cell. This results in the decrease in release of neurotransmitters as the outward current of K+ is dominating the cell.
When the photoreceptors are depolarized, what is their voltage?
Just below 0 mV. While positive ions are entering the cell, there are still always-open K+ channels that ensure the steady stream of K+ out of the cell
How is the phototransduction signal amplified?
For every rhodopsin activated, there will be around 800 transducing molecules cleaved.
For every transducin molecule cleaved, there will be one phosphodiesterase that becomes active, which means that 800 will become active.
This will lead to roughly 4800 conversion of cyclic GMP to GMP, which in turn will lead to the closure of roughly 200 ion channels
Biochemical cascade in phototransduction amplifies the signal many times: one photoreceptor will close hundreds of ion channels.
