Lecture 20- Vision 2 Flashcards
What are the different ‘layers’ of the retina and what type of cells are contained at each part?
- Light sensitive cells (photoreceptors) are furthest from incoming light (at back of retina)
- Retinal ganglion cells exist at the front of the eye but are also the output cells turning into the optic nerve
- Information flows between the photoreceptors and ganglion cells via interneurons which exist in the ‘middle’ layer of the retina
What structure largely makes up the neural component of the eye?
The retina- it is considered part of the brain
What colour is the back of the retina and why?
Black due to choroid pigment
Name 3 types of interneurons and there functions?
- Bipolar cells (2 projections): act as connections between photoreceptors and retinal ganglion
- Horizontal cells: mediate and take charge of connections between photo receptors and bipolar cells
- Amacrine cell: mediate connection between bipolar and retinal ganglion cell
What are the two types of photoreceptors? Compare and contrast them?
- Rods
- A lot (120 million per retina)
- Function in low light (night vision)
- Not colour sensitive (monochromatic) - Cones
- Few than rods (8 million per retina)
- Require high levels of light (day vision)
- 3 types sensitive to red, green and blue light
How does the structure/ look of rods and cones differ according to their function?
- Obviously shape is in their name
- Another difference is while the end piece of rods is large and made of free floating discs. The cone end piece is relatively small and is a folding membrane as opposed to separate discs (less SA).
- Ultimately it’s these structural differences that make rods a lot more sensitive to light and mean they can do night vision while cones need high levels of light and thus can only do day vision.
Broadly what makes photoreceptors sensitive to light?
Photopigments
What are the two components of photo pigments?
1) A membrane spanning protein called an “opsin”
2) A chromophore called Retinal, aVitamin A derivative
How does the membrane spanning pigment opsin differ between rods and cones?
- Rods have Rhodopsin
- Cones have either S(blue),M(green),or L(red) Photopsin (means can detect colour)
What does a vitamin A deficiency cause?
Night blindness= where unable to adapt to low light conditions
Describe phototransduction in the dark…
– In absence of light, Retinal is non-activated (inactive 11-cis isoform)
– Intracelluar [cGMP] is high
– cGMP-gated channels open
– Much Na influx (dark current)
– Photoreceptor depolarised (~ -35mV)
– Lots of glutamate released onto bipolar cells
Describe phototransduction in the light..
– Light energy (photons)
– Retinal changed to active all-trans isoform
– Trans-retinal activates a G protein (transducin)
– Transducin activates cGMP phosphodiesterase, breaking down cGMP
– Less cGMP, cGMP-gated channels close
– Photoreceptor hyperpolarised (~ -60mV)
– Less glutamate release
What is the only light dependent step in vision?
When retinal is changed to it’s active all-trans form as opposed to inactive 11-cis isoform
Why does phototransduction seem ‘backwards’ in the presence versus the absence of light? Does this matter?
- In light would think would have excitation and neurotransmitter release but we don’t. This is instead what happens in the dark.
- Really it doesn’t matter what is important to recognize is that phototransduction is simply different in light and darkness
What feature of cones allow colour vision?
- Cones each have one of 3 types of photopsin, most sensitive to either short wavelengths (S-photopsin, blue), medium wavelengths (M-photopsin, green), or long wavelengths (Lphotopsin, red)
- Light of specific wavelengths is reflected from coloured objects.
- Perception of colour created by relative activation of 3 cone types.
What is meant by the term relative activation when talking about cone types?
In order see colours other than just blue, green and red we activate relative levels of multiple cone types e.g. for yellow activate some green and some red cones.
