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.
How many colours can a human perceive (optimal conditions)?
1 million
True of false colour blindness can be both inherited (congenital) or acquired (from disease)?
True
What are some diseases that damage the optic nerve/ retina?
- Glaucoma, increase in pressure in the anterior chamber (aqueous humour)
- Diabetes
- Alzheimer’s and Parkinson’s diseases
What gender has colour blindness more commonly and why?
- Congenital (inherited) forms affect 8% of males and 0.5% of females (although varies with ethnicities)
- This because the genes encoding for M & L opsins (i.e. green & red sensitivity) exists on the x chromosome, therefore more likely males would get it as they only need 1 dud females would need two.
Is there just one version of colour blindness (R/G)?
No, but R/G is most common
At night, intracellular cGMP levels are high BECAUSE cGMP-gated channels are open
A- Both statements are correct and causally related
B- Both statements are correct and not causally
related
C- First statement is correct, second is incorrect
D- Second statement is incorrect, first is correct
E- Both statements are incorrect
B- Both statements are correct and not causally
related
What is the concept of retinal processing?
- The idea that there is this network of photoreceptors and inter neurons leading to ganglionic cells
- As one photoreceptor is activated and releases neurotransmitter is effects the inter-neurons it is connected to which in turn effect the ones they are connected to and so on
What does the output to ganglion cells critically depend on?
Spatial and temporal pattern of light stimulation on the retina.
Explain the concept of the receptive field in the context of the retina?
- The inputs to each ganglion cell arise from neighbouring photoreceptors in a circumscribed area of the retina, the cell’s RECEPTIVE FIELD
- Basically, each ganglion has a specific set of photoreceptors and interneurons that feed information to it
What is the shape of ganglion receptive fields and the two divisions that exist? What are the receptive fields subsequently called?
-Roughly circular
-Are divided into two parts:
• A circular central zone (the “centre”)
• An annulus around the centre (the “surround”)
-Say they have a center surround receptive field
What do ganglion cells respond optimally to?
-Contrast i.e. need differential illumination of the centre and the surround
How does an on center ganglion receptive field differ to an off center one? What is consistent with both?
- On center: light falling on the center of the receptive fields results in an increase in action potential firing (excites it). Light falling on the annular results in a reduction in action potential firing.
- Off center: opposite occurs. Light falling on the center of the receptive field results reduction in action potential firing. Light falling on annulus results in an increase in action potential firing.
- Both: when there is equal light falling on the center and the surround there is no change in firing. This is called diffuse illumination.
What is acuity?
-Acuity= how strong vision is
When and where is visual acuity the highest?
- Low convergence ganglion cells (not as many inputs to the ganglion receiving less info makes it clearer).
- Small receptive fields, high acuity
- Cones, fovea
When and where is visual acuity the lowest?
- High convergence ganglion cells (there is a lot of different inputs and therefore lots of different information from receptors and interneurons).
- Large receptive fields, low acuity
- Rods & cones, periphery
What is an example of a cell seen in the periphery as opposed to the fovea?
- Periphery= Parasol cell (large dendritic tree= large receptive fields= low visual acuity)
- Fovea= Midget cell (small dendritic tree= small receptive fields= high visual acuity)
Describe the visual pathway…
- Retinal ganglion cells send axons to visual centers in the brain in such as way that preserves spatial relationships between visual stimuli
- Visual info from objects in left visual space is processed in right brain, and vice versa. For this to happen, some info must cross to opposite side of brain, some must remain on same side.
- By this means, information concerning images from visual space falling on equivalent parts of the two retinae is brought together for central processing
How many ganglion cells are there per retina?
1 million (i.e. 1 million nerve cells per optic nerve)
What accounts for our blind spot? How does the brain overcome the blindspot?
- There is a blind spot in each eye due to the axons leaving the retina at the optic disc (no photoreceptors at this point)
- Our eyes compensate for each other meaning we never see the blind spot (unless cover/close one eye), the brain decodes information and attempts to fill gaps in our vision
What are the two parts of the retina and which parts cross at the optic chiasma?
(draw a diagram)
- Nasal retina (closest to nose)= will cross at optic chiasm
- Temporal retina (closest to temples) will not cross at optic chiasm
What words mean cross and don’t cross respectively?
- Contralateral= cross (nasal retina)
- Ipsilateral= don’t cross (temporal retina)
What is the function of the visual cortex in processing information?
-The place where objects in and out of their visual context are analyzed in
detail.
-This analysis is based on form (cells responding to edges and corners), movement, and colour (respond to wavelength). All this done in relation to background (remember cells of the eye work via contrast)
-All this information is used to discriminate between objects and their backgrounds
After the optic chiasm (where cross) where do the fibres go for processing of visual info?
- LGN
- Then to visual cortex
- After visual cortex information can be sent to… Temporal lobe- for identification of objects in visual space (the “what”) or the Parietal lobe- for processing of information regarding the location of objects in space (the “where”)
- This information is then passed to the frontal cortex to assist in making decisions (the action part)
What 4 main subcortical areas do ganglionic cells project to and what is the role of each?
1) Superior colliculus
• Concerned with eye movements and orientation to visual
stimuli
2) Lateral Geniculate Nucleus
• Concerned with the sensation of vision
3) Pretectum
• Control of pupils (constriction during the near response)
4) Suprachiasmatic nucleus
• Control of diurnal rhythms
