Neuro: Vision Flashcards
What are the 2 halves of the retina called?
- Temporal retina - half closest to temporal bone of skull
- Nasal retina - half closest to the nose
When looking at the world you have a temporal field of view and a nasal field of view, define each type of visual field
- Temporal field of vision - field of vision towards temporal bone
- Nasal field of vision - field of vision towards the nose
What part of the retina focuses light coming from temporal field of view and what part of retina focuses light coming from nasal field of view?
- Temporal retina focuse light from nasal field of view
- Nasal retina focuses light from temporal field of view
What is the sclera of the eye?
Sclera is the white fibrous outer layer of the eye
What is the cornea and how is it transparant?
- Cornea is the transparant front part of the eye
- It’s transparant because of the way that the collagen and cells that make up the sclera arrange themselves within the cornea
Why is there a need for the eye to produce intraocular pressure?
Intraocular pressure needed to keep the sclera rigid as it is flexible to a certain degree
How is intraocular pressure created within the eye?
Generated by aqueous humour (fluid that fills the space between cornea and lens)
What part of the eye produces aqueous humour and what part of the eye drains excess aqueous humour away?
- Produced by the ciliary body
- Excess aqueous humour drains into angle of the eye
As you get older what happens to the vitreous humour that fills most of the eye as someone gts older?
- Proteins that make up vitreous humour clump together
- This causes vitreous humour to shrink down and pull away from retina
What parts of the eye make up its optics?
- Cornea
- Lens
- Iris (pupil)
How is the lens able to change shape?
- Lens suspeneded by suspensory ligaments from the ciliary body
- Ciliary body contains ring of muscle and so when that muscle contracts or relaxes it causes the lens to change shape
What is the function of the iris?
Controls the diameter and size of the pupil and thus the amount of light that reaches the retina
Why does the iris need to control the size of the pupil?
- Iris needs to control size of pupil to ensure that light doesn’t hit the edge of the lens
- If light does hit edge of lens it won’t be focused onto retina properly
What are the different parts of the retina?
- Neural retina
- Retinal pigment epithelium
What are the functions of the retinal pigment epithelium?
- Provides biochemical support to photoreceptors
- Prevents retina from peeling away
What does the fact that both the retina and the retinal pigment epithelium developed from the neural tube mean for these structures?
Means they are both Central nervous sytem structures
Describe the pathway of the optic nerve from the retina to the primary visual cortex (Primary visual pathway)
- Axons of retinal ganglion cells within the neural retina run along surface of retina and converge to form optic nerve
- Optic nerve runs from retina down to the optic chiasm
- They continue from optic chiasm down optic tract to lateral geniculate nucleus (LGN)
- Cells of lateral geniculate nucleus project axons through optic radiation to the primary visual cortex
What happens to the axons of the retinal ganglion cells once they reach optic chiasm?
- Axons of retinal ganglion that are formed from the nasal retina cross over at optic chiasm
- Axons of retinal ganglion within nasal retina of the right eye cross over into the left side of the brain
- Axons of retinal ganglion that are formed from temporal retina DO NOT cross over at optic chiasm
What type of vision are the rod and cone photoreceptors responsible for?
- Rod photoreceptor - Night vision
- Cone photoreceptor - Day vision
What are the 2 main segments of the cone photoreceptor? What strictures does each segment contain?
- Outer segment - Phospholipid membrane discs
- Inner segment - Nucleus, axon and synaptic terminal
Why is the phospholipid membrane arranged into discs within the outer segement of the cone photoreceptor?
It arranges the chromophore between the membrane discs into layers perpendicular to light path
Describe what is happening within a cone photoreceptor when it’s a resting potential
- Resting potential of cone photoreceptor around -45mV (slightly depolarised)
- At resting potential K+ moves out of inner segement via K+ channels
- Within outer segment there are Na+ channels open that allow influx of Na+ into outer segement
- This causes slight depolarisation which allows for release of Glutamate from synaptic terminal
Describe what happens when light starts to hit cone photoreceptor
- Light hitting outer segement causes some of the Na+ channels on outer segement to close leading to less Na+ influx and less depolarisation
- K+ channels of inner segement still open so cone photoreceptor becomes hyperpolarised due to K+ efflux
- Hyperpolarisation prevents glutamate release from synaptic terminal
Within the outer segment what molecule is responsible for keeping the Na+ channels open?
cGMP
Within the membrane discs of the outer segement of the cone photoreceptor are photopigments. What 2 molecules make up photopigments?
- Opsin
- Retinal: 11-cis retinaldehyde
Explain how light hitting the outer segment causes the closure of the Na+ channels within it
- Light hitting membrane discs of outer segment causes 11-cis retinaldehyde within the photopigment to be converted into all-trans retinaldehyde
- This change causes activation of the photopigment
- Activated photopigment then activates G proteins and the G proteins will then go on to activate particular enzymes
- These enzymes will digest the cGMP within the outer segment
- This results in cGMP not being present to keep Na+ channels open causing them to close
Explain how the Na+ channels are reopened and how photopigments restored once light stops hitting cone photoreceptors
- G-proteins get inactivated - enzymes responsible for degradtion of cGMP can’t get activated
- All-trans retinaldehyde removed from opsin of photopigment to prevent activation of more G-proteins
- Enzyme then works to produce cGMP which result in re-opening Na+ channels
- New molecule of 11-cis retinaldehyde attches to opsin of photopigment
Why is peripheral vision quite blurry?
- Within the peripheral retina there are 10 times as many rods as there are cones so there big gaps between cone photoreceptors in peripheral retina
- Multiple cone photoreceptors connected to one bipolar cell which connects photoreceptors to retinal ganglion - this convergence increases pixel size making image less refined
- Also, light has to pass through many layers of retinal tissue before it reaches peripheral retina so even before light hits retina image is blurry
What part of the retina is responsible for central vision?
- Fovea centralis within the central retina
How is the fovea centralis adapted to produce very clear/fine central vision?
- Within fovea centralis there’s a region called the foveal pit
- In the foveal pit there’s no retinal tissue so light directly hits the photoreceptors in this region
- There are also only cone photoreceptors in foveal pit - Specifically only red and green cone photoreceptors as these are associated with fine detail
- Cone photoreceptors are thin and closely packed
- Each cone photoreceptor is connected to its own bipolar cell and therefore its own retinal ganglion (no convergence)
When a person is looking at an image and the signals from the retinal ganglion reach the LGN and the primary visual cortex what do they form?
- They form a retinoptopic map within the LGN and primary visual cortex based on what the person is looking at
- These retinoptopic maps will have expanded regions for what is within the persons central vision and much smaller regions for what is in their peripheral vision
Explain how the cone photoreceptors are able to respond to even small changes in illumination from one moment to another
- When eyes first move to new location brightness of light hitting particular cone photoreceptor causes hyperpolarisation or depolarisation within that cone
- However, when eye rests in one position for a particular amount of time cone photoreceptor adapts to amount of light hitting it by resetting itself to its resting membrane potential
- This allows that cone photoreceptor to then respond to another change in illumination
Explain how lateral inhibition is able to allow cone photoreceptors to adapt to changes in illumination
- Cone photoreceptors in central retina are connected to single retinal ganglion via single bipolar cell
- Surronding the cone photoreceptors in central retina are cone photoreceptors in peripheral retina
- These peripheral cone photoreceptors are connected to inhibitory interneurones which send signals to retinal ganglion
- Differences in illumination between central and peripheral cone photoreceptors will result in differing amounts of excitation and inhibition resulting in a response in the ganglion cell
- If illumination is the same then inhibition from peripheral cones and excitation from central cones cancel each otehr out leading to no response in ganglion cell
What type of retinal ganglion cell respond to increases in illumination and what type of retinal ganglion cell responds to decreases in illumination?
- “off” centre retinal ganglion respond to decreases in illumination
- “on” centre retinal ganglion respond to increases in illumination
How are “on” centre retinal ganglion able to respond to increases in illumination?
- Central photoreceptor hyperpolarised by increased illumination
- Bipolar cell depolarised by inverting synapse which causes excitation of ganglion cell
How are “off” centre retinal ganglion able to respond to increases in illumination?
- Central photoreceptor depolarised (red) by decreased illumination
- Bipolar and ganglion cells depolarised by excitatory synapses
What are the 2 main types of retinal ganglion cell? Give some characteristics for each type
- Magncellular - lots of convergence, large receptive field, low resolution
- Parvocellular - no converge, small receptive field, high resolution
What wavelengths of light do the parvocellular ganglion cells detect?
- Parvocellular ganglion cells detect/compare red and green wavelengths of light as they are connected to red and green cone photoreceptors
What other wavelengths of light can retinal ganglion cells detect and what type of retinal ganglion is responsible for this?
- Detect/compare blue and yellow wavelengths of light
- Bistratified retinal ganaglion responsible for this
Once the primary visual cortex receives information from the axons of the retinal ganglion about an image it sends that information to different areas of the brain. What are some of these brain areas?
- Sends info to cortical area which processes colour
- Sends info to inferotemporal region which processes info about object identity
