Vision Flashcards
what is the sclera and its role?
- non stretchy outer layer, at the front of the eye it becomes the transparent cornea - this is inflated by fluid
- rigid enclosure
why does the production of the aqueous humour have to be carefully controlled?
to keep the eye rigid without causing too much of an increase in pressure - the aqueous humour controls intra-ocular pressure
what can an increase in intra-ocular pressure cause?
glaucoma
what is the cornea and its role?
cornea is transparent but highly curved - acts as the most powerful lens in the eye, as it refracts the light rays the most
what is the lens and its role?
Lens comes after the cornea it bends light rays further and produces fine focus, allowing you to focus on different distances
- the lens is suspended on a ring of suspensory ligaments which arise from the ciliary body
- the ciliary body includes a muscle which can contract and relax and in doing so make the lens fatter or flatter
making the lens fatter has what effect?
retracts the light rays more
the role of the iris?
the iris controls how much light enters the eye via the pupil size
-when you enter a dim room, the iris opens up the pupil
what is the pupils job?
to stay as small as possible whilst also having the brightest image
- staying small improves focus by cutting out the light rays that otherwise go through the edge of the lens
- as you go into a darker environment, your image will become too dark to see properly, so the pupil opens up to let a bit more light in, but at the expense of a bit of fuzziness in the image
what is the neural retina?
an outpost of the brain, generated from the neural tube
- contains a whole neural circuit, which links the photoreceptors (detect the light) to retinal ganglion cells
- retinal ganglion cells have axons that run out via the optic nerve to take the signal to the rest of the brain
what is the retinal pigment epithelium?
important supporting structure which keeps the neural retina alive
where do the 2 optic nerves meet?
the optic chiasm (some of the axons swap over at this point)
after crossing over, what is the route for the optic nerves?
they run through the optic tract, which dives up into the brain
- many of the axons have branches which go down into the brainstem, reaching a variety of brainstem structures that are associated with the control of movement
- the main branch goes to the LGN in the thalamus – specific nucleus in the primary visual pathway
- there, they activate relay cells that carry the signal up to the primary visual cortex, which is in the occipital cortex
- they run in a part of the sub-cortical white matter known as the optic radiation
what is the thalamus?
a specific nucleus in the primary visual pathway
name the 2 photoreceptors?
rods and cones
what are rods?
super sensitive photoreceptors, used for night vision
when the light is too bright what happens to the rods?
the rods simply saturate and become non-functional (anything above twilight levels = rods stop working)
what are cones?
cones are less sensitive, but have the high advantage of being able to work well in high light levels
structure of a cone photoreceptor?
made up of inner and outer segment
-nucleus and protein making machinery – inner segment
- axon which isn’t really an axon as it doesn’t fire AP’s, doesn’t contain an voltage gated channels at the end of the axon
- synaptic terminal releases glutamate as its neurotransmitter – fast excitatory synapse
- outer segment = bag containing tightly packed layers (inholdings) of phospholipid membrane - hold the chromophore in neat layers
why does the cone photoreceptor not fire AP’s?
doesn’t need to fire AP’s because its such a small cell, it can use electrotonic potentials to transmit info from 1 end to another
what is the relevance of the outer segment infoldings holding the chromophore in neat layers?
hold the chromophore in neat layers perpendicular to the light path, ensuring efficient trapping of the light right
why is the resting MP of a cone photoreceptor negative?
because the cell is leaking potassium all the time
resting MP of -45mV
when are the cone photoreceptors depolarised and why?
at rest
-due to sodium channels in the outer segment being open by default
what happens when the light striking the outer segment becomes brighter?
- the cell hyperpolarises
- sodium channels close, cell becomes more negative inside
- prevents glutamate release
what happens when the light striking the outer segment becomes less bright eg. shadow?
- more of the sodium channels open in the outer segment
- depolarisation
- more glutamate release
explain transduction fully
- occurs in membrane discs in outer segment
- sodium channels held open by cGMP, sodium leaking in, cell depolarisation
- photopigment made of opsin (protein) and 11-cis retinaldehyde (light sensitive bit)
- light hits the retinal and converts it from 11-cis to all-trans retinal
- all trans retinal acts as an agonist coupled to a GPCR
- opsin activated
- g-proteins activated, activate enzymes which destroy cGMP
- sodium channels close
So when 11-cis retinal turns into all-trans retinal, it activates the g-protein which activates the enzyme which eats up the cGMP. The cGMP concentration falls, and some of it diffuses away from the sodium channel, allowing them to close.
11-cis retinal vs all-trans retinal - which is more stable and why?
all trans is more stable
-in 11-cis retinal all of the carbon-carbon links that make the tail are in the trans configuration, except for the 11th, which is in the cis-configuration
the cis configuration is less stable. when light hits the retinal, cis bond ruptures and the more stable all trans retinal is formed
termination of transduction
- all-trans retinal is taken away to the retinal pigment epithelium to be reformed as 11-cis retinal
- opsin is enzymatically capped off
- stops activating g-proteins
- another enzyme rebuilds the cyclic GMP
- new molecule of 11-cis retinal attached to opsin
what happens to the photoreceptor when the brightness changes but then remains constant?
- photoreceptor changes its sensitivity
- photoreceptor adapts, membrane potential goes back down to resting - means it can respond rapidly to any tiny change around the new level
-they can constantly respond to minute changes in brightness without becoming saturated
Retinitis pigmentosa
genetic disease that wipes out photoreceptors, starting in the peripheral vision but moving inwards (fovea is kept for the longest time)
age related macular degeneration
lose the centre of vision first, then lose the ability to read and recognise faces at the same time.
lose the fovea = lose most of your visual function
photoreceptors positions in the retina
cones sit far apart with loads of rods in between
-in daylight, rods are non functional so during the day there are big gaps in the sampling array
what gather info from photoreceptors?
bipolar cells gather info from a whole pool of photoreceptors (convergence) before passing it on to the ganglion cells
what gather info from photoreceptors?
fovea
region where all of the retina layers, apart from the photoreceptors, have been pushed to one side
- no image blur, no convergence
- no rods. There are only red and green cones here, super thin and packed super tightly together
- excellent sampling array
why are there no blue cones in the fovea?
No blue cones because red and green are associated with fine detail
is there convergence in the fovea?
no, the ganglion cells cells that gather info from these photoreceptors gather info from a single cone each – signals are kept completely separate all the way back to the primary visual cortex
-sampling info remains uncontaminated all the way back to the visual cortex
what does the majority of the retina serve?
coarse vision
peripheral vs central vision
peripheral
- the visual image is optically blurred
- cone photoreceptors are large and widely spaced (separated by larger number of rods)
- the signals from many cones converge onto single ganglion cells
central
- good focus – overlying layers are absent
- only red and green cones - narrow and closely packed
- signals from photoreceptors kept separate throughout the primary visual pathway
what does the left side of the image correlate to?
the right side of both retinae
what do retinal ganglion cells report?
changes in illumination from one location to another
action of inhibitory interneurons?
- ganglion cell receives input from a single cone somewhere in central vision
- the ganglion cell also receives information from inhibitory interneurons - pick up information from the pool of cones around that cone
- ganglion cell is not responding to absolute brightness, but instead responding to contrast/changes in brightness/centre vs the surround
what does the ganglion cell respond to?
doesn’t respond to absolute brightness, but instead responds to changes in brightness/centre vs the surround
on centre ganglion cells
central photoreceptor hyperpolarised (blue) by increased illumination
bipolar cell depolarised by inverting synapse, excites ganglion cell
off centre ganglion cells
central photoreceptor depolarised (red) by decreased illumination
bipolar and ganglion cells depolarised by excitatory synapses
parvocellular vs magnocellular
parvocellular
small field with strong surround
fine resolution
accurately follows changes in light
needs stable image
magnocellular
large field with weak surround
coarse resolution
transient responses to change
responds well to fast movement
so, the visual cortex gets this coarse image first, and then the parvocellular comes and fills in the detail.
wavelengths and bipolar cells
-bistratified - comparing an input from blue cones with an inhibitory input from red plus green cones
BLUE vs YELLOW
-parvocellular - comparing the signal from red cones in the middle compared with green cones around the outside
RED vs GREEN
what do Inferotemporal visual areas encode?
information about object identity
what do Parietal visual areas encode?
information about location and movement
