lecture 7 - evelyne sernagor Flashcards
definition of vision
the transduction of visible light into neural signals, creating a perception of ‘what’ is ‘where’
where does light enter the eye through
through the pupil
once the light goes through the pupil where does it fall
on the retina
light will cause impulses to for generated in the output cells which form what
the optic nerve
where will the signals eventually reach
the primary visual cortex/V1
where is the primary visual cortex located
in the occipital lobe at the back of the cortex
where do the signals go after V1
V2
then it will split into what two streams
ventral and dorsal streams
what does the ventral stream deal with
object identification
what does the dorsal stream deal with
spatial location
what does the external layer of the eye consist of
the sclera, cornea and limbus
what is the sclera
the white of the eye which is continuous with the dura mater
what is the cornea
translucent surface
what is the limbus
connects the cornea to the sclera
what is the intermediate layer of the eye called
the uvea
the anterior part of the uvea consists of…
the iris, ciliary body, pupil and lens
the posterior part of the uvea consists of…
choroid
what is the choroid
layer that lies between the sclera
and the retina.
It is made up of blood vessels and capillaries that bring oxygen and nutrients to the outer retina
what is the ciliary body
consists of the ciliary muscles that will control the shape of the lens
what does the internal layer of the eye consist of
the retina
which part of the eye is the sensory part of the eye
the retina
behind the retina there is the RPE which stands for
the retinal pigment epithelium which helps the health and survival of the retina
the other most layer of the sclera is called the…
episclera
what does the episclera contain
blood vessels
the episclera appears yellow in old people, why?
due to fatty deposits in the blood vessels
what covers the anterior of the eye
the cornea
the sclera merges with the cornea at the…
limbus
where is the lens located
directly behind the iris
the iris is connected to the ciliary body by…
suspensory ligaments
ciliary muscles enable the elastic lens to…
change shape (accommodation)
what does accommodation allow
change of focus depending viewing distance
images are formed on the retina by light refraction (bending). what parts of the eye cause this
the cornea and the lens
refraction and focusing forms an _________ image on the retina
inverted
what supplies 2/3 of the refractive power
the cornea
why are images blurred in water
because refraction index of water and cornea are similar
define focal length
distance from refractive surface to point where parallel light rays converge
what is the focal length for ideal eye
around 22mm
what is the distance from the lens to the retina
17mm
what is accommodation
dynamic changes in the refractive power of the lens
the ciliary muscles contract and release tension on suspensory ligaments
focus on a far object
the ciliary muscles contract making the lens flatter
focus on a near object
the ciliary muscles will relax and the lens will be rounder and bend the light more
myopia
the eye is longer causing the light to be focussed in front of the retina
hyperopia
eye is too short
or the lens is losing elasticity (aging) (need reading glasses)
astigmatism
the cornea or lens have an irregular shape so you get focus at different points
1 degree visual angle ~
300 um on the retina
the entire retina covers about 200 degrees (in diameter)
retinal organisation
ONL - photoreceptors
INL - horizontal, bipolar, amacrine cells
GCL - ganglion cells
glial cell - muller cells (spans the entire thickness of the retina and provides support)
synaptic layers of the reteina
OPL - synaptic contacts between photoreceptors, horizontal and bipolar cells
IPL - synaptic connections between bipolar amacrine and ganglion cells
retinal neurotransmitters:
vertical connects are carried by…
glutamate (photoreceptors, bipolar cells, ganglion cells)
horizontal connections are mostly…
GABA (horizontal and amacrine)
some use glycine and dopamine,
starburst amacrine cells use ACh
why is the retina inverted
receptors are extremely metabolically active.
blood vessels provide nutrients and oxygen and the receptors are directly underneath as the retina is inverted.
what is the fovea
a depression in the retina which is the centre of focus for images
why are inner layers push aside in the fovea
so when the light comes in it will hit the photoreceptors straight away
in the middle of the fovea (the foveola) there is only…
cones
where is the blind spot
just above the optic disc (no photoreceptors)
the outer segments of the photoreceptors are stacks of membrane discs containing…
the visual pigment molecules (rhodopsin and cone opsins)
disks are constantly renewed (eaten) by the,,,
RPE
morning (light) triggers ______ disk phagocytosis
rod
evening (darkness) triggers _____ disk phagocytosis
cone
inner segment of photoreceptors
mitochondria, ribosomes and membrane where opsins are assembled to become part of outer segment discs
synaptic terminal of photoreceptors is in the…
OPL where glutamate is released to bipolar cells and horizontal cells
peripheral vision:
rode mediate phototransduction and they’re responsible for our ability to see…
dim light (scotopic vision)
rods are highly sensitive meaning they…
can detect tiny changes in light intensity
scotopic vision is monochromatic meaning…
colourblind
produces low resolution images (poor spatial acuity and respond slowly to changes in illumination
central vision:
cones mediate responses, they’re concentrated around the…
fovea
cones work only in…
day light (photopic vision)
cones have ____ sensitivity
low
operate over wider range of light intensities.
photopic vision is chromatic meaning..
colour vision
cones have ____ spatial acuity
high
3 types of cones:
S (blue), M (green), L (red)
phototransduction
enabled by a visual pigment in the outer segment discs
pigment: opsin + chromophore
the chromophore is what is sensitive to light, when light hits it at the right wavelength the chromophore will change conformation and that will trigger a change in the protein opsin which will then trigger the whole process
phototransduction cascade:
in the dark, the chromophore is in the 11-cis conformation and when a photon of light hits it, is will isomerise into…
all-trans conformation.
this will activate the rhodopsin to become…
metarhosopsin II
this activated GTP binding protein called…
transducin
this then activated phosphodiesterase (PDE) to hydrolyse…
cGMP which is bound to cationic channels in the outer segment membrane
this causes closure of the membrane bound
cGMP-gated cation channels (sodium and calcium)
the cyclic GMP-gated channel is open in the dark and allows _____ and ______ into the receptor
sodium
calcium
sodium moves into the cell down its gradient, there is a constant flow of sodium into the cell in the dark and this is called the
dark current
when light hits the photoreceptor, the channel will close and stop the influx of sodium causing membrane…
hyperpolarisation
photoreceptors, in dark, release glutamate onto bipolar cells and horizontal cells in the OPL. hyperpolarisation will lead to…
less glutamate release
these are graded potentials not APs so the stronger the light flash,,,
the stronger the response
due to the constant influx of sodium the resting potential is less negative than other cells around…
-40mV
amplification system
rods have so many discs with photopigment that are stacked on top of each other so each one of them will react when light hits it so all the responses will accumulate and cause a very big response
melanopsin expressing ganglion cells respond directly to light. these cells project to the SCN in the hypothalamus via the retina hypothalamic tract which is made of the axons of these cells. they fire during day time controlling the…
body clock
photosensitivity in ganglion cells:
small percentage of ganglion cells contain pigment in their membrane called…
melanopsin
what is the mesopic range
the range where rods and cones are both active (scotopic and photopic vision overlaps)
surface reflectance =
fraction of incident light reflected by a surface
surface luminance =
absolute amount of light emitted from a particular area
for example
if the source of intensity is 1000 and the surface reflectance is 10% the surface luminance will be 100. if the surface reflectance from a darker surface was 5% the surface luminance would be
50
light adaptation
- happens in cones
- occurs when we move from dark into bright light
- it is a temporal adjustment of photoreceptor responsiveness to changing illumination
- shifts the entire operating range of the cones
slope of the response curve is steep in the middle (high gain) and shallow at the ends (low gain)
gain =
change in receptor response / change in light level
at any given adaptation level, cones rapidly adjust their response over a range of _____ intensity levels
10^4
on a slower time scale cones can shift their operating range to the right or left over a range of ____ intensity levels
10^9
rods do not shift their operating range
they saturate
light adaptation resets the operating range to be around the current….
average light level
RGCs have responses mostly determined by…
stimulus contrast rather than by differences in absolute luminance levels
it allows good contrast discrimination near that average level, at expense of…
poor discrimination between objects that are much darker or much brighter
role of calcium in light adaptation:
high intracellular calcium inhibits cGMP synthesis (the cationic channels in the outer segment of the photoreceptors are gated by cGMP. cGMP is bound to the inner side of the channel in the dark and the channel is open allowing sodium and calcium to flow)
light induced closure of cGMP channels leads to a net decrease in…
intracellular calcium concentration
adaptation mechanisms are triggered by decrease in intracellular calcium:
there will be increased activity of guanylate cyclase (the enzyme that synthesises cGMP) leading to an increase in cGMP levels causing the channels to open again
a decrease in calcium will increase the affinity of the cGMP-gated channels for cGMP (through calmodulin), weakening the impact of light induced decrease of cGMP levels
dark adaptation occurs when…
we move from bright light into darkness
shifts operation from…
cones to rods
initially we only see blackness because cones stop functioning in low light intensity and the rods are not yet function because…
rhodopsin has bleached in the light
rhodopsin starts regenerating in the dark and retinal sensitivity increase with time (can take up to 30 mins)
the pupil also adapts to changing light levels
the rod cone break is the point when the rods take over from the cones
each part of the visual scene is detected by a number of photoreceptors.
each RGC gathers information (via bipolar cells) from that part of the visual fields, this is called the…
receptive field
the centre surround receptive field:
off centre ganglion cell
when the light is off in the centre you get the strongest response
on centre ganglion cell
when the light is on in the centre you get the strongest response
the centre signal (where its on or off) is mediated by bipolar cells.
centre surround antagonism is due to lateral inhibition provided by the…
horizontal cells
when the light is on in the centre and off in the surround:
light hits the photoreceptor in the centre of the field causing…
hyperpolarisation there
therefore less ________ is released from the cone to the bipolar cell
glutamate
there are ON bipolar cells and OFF bipolar cells.
on the dendrites of the ON bipolar cell the is ___________ ___________ receptors
metabotropic glutamatergic receptors (mGLUR6)
what do these receptors do
invert the signal therefore the ON bipolar cells depolarise if there is less glutamate release
the OFF bipolar cells have what type of receptors
ionotropic AMPA kinase receptors which preserve the signal.
so less glutamate means there is hyperpolarisation of the OFF bipolar cell
ON centre ganglion cell receives input from an ON centre bipolar cell
the ON bipolar cell will release more glutamate as they have been depolarised and the ganglion cell will fire. this is why ON centre ganglion cells fire when there is light on the centre
the OFF centre will not fire as the bipolar cell is inhibited and it is hyperpolarised
when light is off in the centre and on in the surround:
light hits to photoreceptor in the surround of the field causing the centre to _________ therefore there is more glutamate release
depolarise
this will inhibit the ON bipolar cell as it inverts the signal (mGLUR6) and it will excite the OFF bipolar cell (AMPA receptors)
therefore the OFF centre ganglion cells will fire when there is dark in the centre and the ON centre ganglion cell will be…
inhibited
lateral interactions from horizontal cells:
horizontal cells from the surround release GABA onto photoreceptor terminal in centre.
Is GABA inhibitory or excitatory
inhibitory
if there is dark in the centre (more glutamate release and excite the OFF centre bipolar and ganglion cells) and if the surround is in the light (less glutamate release onto the horizontal cells meaning theyre less depolarised) the horizontal cells project back into the terminalc of the adjacent photoreceptors in the centre. therefore, the horizontal cells will release ____ GABA to the terminal which will enhance the glutamate release in the centre photoreceptors even more
less
the stronger the difference between the centre and the surround the…
stronger the response will be
