Vision Flashcards
the eye is design to
focus light on specialised nerve endings that translate light wave and photons into pictures our brink an understand
light which reaches photoreceptors of the retina
and sends signals via the visual pathways tot he usual cortex in the occipital lobes
human visible light wave lengths
400>550nm
what makes up the ouster fibrous layer of the eye
cornea, conjunctiva and sclera
middle vascular layer
iris, ciliary body and chorois
inner neural layer
retain and single layer of retinal pigmented epithelium
the lens
focuses light waves onto the retina and specifically the fovea
iris
is a circular structure with a central opening - the pupil
what divides the internal space into an anterior chamber in the front of the iris and a posterior chamber between the iris and the lens
iris
posterior and anterior chambers
contain aqueous humour secreted by ciliary body
vitreous humour
fills the main cavity of the eye behind the lens and helps maintain the shape of the eye
central retinal artery
provides nutrients and oxygen for the retina
central retinal vein
take away oxygen poor blood to be oxygenated
tetina
light sensitive part f the ye containing photoreceptors
fovea
is the most light sensitive part of the eye- most colour snesiitve
optic nerve
where photoreceptors of the retina send signal via the visual pathways to the visual cortex i the occipital lobe
optic nerve is famous as being
a blind spot- no photoreceptors
the retina contains
the neuronal tissue of the eye
5 types of neurones in the retina
rods, cones, amacrine, horizontal, bipolar cells
the fovea only contains
cones- an area which provides high visual acuity
rods are
Monochromatic vision Low intensity light Single photon Equally distributed throughout the retina rhodopsin as visual pigment
cones
Colour vision High light threshold Higher acuity Hundreds of photons Clustered in the macula lutea Within the macula lutea is a clustered pit called the fovea, which only contains cones Highest visual acuity is found here
photoreceptors are locate
at the back of the retina
order of neurones in the retina
photoreceptor synapses with bipolar cell with synapses with ganglion cell
ganglion cells and bipolar cells are neurones where
signals from rods and cones are integrated
rhodopsin
rods
iodopsin
cones
- colour blindness results from defects in cone opsins
phototransduction occurs via which G protein
Gt
what does absorption of light do to colour pigments
isomerise them
-starting a cascade of chemical reactant
photo transduction in rods
1) in the dark Sodium (ad calcium) ion influx though cGMP- controlled cation channels which depolarised the rod cells to about -40mV
2) when photon is absorbed by 11-cis retinal it is isomers to all trans retinal (photoisomerisation)
3) rhodopsin is activates and bind to a GTP-bound protein called transducin
4) The GTP-bound alpha submit of transducer dissociates and activates a phosphodiesterase with hydrolyses cGMP- 5’-CMP
5) this closes the cation channels causes hyperpolarisation
when photoreceptors aren’t stimulated by light
depolarisation
- open cGMP channels
when photoreceptors are stimulated by light
hyperpolarisation
-cGMP channels closed
Bipolar cells connect photoreceptors to
ganglionic cells
ganglionic cells
nerves leave the eye at the optic disk dn become the optic nerve
horizontal cells
revive input from multiple photoreceptor cells
- regulated activity of photoreceptor cells and makes adjustments to signals sent to bipolar cells
amacrine cells
receive signals form bipolar cells
-regulate and integrate bipolar and ganglionic cells
which is the transmitter of the visual pathway
glutamate- the main excitatory nt in the brain
phototransduction pathway
1) light travels through layers of transparent neurones- ganglion, amacrine, bipolar and horizontal
2) and is absorbed by the rods and cones a the back of the retina
30 visual information process through several layers of neurones
4) and finally converges on ganglion cells,which send their axons to the brain
smaller receptive fields
e. g. 1 ganglion per photoreceptor
- lots of detail
large receptive filed
e. g. 3 cones synapse with 1 ganglion
- summarises info from 3 cones
- less detail
when photoreceptors are not stimulated by light
constant state of depolarisation
-constnat release of Glu
when the centre of the cone is stimulated by light
the photoreceptor becomes activates
-stimulation result in hyper polarisation due to national channels being close– > less glutamate produced
on centre cells
Glu is inhibitory
1) decrease in Glu release from cone due to hyper polarisation caused by light stimulation
2) due to less Glu inhibition, there is increased depolarisation of the bipolar cells
3) therefore the bipolar cells will release more Glu
4) glu is always excitatory for ganglionic cells
5) therefore INCREASE IN FIRING RATE
off centre cells
Glu is excitatory
1) less Glu due to hyper polarisation of cone
2) therefore bipolar cell become hyper polarised
3) decrease in Glu release form bipolar cells
4) decreased firing rate of off-metre ganglionic cells
direct pathway in darkness- on centre
Increase in glutamate is inhibitory
Therefore increase in MetabotropicGluReceptor
Therefore hyperpolarisation
Decrease in Glu release
Therefore ganglionic cell synapsing with it are less likely to reach threshold
Action potential is less likely.
direct pathway in darkness - off centre
Excited by Glu
Therefore becomes more depolarized
Ganglion which synapses with cell is more likely to reach threshold
Increase in likelyhood of AP
APs are less likely in
positive centre
APs are more likely in
negative centre- tells us it is dark
Direct pathway light- On-centre
Decrease in glutamate is excitatory
Therefore decrease in MetabotropicGluReceptor
Therefore more depolarization
Increase in Glu release
Therefore ganglionic cell synapsing with it are more likely to reach threshold
Action potential is more likely.
Direct pathway light- Off-centre
Inhibited by decreased Glu
Therefore becomes more hyperpolarized and releases less Glu
Ganglion which synapses with cell is less likely to reach threshold
decrease in likelihood of AP
indirect pathway no light
horizontal and amacrine cells release GABA- inhibitory- contributing to telling us it is dark
indirect with light
Horizontal and amacrine cells release less GABA
