vision B Flashcards
how many photoreceptors are in the eye
130 mill
what signals do photoreceptors transform light, intensity, colour pattterns into
chemical and electrical signals that trigger activity of retinal cells
photoreceptors to ganglion cells
130 to 1
what do we use to see the retinal inner surface
clinical opthalmoscope
where is the optic disc located
nasaly
what is the optic disc
our blind spot
where isthe macula/ fovea located
temporally
what do rays not have to pass through
retinal pigment epithelium, choriocapilllaris, sclera to reach the epithelium
cones
colour specific and high acuity
concentrated in the fovea
end stage diferentiated and non proliferative
rods
no colour specificity and low acuity
high sensitivity
in the peripheral retina
end stage diferentiated and non proliferative
3 classes of cones
trichromat person has normal colour vision
s cones- short wavelength- blue and purple
m cones- middle wavelength- green
l cones- long wavelength- yellow and red
monochromats
totally colour blind with one cone pathway plus rod pathway
dichromats
a cone photopigment missing thus only has two cone channels
anomalous trichromats
all three cone photopigments with one photopigment anomalous in having a shifted peak sensitivity
where are the retinal photoreceptors located
anterior to retinal pigment epithelium and behind all other transparent neural retinal cell layers
retinal photoreceptors relationship with RPE - retinal pigment epithelium
have a close physical nad functional relationship
what do retinal photoreceptors convert
photonic energy to chemical signals by virtue of thier special cellular structure consisting of an inner cell body and an outer segment of photopigment
photoreceptor renewal
stacks of discs of visual pigment molecules pf photoreceptors are constantly renewed with new discs added at the base of the outer segment displacing the old discs up the outer segment and pinched off at the tips and engulfed by retinal pigment epithelium on a diurmal cycle peaking in the morning
rhodopsin in the dark
unstimulated, cgmp is available and binds to cation channels and those channels and kep t open by the cgmp wihch depolarizes the photoreceptor
rhodopsin in light
stimulated starting a cascade involving transducin and phosphodiesterase PDE becomes more activated which then degrades cgmp. cgmp levels begin to decrease and cation channels begin to close and start to hyperpolarize the photoreceptor
what sends impulses to the visual cortex
Bipolar cells and ganglion cells
what does photoreceptor hyperpolarization lead to
decreased release of inhibitory neurotransitter such as glutamate onto downstream retinal neurons which leads to increased action potentials in these downstream neurons
rod to ganglion cell connections
high convergence- less cell to cell connections
cone to ganglion cell connections
low convergence- more cell to cell connections
why is rods resolution low
bc of the way the wiring is done and rods have high convergence so lower resolution
what do horizontal cells do
connect photoreceptors laterally and dampen cells surrounding the centre of maximal receptive stimulation. inhibit surrounding less stimulated cells and effect lateral inhibition
cone cells are
receptive fields for GC
ganglion cells serve
cone based receptive fields
what are the two types of ganglion cells that cone photoreceptors synapse with
on and off centre ganglion cells
on centre, off surround field
excites ganglion cell that response to on centre light
inhibit off centre ganglion cell
off centre, on surround field
inhibits ganglion cell that responds to on centre light
exites off centre ganglion cell
diffuse light on both centre and surround
weak response from both on and off centre ganlgion cells
what kind of inhibition do peripheral cones do
compared to foveal cones their connect to more horizontal cells which further inhibit neighbouring cells getting weaker stimuli which optimizes acuity
temporal and nasal axons crossing
temporal stay temporal and nasal cross at optic chiasm
retinal pathology
showed by clinical opthalomoscope
retinopathy
what is the macula
extension of the fovea
optic disc location
nasal and fovea is temporal
retinal pigment epithelium purpose
to take up the garbage made by the photoreceptors
what do horizontal cells do
allow photoreceptor cells to modulate each other
rhodopsin
a type of opsin
