the eye Flashcards
describe the pathway of the retina
photoreceptors –> bipolar cells –> ganglion cells
what is a photopigment-
absorb light and will trigger changes in membrane potential
describe Photoreceptor convergence
Many photoreceptors feed (convergence) into individual ganglion cells in the periphery but Individual photoreceptors feed into individual ganglion cells in the central retina
what are the steps of visual processing
retina –> lateral geniculate nucleus (LGN, thalamus) –> visual cortex
what is the Receptive field centr
direct connection from photoreceptors
Whatever response is triggered in the centre, the opposite will be triggered by the surround
what so amacrine cells allow for
modulate transmission onto ganglion cells – lateral inhibition
what is in cones
– 3 photopigments
colour vision
what is in rods
– one photopigment- Rhodopsin
black/white vision
how do you fix longsightedness?
convex lens
describe depolarisation of bipolar cells
Bound to glutamate (during dark), mGluR6 causes closing of Na+ channels.
what are the layers of the retina
Pigmented Epithelium
Photoreceptor Outer Segment
outer nucleus layer
outer plexiform
inner nucleus layer
inner plexiform
ganglio cell layer
why is there 3 Photopigments in cones
Photopigments of cones require more energy to be activated
how do you fix shortsightedness?
concave lens
what is Phototransduction
Conversion of light energy into membrane potential changes – NOT Action Potentials
what is Receptive field surround
connection from photoreceptors through horizontal cells
what is the role of photoreceptors
transform electromagnetic radiation into electrical signals - Phototransduction
what happens on-centre in ganglion cell receptive field
ganglion produces APs when light is shone on the photoreceptor that directly innervates it
ganglion decreases APs
when light is turned off
describe Phototransduction in light (rods)
Photoreceptors will hyperpolarize with light
- Light activates rhodopsin and a G protein called transducin
- Transducin-binding GTP activates PDE (phosphodiesterase)
- Active PDE breaks down cGMP
- Low levels of cGMP promote channel closing
- Na+ influx decreases and the membrane hyperpolarizes
- Membrane hyperpolarization reduces glutamate release
what do horizontal cells allow for
modulate transmission onto
bipolar cells - lateral inhibition
why do ganglion cell responses reflect differences in contrast
there is No change in firing rate if both centre and surround are in same level of illumination
the Greatest difference in firing is when you have most contrast between the centre and surround
describe Phototransduction in dark (rods)
Photoreceptors are depolarised in darkness (-30mV) due to a “dark current” of Na+
Na+ channels open, rate of transmitter release is high
- Guanylyl Cyclase produces cGMP in the dark
- cGMP binds and activates Na+ channels
- Na+ influx depolarizes the
membrane (not AP) - Membrane depolarization induces liberation of Glutamate
what are the steps of vision
- transmission and refraction of light by the optics of the eye
- the transduction of light energy into
electrical signals by photoreceptors - the refinement of these signals by synaptic interactions within the neural circuits of the retina
describe hyperpolarisation
Hyperpolarisation of photoreceptor by light reduces glutamate release, reduces mGluR6 binding and allows Na+ channels to open on the bipolar cell
describe Phototransduction
Graded changes in membrane potentials → change in rate of transmitter release
Light: hyperpolarisation, Na+ channels close, decrease in rate of transmitter release
what can bipolar cells be depolarised or hyperpolarised by
glutamate
what is the role of ganglion cells
Ganglion cells are the only ones generating AP in the retina, and the only output of information to the rest of the brain
what is in the fovea
only cones
what happens off-centre in ganglion cell receptive field
ganglion decreases APs when light is shone on photoreceptor that innervates it
ganglion increases APs when
light is turned off
what are the main differences between rods and cones
rods are highly sensitive to light but cones require bright light
120 million roda in retina nad 6 million cones
rods are for night vision (black and white), cones are for colour vision
rods are in peripheral retins and cones are in fovea (central)
rods have high convergence with bipolar cells
what is is hyperopia
longsightedness
