NEUS 609 - Visual Processing & the Retina

pupil, cornea, lens, retina

light entre the eye through the __, and get refracted by the ___, before the __ refract it a bit more to focus it on the ___ aka the back of the eyeball where the photoreceptors are. 

phototransduction

transforming light energy into neural transmission

• type of transduction used by photoreceptors

pigmented epithelium

layer past the outer segments of photoreceptors, at the very back of the eyeball

• colored layer

pigmented epithelium funcitons

• phagocytosis of old photoreceptor discs as new ones get made and added to the bottom of outer segment

• recycling of 11cis-retinal light absorbing molecule

• reducing back scattering of light

fovea

dip in the retina located at the center where most of the light is focused, and there is an increase in packing of photoreceptors, bipolar cells, and ganglion output neurons

• increase visual acuity

faveola

center of the fovea where any vasculature and other cell types are pushed to the side to limit light diffraction by overlaying cells

• entirely made of cones, thus only active during daytime

• rods start taking over the further from it you go

inside out

retina has an ___ structure: photoreceptors are at the back of the eye near the pigmented epithelium while the ganglion cells are closer to the vitreous components/chamber

optic disc, optic nerve

axons of ganglion cells coalesce/converge to pass through the retina and reach the brain; the convergence happens at the ___ which then form the optic nerve

• ___ occurs on the nasal side of the retina, creating a theoretical blind spot, however, because it happens on both eyes, there is no blind spot as the potential missing visual info is picked up by the other eye. AKA it is never in BOTH blind spots at once.

structure of retina

___ from the back of the eye to the vitreous chamber:

• pigmented epithelium

• photoreceptor outer segments

• outer nuclear layer (cell bodies of photoreceptors)

• outer plexiform layer (synapses between photoreceptors and bipolar cells)

• inner nuclear layer (cell bodies of bipolar cells)

• inner plexiform layer (synapses between bipolar cells and ganglion cells)

• ganglion cell layer (cell bodies and extending axons of ganglion cells)

outersegment

outmost part of rods and cones that is closer to the pigmented epithelium/the back of the eye

rods

• photoreceptor type

• more sensitive to light (can detect a single photon) = night vision

• intracellular discs containing rhodopsin and 1-cis-retinal

• longer, straight, and thinner

• response is in the seconds range

cones

• photoreceptor type with 3 subtypes

• less sensitive to light = daytime vision 

• discs with opsin and 11-cis-retinal are part of plasma membrane

• shorter, thicker, and cone shaped

• response is in the milliseconds range

inner segment

link between outer segment of photoreceptors and cell body; contains protein synthesis machinery and mitochondria

visual pigment

opsin protein and its covalently bonded 11-cis-retinal light absorbing molecule

rhodopsin mechanism

1. light absorbed by 11-cis-retinal causing change in conformation to all-trans retinal

2. conformation change activates opsin molecule into metarhodopsin II state which forces GDP out of associated G-protein transducin allowing it to bind GTP.

3. GTP-bound G-protein dissociates into alpha & beta-gamma subunits, and activates cGMP phosphodiesterase which decreases cAMP concentration

4. lower cAMP levels close cation channels, causing the cells to hyperpolarize

3 types of cones

• S-wave = short wave = blue absorbing

• M-wave = medium waves = green absorbing

• L-wave = long wave = red absorbing

cones terminals

___ contain multiple invaginations to connect with multiple parts of the bipolar/horizontal cell

• synaptic ribbon for continuous release

rods terminals

__ contain only 1 invagination and thus can only connect with 1 part of the following cell

• synaptic ribbon for continuous release

lateral inhibition

bipolar cell bodies share the inner nuclear layer with interneurons (amacrine and horizontal cells) that mediate ___

AII amacrine cells

Rods always synapse on ON bipolar cells, which DO NOT directly connect with retinal ganglion cells. Instead, they connect to ___, which then connect to the ON or OFF cones bipolar cells

• rods use cones bipolar machinery to communicate with ganglion cells

OFF bipolar cell

• AMPA/Kainate ion channels

• DARK: high GLU release from photoreceptor, cation channels open, cell depolarizes and release GLU

• LIGHT: low GLU release from photoreceptor, cation channels not as active, cell hyperpolarize and is silent (do not release NTs)

ON bipolar cells

• mGluR6 metabotropic receptors

• DARK: high GLU release from photoreceptor, mGluR6 active and inhibiting TRPM1 cation channels, cell hyperpolarize and is silent

• LIGHT: low GLU released by photoreceptor, mGluR6 less active, TRPM1 cation channels open, cell depolarizes and releases GLU

superficial, deep

ON and OFF bipolar cell synapses are segregated within the inner plexiform layer:

• OFF bipolar cells synapses are in the ___ IPL

• ON bipolar cells synapses are in the ___ IPL

midget cells

bipolar cells receiving input from ONLY ONE cone

• visual/spatial acuity

diffuse bipolar cell

bipolar cells receiving input from MULTIPLE cones

• color vision, movement

receptive field, opposite

ganglion cells have a preferred ___ with a center zone and surround region

• maximum response happens when the center and the surround regions have ___ signals

• same signal in both center and surround regions = weak response

changes

ganglion cells in the retina respond better to ___ in light levels

• some cells have a sustained response (seconds)

• some cells have a more transient response

• stronger response to rapid changes

continuous

___ illumination of an object in same position on the retina will cause it to fade from consciousness

• why eyes make small movement automatically

horizontal, amacrine

surround effect of receptive field is mediated by lateral inhibition from ___ and ___ cells

• focus on passing signal from center zone of stimuli

light adaptation

process during which retina adjusts its response to match and maximize the differences in light intensities in reflected image