Visual System
Structures
Retina
Fovea
cotains ONLY cones
no blood vessels @ center so photons hit it directly
Axons of ganglion cells leave retina through optic disk
enter optic nerve
no receptor @ optic disk = blind spot
cornea & lens project physical image of world onto retina
retina converts optical image –> neural image
Optical Image –> Neural Image
3 interconnected layers & 5 classes of neurons
Lateral Info Flow
Vertical Info Flow
1) transduction of image by photoreceptors
2) photoreceptors synapse on bipolar cells
3) bipolar cells synapse onto ganglion cells
each synaptic stage, laterally connecting neurons = horizontal cells & amacrine cells
modify forward transmission across synaptic layer (largely by inhibition
Several Cell Types
impossible to encode all info in optical image using single neural image
parallel pathways
different cells used to create parallel circuits for simultaneous transmission of multiple neural images
Photoreceptors (Rods & Cones)
transduce light into electro chemical energy
Rods
Cones
more sensitive to light
more disks
more photopigment to absorb light
less sensitive to light
use only cones @ high light levels
3 types of cones for short, medium & long wavelengths
only 1 rod pigment
use only rods in starlight conditions (scotopic vision)
achromatic: 1 channel responding only to intensity (grayscale)
greatest density outside the fovea
Light & Dark Reactions
Dark Current
rods & cones depolarized in darkness
Na+ ions flow in through channels in outer segment
K+ ions flow out through channels in inner segment
In light, channel in outer segment close –> hyperpolarization
Visual Pigment Molecules
w/in membranous discs of rods & cones
absorb ee & divert energy into biological process
formed by combining chromophore w/ opsin
rhodopsin = GPCR; ligand = 11-cis retinal (chromophore)
mechanism for phototransduction = same for rods & cones
involves cGMp as intracellular messenger; cGMP "gates" Na+ channels
chromophore: 11-cis retinal, changes shape when absorbs photon of light
opsin: large, membrane-bound protein, regulates biochemical processes w/in cell
In Darkness:
In Light:
1) high concentration of cGMP
2) inward Na+ current through cGMP-gated channels = dark current
3) depolarization causes transmitter release (glutamate) @ terminal
approx. 50pA (inward current) keeping Vm @ -40 mV
1) light activates rhodopsin
2) transducin (G-protein) = active
3) PDE (effector enzyme) = activated
4) PDE activity reduces cGMP level
5) cGMP-gated Na+ channels close – no Na+ current (dark current)
6) cell hyperpolarizes to -70 mV –> transmitter release stops
Retinal Ganglion Cells
Receptive Fields
1) set photoreceptors to which cell is connected
2) part of visual field in which various stimuli can affect discharge rate of cell
Vertical Pathway
Lateral Pathway
photoreceptor–>bipolar cell –>ganglion cell
photoreceptor –>horizontal cell –>bipolar cell –>ganglion cell
horizontal cells mediate inhibition
forms inhibitory surround RF
forms center of RF
Center-surround RF Organization
ON-center
OFF-center
Vertical Pathway
Lateral Pathway
light in RF center –> photoreceptor hyperpolarized –> bipolar cell depolarized
light in receptive field surround –> photoreceptor hyperpolarized –> bipolar cell hyperpolarized
inhibition
RF of ganglion cell – 2 parts = center & surround = mutually antagonistic
surround mediates inhibition of response to stimuli that is effective in center
center = much more sensitive, but surround = much larger
center = generated by vertical connections photoreceptors –> bipolar cell –> ganglion cells
surround = lateral pathway between cones & horizontal cells
Vertical Pathway
Lateral Pathway
light in RF surround –> photoreceptor hyperpolarized –> horizontal cell depolarized –> bipolar cell depolarized
light in RF center –> photoreceptor hyperpolarized –> horizontal cell hyperpolarized –> bipolar cell hyperpolarized
center surround receptive fields cause signals to be contrast signal (difference between center & surround)
M & P Cells
P cells
small receptive fields
M Cells
selective to particular wavelengths of light (color)
connected w/ analysis of fine detail & color
terminate in P (parvocellular layers of LGN
large receptive fields
not wavelength selective
responds well to large objects & movement
terminate in M (magnocellular) layers of LGN
Non-M Non-P (blue-yellow) Cells
small receptive fields
selective to particular wavelength of light (blue center/yellow R+G OFF surround)
terminate K (koniocellular) layers of LGN
3 Signals
Bipolar Cells
OFF Bipolars
ON Bipolars
Glutamate
depolarized by glutamate/hyperpolarized by light
hyperpolarized by glutamate/depolarized by light
ionotropic receptors –> opens cation channels & depolarizes OFF bipolars
metabotropic receptors –> closes cation channels –> hyperpolarizes ON bipolar
1) luminance signal: black n& white from red & green
red-green color signal: compare activity of red & green cones
blue-yellow color signal: compare activity of luminance signal & blue cones