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

on:off g-cell

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