BIOL 2480 - Visual Neuroscience

Light comes into retina and hits

photoreceptors

Photoreceptor turn OFF when lights come

ON

•Cellular mechanisms

Photoreceptors send into to

bipolar and then ganglion cells

Relevant visual pathways from

retina to the brain

Light comes through the

cornea and then lens

Cornea and lens both

•refract light to focus image on retina

Lens focuses by

changing shape by ciliary muscles

Iris changes pupil size to

regulate amount of light

Retina is where

phototransduction occurs

When focusing on a distant object the lens becomes

thinner and flatter

When focusing on a distant object the lens becomes thinner and flatter and does little

refraction

When focusing on closer object, lens

thickens and refracts more

Phototransductoin = turning photon (light) stimulation into

neural impulses

Image projection is

inverted and topographic

Retina is split into

nasal vs temporal

Retina is split into nasal vs temporal and

superior vs inferior

Right visual field is seen by

right nasal retina

Right visual field is seen by right nasal retina and left

temporal retina

Binocular visual field is where

two eyes overlap

Each eye has both

contralateral and ipsilateral projectionsq

Nasal is toward

middle/nose

temporal is on

outside

Nasal retinal axons go

contralateral,

Nasal is toward middle/nose, temporal is on outside

Nasal retinal axons go contralateral, axons from temporal go

ipsilateral

Info from left visual field goes to

right optic tract and vise versa

•Binocular visual field is where two eyes overlap, making the

optic chiasm

Lateral geniculate nucleus -

visual part of thalamus

Superior colliculi

coordinate rapid eye movements (saccades)

Suprachiasmatic nucleus of hypothalamus

circadian rhythyms

Visual pathways: last location of vision

•Visual cortex

Lateral geniculate nucleus, is the 1st stop for axons, the main area of

initial innervation

Photoreceptors (PR) - detect

•light

•Rods & Cones

Bipolar Cells - synapse with

PR

Ganglion cells (GC) - synapse with

bipolar and send info to brain

•Horizontal cells - at PR/bipolar

synapse

Amacrine cells - at

bipolar/GC synapse

Basic visual transmission follows 3-neuron pathway of

PR ->BC ->GC; other two cell types modify input

Horizontal cells are inhibitory interneurons that span several

photoreceptor cells

Horizontal cells are inhibitory interneurons that span several photoreceptor cells and can inhibit PR to form

center-surround organization

Horizontal cells are inhibitory interneurons that span several photoreceptor cells and can inhibit PR to form center-surround organization important for

light contrast and edge detection.

Amacrine cells are inhibitory interneurons that can span multiple

BC/GC synapses

Amacrine cells are inhibitory interneurons that can span multiple BC/GC synapses and can provide

directionally-sensitive motion cues (inhibit GC that detect movement in "wrong" direction).

Changing light stimuli (photons) into

neural impulses

PR have

•graded potentials

PR have graded potentials and are

hyperpolarized in presence of light.

A very few GC (<5% of total) are actually

photosensitive

A very few GC (<5% of total) are actually photosensitive and are

depolarized by light

A very few GC (<5% of total) are actually photosensitive and are depolarized by light, they send input directly to

hypothalamus

For photoreceptors, a less intense light flash causes a few

Na+ channels to close

For photoreceptors, a less intense light flash causes a few Na+ channels to close, the most intense flash causes all the

Na+ channels to close

Light sensitive GC project to suprachiasmatic nucleus for

circadian rhythm control

Rods & Cones have outer segments that contain

photopigments

Outer segments have a transmembrane protein called

opsin

Outer segments have a transmembrane protein called opsin surrounding a chemical called

retinal

Photons hitting retinal change its

conformation

Photons hitting retinal change its configuration, which changes opsin

configuration

Opsin change sets off

cellular cascade, in photoreceptors

Outer segment is

photopigments

inner segment is

cell nucleus and synaptic terminals

Rods and different types of cones have differences in

opsin configuration, which affects retinal response to different wavelengths of light

When light hits opsin, changes conformation which makes the

TM protein change shape, setting off transduction

Opsin configuration change sets off

G-protein cascade

Opsin activates

transducin,

Opsin activates transducin, turns GDP into

GTP

Transducin activates

phosphodiesterase

Phosphodiesterase reduces concentration of

cGMP

Reduction in cGMP closes

cGMP-gated ion channels

Inactive transducin (a G-protein) is bound to

GDP.

opsin reconfiguration activates transducin and it exchanges GDP for

GTP

Activated transducin then activates

phosphodiesterase

Activated transducin then activates phosphodiesterase, which hydrolyses

cGMP, lowering its concentration.

Reduction in cGMP then closes

cGMP-gated ion channels on disk membrane

In the dark, cGMP channels are

open,

In the dark, cGMP channels are open, allowing

•allowing Na+ and Ca2+ in, with some K+ leak channels

In the light, cGMP decreases, channels close so cell

•becomes more negative as K+ channels stay open

Causes decrease of neurotransmitter release in the

light in a graded fashion

•Causes decrease of neurotransmitter release in the light in a graded fashion, Allows

signal amplification

Single activated rhodopsin can activate

transducins

Single activated rhodopsin can activate transducins and each phosphodiesterase can break down up to

6 cGMP resulting in closure of about 200 ion channels, about 2% of the ion channels in the outer segment

This cascade can reset itself fairly

quickly

More positive ions close the more

light there is

•In the light, cGMP decreases, channels close so cell becomes more negative as K+ channels stay open, with the channels being

hyperpolarized as positive ions are not flowing through it

•So in light, photoreceptor NT release stops. How do bipolar cells react?

•On-Center vs Off-center bipolar cells

Off-center cells receive more

•glutamate when center is dark, so they depolarize

On-center cells have 2nd messenger pathway that closes

depolarizing channels.

When light shines on them they

reopen channel and get depolarized

in the dark, the depolarized rods and cones release glutamate continually onto the

bipolar cells they connect to.

For an off-center bipolar cell, a dark patch of a visual scene falling in the center of the receptive field will

will depolarize the receptors and lead to the release of glutamate

The receptor for glutamate in off-center bipolar cells depolarizes the cell, so if the photoreceptors in the center are in the dark, the off-center bipolar cell receives

receives transmitter and is depolarized: an off response.

On-center bipolar cells have a different receptor for glutamate, one that leads to activation of a

second messenger that closes depolarizing channels.

Light on the center of the receptive field interrupts the release of transmitter from

photoreceptors

Light on the center of the receptive field interrupts the release of transmitter from photoreceptors, which stops activation of the messenger that

closes depolarizing channels in the bipolar cells.

Those channels then open, and the bipolar cell depolarizes, producing an

on-response

center surround activity - center has different activity than the

surrounding

Off-center cells release more glutamate when it is

dark, being excited in the dark

GC (ganglion cells) also follow

center-surround organization

Horizontal and amacrine cells form

network across retina to modify activity

GC: Entire system works to emphasize

contrast, rather than absolute brightness

on center GC's are most excited in

light

Off center GC is most excited in

dark