VSP 2 Lecture 2 Retinal Physio Part 2

Neural convergence

compression of information needs to occur

(over a hunfred million PR and only a million GC)

100:1 ratio

some info is lost (blurred vision)

Spontaneous activity/ maintained discharge

RGC are always active -when not stimualted they send signals at 5-40/sec

visual signals are superimposed on this background

In the periphery

Rods

- large spacing

- lower density

large convergence 300:1

GC intergrate information from a large area 3 degrees

lower acuity

In the fovea

cones

- small spacing

- highest density

small convergence 1:1

GC integrate information from a small area (0.03 degrees)

higher acuity

There are ___ type of Retinal ganglion cells

18

Ganglion cells connect

bipolar

amacrine

each type of ganglin cell specialized for encoding a particalar aspect of the visual world

contrast

color

size

direction

movement

GC produce

full action potentials

all or none

Action potential

do not degrade over distance

slow conduction speed

Action potential conduction speed is increased by

myelin

saltatory transmission --> action potential is regenerated at gaps in the myelin sheath

gaps in the myelin sheath are called

Nodes of Ranvier

On Ganglion cell

turned on by light (increase AP)

turned off by darkness (decrease AP)

synapse with On BP cell

Off GC

turned on by darkness (increase action potential)

turned off by light (decrease action potential)

transient GC

respond strongly just as light is turned off or on

weakly if light stays on

conencted to both On and Off bipolar cells

is stimualted and inhibited by white light

Ganglion Cell 2 classes

Midget (Retinal Parvo) GC

Parasol (retinal Magno) GC

Midget (retinal parvo)

project to the parvocellular layers of the LGN (layers 3-6)

Parasol (retinal magno) GC

project to the magnocellular layers of theLGN (1-2)

On center midget BP cells synapse with

on center midget (retinal parvo) ganglion cells

Off center midget BP cells synapse with

off center midget (retinal parvo) ganglion cells

On center diffuse bipolar cells synapse with

on center parasol (retinal magno) ganglin cells

Off center diffuse bipolar cells synapse with

Off-center parasol (retinal magno) ganglion cells

Midget Ganglion Cell

in the fovea: 1 cone to midget1 Bp to 1 midget GC

mostly input from cones

sustained response to light

high spatial resolution

poor temporal resolution

color sensitive

Parasol (magno) Ganglion cell

larger receptive fields and bodies

synapse with multiple BP and PR

mostly input from rods

transient response

good temporal resolution

- sensitive to motion

- poor spatial resolution

not color sensitive

Parasol Ganglion cells are best associated with

Good temporal resolution

transient responses

Midget ganglion cells are best associated with

Good spatial reolution and sustained responses

S cone bipolar cells synapse with

small bistratified cells

on center receptive field

Bistratifed layer of the LGN prjects to

koniocellular layer

Which ganglion cell show transient response to light

on/off cells

Where are receptive fields larger

periphery

spot mapping

receptive fields overlap

--> small spot of light will excite or inhibit many ganglion cells

how we determine shapes

perceive edges

SEE drawings

Spatial tuning

the maximal response can be evoked only with an optimally sized spot

ganglion cell is more responsive to

centered illumination

see fiagram

Axons leave the GC and -->

Optic nerve

Synapse in LGN

Some to to superior colliculus ( eye movements)

some to pulvinar region (visual attention, motino processing, visually guided behavior)

ipRGC (intrinsically photosensitive retinal ganglion cells)

photopigment: melanopsin

directly responsicve to loght

peak sensitivyt (480 nm mesopic blue light)

even when no rods or cones pressent

ipRGC project to

rats: Suprachiasmatic nucleus via retinohypothalmic tract (ciracidan rhythm)

monkeys: LGN

SUMMARY

Graded potentials

Photoreceptors

horizontal cells

BP cells

Action potential

amacrine cells

What cells have center-surround

Bipolar

Ganglion

amacrine

direct neuronal pathway

PR --> BP -- GC

Indirect (lateral) pathway

horizontal cells and amacrine cells

1st neuron in pathway to have center surround

bipolar cells --> spatial antagonism