13- Center Surround and RGCs

What is the center surround pathway of BPC/Horizontal cells in the dark @ the fovea?

@fovea every cone connects to one ON center and one OFF center BPC

1. PRs depolarize + release glutamate

2. ON-center BPC hyperpolarizes and OFF-center BPC depolarizes

3. Horizontal cells depolarize

4. Horizontal cells release inhibitory compound (GABA) once depolarized

5. 'GABA' causes the PR to become more hyperpolarized/less depolarized

What is the resting membrane potential of a PR in the dark? What would it be without horizontal cells?

-40mV usually

-30mV (more depolarized) without horizontal cell

How does 'GABA' from horizontal cells cause the PR to become more hyperpolarized?

GABA receptors are Cl- channels that when stimulated open and let Cl- enter the cell, making it more negative

What is the center surround pathway of BPC/Horizontal cells in light @ the fovea?

@fovea every cone connects to one ON center and one OFF center BPC

1. PR hyperpolarize and release less glutamate

2. ON center BPC depolarizes, OFF center hyperpolarizes

3. Horizontal cell hyperpolarizes and releases less GABA

4. decrease in GABA results in stimulation of PRs

5. PRs are more depolarized/less hyperpolarized and release more glutamate

What is the function of horizontal cells?

lateral inhibition which allows for central-surround antagonism

central surround antagonism enhances edges and borders and enhances contrast

What do BPC responses depend on? What does this make humans good at?

relative difference between center + surround luminance

(makes absolute intensity irrelevant) makes us good at contrast detection

What is simultaneous contrast? What causes this?

apparent brightness of an object is inversely related to the luminance of its surroundings

this is due to center surround antagonism

What is the Mach Band effect?

When looking at mach bands (dark to light from left to right), the right edge appears darker and the left edge appears lighter

What kind of synapse to BPCs have?

ribbon synapse with two processes --> dyad synapse

What is usually on the two process lateral to the ribbon synapse of a BPC?

one RGC and one amacrine cell

What receptors do RGCs have? Does this make RGCs sign conserving or sign inverting?

inotropic AMPA/kainate + NMDA receptors therefore they are sign conserving

OFF-center BPCs synapse onto what?

OFF-center RGCs

ON-center BPCs synapse onto what?

ON-center RGCs

Where do OFF-center bipolar cells synapse with OFF-center RGCs?

outer IPL/off sublamina

Where do ON-center BPCs synapse with ON-center RGCs?

inner IPL/on sublamina

What type of potentials do RGCs generate?

action potentials

How does increased depolarization affect RGCs action potentials?

increased depolarization=increased spike frequency

What do extracellular recordings of the retina record?

only detect action potenial

What can we use extracellular recordings of the retina to detect?

detect changes in spiking frequency in RGCs

What are the three types of RGCs based on light responses?

ON-Center

OFF-Center

ON-OFF (responds to both light onset + offset)