Retinal Circuit and Processing March 23, 2007 Mu-ming PooSlide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14How does glutamate activate or inhibit bipolar cells?Slide 16Slide 17Slide 181Retinal Circuit and ProcessingMarch 23, 2007 Mu-ming Poo •Overview of the retinal circuit•Receptive field (RF) of retinal ganglion cells (RGC)•Neural circuitry underlying the RGC receptive field2Lens defects:Myopia (nearsightedness) – focusing in front of retina, corrected by concave lensHyperopia (farsightedness) - focusing behind the retina, corrected by convex lensPrespbyopia – hardening of the lens due to aging (reduced accommodation), cannot have near vision, corrected by convex len.Astigmatism:- nonuniform curvature (in one axis), image blurred, corrected by cylindrical lens3lightFovea: high spatial resolutionPeriphery: low spatial resolution (Cell bodies)(dendrites)4Structure of the eye The Basic Retinal Circuit1. Receptor Cells(Graded potential)(input)2. Bipolar Cells(Graded potential)3. Ganglion Cells(action potential)(Output)Different cells in the retinaBack of eyeFront of eye4. Horozontal Cells(Graded potential)5. Amacrine Cells(Graded/action potential)6. Pigment cells2100 x 106106Convergence and divergence of connections5Receptive field of RGCMicroelectrodeStephen Kuffler6Receptive field of RGCcab78Features of RGC center-surround interactions1) There are two types of RFs: ON center/OFF surround, OFF center/ON surround.2) Surround stimulation actively inhibits response to center stimulation; maximal response can be evoked only with an optimally sized spot.9Receptive field•Definition: the area of the retina (or visual field) in which light signals evoke responses•It depends largely on the synaptic inputs to the cell, which in turn depends on dendritic field of the cell•Fovea: small RF (~0. 1o), high resolution; Periphery: large RF (3-5o), low resolution(1o = 0.25 mm on the retina)Ganglion Cell types:•M cells: large RF, fast response, color blind;•P cells: small RF, slower response, color sensitive.10Center/surround RF explains some visual illusionsMach band11More illusions…Hermann’s grid12Center/surround RF also explains spatial frequency tuning13M and P RGC cells have different spatial frequency selectivity14Direct pathway: Photo receptor  Bipolar  RGCP h o t o r e c e p t o rO n b i p o l a rO f f b i p o l a rO n R G CO f f R G C+-++Direct pathway is responsible to the RF center+: excitatory synapse, preserve response direction-: inhibitory synapse, flip response directionglutamateglutamate15How does glutamate activate or inhibit bipolar cells?•Off center bipolar cell – glutamate is excitatory, opens glutamate receptor-channel similar to the excitatory synapse. •On center biopolar cell – glutamate is inhibitory, opens K+ channel or closes cGMP-gated channel, by reducing cGMP through a metabotropic glutamate receptor (G-protein coupled receptor) that activate cGMP phosphodiesterase.16Indirect pathwayP h o t o r e c e p t o r P h o t o r e c e p t o rH o r i z o n t a l c e l l+ -Indirect pathway mediated by horizontal is responsible to the antagonistic RF surroundThere are other indirect pathway mediated by amacrine cells (use dopamine, ACh, indolamine)glutamate GABA1718P h o t o r e c e p t o rO n b i p o l a rO f f b i p o l a rO n R G CO f f R G C+-++Direct pathwayP h o t o r e c e p t o r P h o t o r e c e p t o rH o r i z o n t a l c e l l+ -Indirect pathwayBipolar cell → amacrine cell → ganglion cellSummary of retinal circuit+