The Visual System• Basic anatomy of the eye• The retina– Organization of cell types– Rod and cone photoreceptor systems• Phototransduction– Conversion of energy of light into changes in Vm– Adaptation and expansion of dynamic range• Information processing– Retina– LGN– CortexThe Visible SpectrumWe detect only a small portion of the electromagneticspectrumlight from 740 nm (red) to 370 nm (blue)The Light Gathering Parts of the EyeFocusing light on the retinaThe retina is a point-to-point map of the visual fieldBut the visual field is inverted!Structure of the eye The Basic Retinal Circuit1. Receptor Cells(rods and cones)2. Bipolar Cells3. Ganglion CellsDifferent cells in the retinaBack of eyeFront of eye4. Horizontal Cells5. Amacrine Cells6. Pigment cellsThe Fovea• Packed with cones - high density of sensors, divergent connections• Minimal light scattering (depleted of other cell types) => High acuity (high resolution) visionDistribution of Rods and ConesRod and Cone Photoreceptor Cells Are the Light SensorsBack of eyeFront of eyeLight absorption andphototransduction occurin the outer segment discsOpsinDiscsFundamental Differences between Rod & Cone SystemsHigh acuity (in the fovea,divergent connections)Low acuity (not in the fovea,convergent connections)Chromatic (color)AchromaticFast responseSlow responseLower AmplificationHigh AmplificationLow sensitivity, specializedfor day visionHigh sensitivity to light,specialized for night visionConesRodsLight Hyperpolarizes the Photoreceptor Cell(graded potentials)• In the dark, there is a large sodium current (gNa).• Light causes Na+ channels to close (gNa), bringing Vm closer to Ek.Na+ Channels Are Open in the DarkCurrent Flux in the Photoreceptor Cell & theCorresponding Equivalent CircuitSegregation of K+ channels in RIS allows changes in Vm in ROS to be transmitted with minimal degradation to synaptic terminalRhodopsin/Opsins: The Light Receptors11-cis retinal: the chromophore or “light catcher”Photoisomerization ->Retinal is attached to rhodopsin via a protonatedSchiff’s base @ Lysine-296:Rhodopsin/opsins areG protein-coupled receptorsRod opsin: “Rhodopsin”Cone opsins:“S opsin” (blue)“M opsin” (green)“L opsin” (red)Different Opsins Recognize Different WavelengthsWe have 4 different opsins - each provides a different environment for 11-cis retinal, resulting in different Absmax for each receptor.Rods: Rhodopsin: blue/green-sensitiveCones: S opsin: blue-sensitiveM opsin: green-sensitiveL opsin: red-sensitivePhotoisomerization of Retinal -> Opsin Activation• Retinal is covalently bound to opsin @ Lys-296, also otherinteractions in transmembrane regions of receptor•11-cis: an opsin antagonist (-> receptor inactive)•all-trans: an opsin agonist (-> receptor active)<-- active<-- inactiveSlow recycling of opsin and retinal+The Phototransduction CascadeRh* Gtα* PDE*cGMP5’GMP[cGMP] -> cGMP-gated channels close• Photoconversion of Rh(11-cis retinal) to Rh*(all-trans-retinal) is the onlylight-dependent step in the cascade!• “Dark current” is carried by the cGMP-gated channels; activation ofcascade leads to channel closure.• High level of amplification (especially in rods):– One Rh* molecule can activate up to 100 transducin molecules in ~1 sec(= number of PDE molecules activated)– PDE can cleave cGMP @ 1,000/sec– => 100,000 cGMPs cleaved/sec for 100,000-fold amplification (!!)(cGMP-gated channels flux both Na+ & Ca2+)In the dark…In the light…Wide Dynamic Range in Vision System responds to light intensities over 6 logs (1 q/sec -> 106 q/sec) 1. Rods vs. cones 2. Long-term adaptation-pupil size -receptor photobleaching 3. Short-term adaptation/densensitization (Ca2+-dependent mechanisms) -deactivation of receptors -recovery of membrane potentialAdaptation of Photoreceptor Cellslight:VmVm returns to resting level-40 mV-70 mVPhotoreceptor Cell Adaptation is Ca2+-DependentRh* Gtα* PDE*cGMP5’GMPGTP[cGMP] -> cGMP-gated channels close[Ca2+]iGuanylyl CyclaseCa2+Rh kinaseArrestinCa2+Decrease in intracellular calcium in response to light:• activates guanylyl cyclase• activates rhodopsin kinase/arrestin Also: • increases cGMP channel’s affinity for cGMPNet effect: moves Vm back toward resting levels(cGMP-gated channels flux both Na+ &