The Visual System. Organization of cell types Rod and cone photoreceptor systems

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1 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 V m Adaptation and expansion of dynamic range Information processing Retina LGN Cortex

2 The Visible Spectrum We detect only a small portion of the electromagnetic spectrum light from 740 nm (red) to 370 nm (blue)

3 The Light Gathering Parts of the Eye

4 Focusing light on the retina

5 The retina is a point-to-point map of the visual field But the visual field is inverted!

6 The Basic Retinal Circuit Different cells in the retina Back of eye 6. Pigment cells 1. Receptor Cells (rods and cones) 2. Bipolar Cells 3. Ganglion Cells 4. Horizontal Cells 5. Amacrine Cells Structure of the eye Front of eye

7 The Fovea Packed with cones - high density of sensors, divergent connections Minimal light scattering (depleted of other cell types) => High acuity (high resolution) vision

8 Distribution of Rods and Cones

9 Rod and Cone Photoreceptor Cells Are the Light Sensors Back of eye Discs Light absorption and phototransduction occur in the outer segment discs Opsin Front of eye

10 Fundamental Differences between Rod & Cone Systems Rods High sensitivity to light, specialized for night vision Cones Low sensitivity, specialized for day vision High Amplification Lower Amplification Slow response Fast response Achromatic Chromatic (color) Low acuity (not in the fovea, convergent connections) High acuity (in the fovea, divergent connections)

11 Light Hyperpolarizes the Photoreceptor Cell (graded potentials) In the dark, there is a large sodium current (g Na ). Light causes Na + channels to close (g Na ), bringing V m closer to E k.

12 Na + Channels Are Open in the Dark

13 Current Flux in the Photoreceptor Cell & the Corresponding Equivalent Circuit Segregation of K + channels in RIS allows changes in V m in ROS to be transmitted with minimal degradation to synaptic terminal

14 Rhodopsin/Opsins: The Light Receptors Rhodopsin/opsins are G protein-coupled receptors 11-cis retinal: the chromophore or light catcher Photoisomerization -> Rod opsin: Cone opsins: Rhodopsin S opsin (blue) M opsin (green) L opsin (red) Retinal is attached to rhodopsin via a protonated Schiff s Lysine-296:

15 Different Opsins Recognize Different Wavelengths We have 4 different opsins - each provides a different environment for 11-cis retinal, resulting in different Abs max for each receptor. Rods: Rhodopsin: blue/green-sensitive Cones: S opsin: blue-sensitive M opsin: green-sensitive L opsin: red-sensitive

16 Photoisomerization of Retinal -> Opsin Activation Retinal is covalently bound to Lys-296, also other interactions in transmembrane regions of receptor 11-cis: an opsin antagonist (-> receptor inactive) all-trans: an opsin agonist (-> receptor active) <-- active Slow recycling of opsin and retinal + <-- inactive

17 The Phototransduction Cascade 5 GMP Rh* G tα * PDE* cgmp [cgmp] -> cgmp-gated channels close (cgmp-gated channels flux both Na + & Ca 2+ ) Photoconversion of Rh(11-cis retinal) to Rh*(all-trans-retinal) is the only light-dependent step in the cascade! Dark current is carried by the cgmp-gated channels; activation of cascade 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 1,000/sec => 100,000 cgmps cleaved/sec for 100,000-fold amplification (!!)

18 In the dark

19 In the light

20 Wide Dynamic Range in Vision System responds to light intensities over 6 logs (1 q/sec -> 10 6 q/sec) 1. Rods vs. cones 2. Long-term adaptation -pupil size -receptor photobleaching 3. Short-term adaptation/densensitization (Ca 2+ -dependent mechanisms) -deactivation of receptors -recovery of membrane potential

21 Adaptation of Photoreceptor Cells light: -40 mv V m -70 mv V m returns to resting level

22 Photoreceptor Cell Adaptation is Ca 2+ -Dependent 5 GMP Rh* G tα * PDE* cgmp [cgmp] -> cgmp-gated channels close Rh kinase Arrestin Guanylyl Cyclase [Ca 2+ ] i Ca 2+ Ca 2+ GTP (cgmp-gated channels flux both Na + & Ca 2+ ) Decrease in intracellular calcium in response to light: activates guanylyl cyclase activates rhodopsin kinase/arrestin Also: increases cgmp channel s affinity for cgmp Net effect: moves V m back toward resting levels