Retinal Physiology : from photon capture to spike trains, an overview

1. Retinal Physiology: from photoncapture to spike trains, an overview • Cast of characters and personalities, • Who’s on first . . . Wiring diagrams of the most studied of human neural circuits. • Themes, patterns and guiding principles; some views of the forest.

2. Building a light sensor

3. Tartuferi 1887 Ganglion cell layer Amacrine cells Bipolar Cells Horizontal Cells Rod and Cone Photoreceptors

4. Key points to remember • Duel photoreceptors system (rods & cones) extend the range of visual function. • The minimal direct pathway for a signal is photoreceptor to bipolar cell to ganglion cell. • Horizontal and amacrine cell form lateral connections and are critical for lateral inhibtion and center-surround organization.

5. Key points to remember • The retina is interested in contrast differences, edges, light vs. dark. • The on-off pathways are critical for making these comparisons. This division begins at the bipolar cell level. • The center-surround receptive field is a key feature of retinal output. • It starts at the bipolar cells.

6. LIGHT

7. Local specializations • ON head • Major blood vessels

8. Photoreceptors • The only neurons of the visual system that sense light.. • Duality (rods and cones) permits specialization into two systems (1) for high sensitivity and (2) for spatial and temporal sensitivity (plus color). • Metabolically very high maintenance cells.

9. v. Photoreceptor distribution Fig 15.12

10. Rod photoreceptor (system) • 95% of photoreceptor in human eye • Single photon sensitivity (amazing feat) • High degree of spatial summation . . . Therefore LOW spatial acuity • Slow time-to-peak, slow recovery . . . Therefore LOW temporal resolution • Dark, starlight, moonlight (2.5 log units)

11. Cone(system) function • In foveate animals the overwhelming majority of visual behavior depends on this minute (<1%) patch of retina. (Tiny area of high spatial acuity). • Fast response time, but insensitive. • Wide range of adaptation (6+ Log Units) • . . . And in living color too.

12. Light is the ligand that triggers activation of the enzyme.

13. Biochemistry of Phototransduction • Rhodopsin is the classic example of a 7-trans-membrane spanning G-protein coupled receptor. (ligand = photons) • High gain means high sensitivity (But takes time to develop). • Smaller Responses , due to lower sensitivity (low gain) are over faster resulting in higher . . .

14. Photocurrent & Photovoltages Graded responses (no spikes) • Photoreceptors are partially depolarized in the dark (due to an influx of Na+ and Ca2+ ions). • Light shuts off the influx, thus the cells hyperpolarize and . . . • Neurotransmitter is released constantly in the dark and this release is attenuated by light!!!! (GLUTAMATE)

18. Themes ofretinalcircuitry(RECEPTIVE FIELDS) • Highest visual acuity and fidelity of signals carrying that message requires requires a private line (Midget system). • SPLITTING (push-pull or on-off systems) Divergent wiring also Midget. • Highest sensitivity (Greatest summation) Convergent wiring.

19. Private Line • Divergence • Convergence

20. Themes of retinal circuitry • Horizontal Cells and Amacrine cells provide lateral pathways in the retina. • Feedback and feed forward synaptic interactions add flexibility and complexity • Spatial filters (Lateral inhibition) • Temporal filters (Directional selectivity) • Network gain control (light/dark adaptation)

22. Divergent wiring Same neurotransmitter different responses.(receptor biochemistry) On bipolar :sign inverting feeds onto ON ganglion cells (SPIKING INCREASES) Off bipolar :sign conserving feeds onto Off ganglion cells (SPIKING Diminishes) On and Off pathways

24. Light hyperpolarizes photoreceptors. Transmitter release goes down. Off bipolar cells hyperpolarize. Transmitter release goes down. Ganglion cells hyperpolarize. Spike frequency (rate) goes down. The Off pathway

25. Light hyperpolarizes photoreceptors. Transmitter release goes down. On bipolar cells depolarize. Transmitter release goes up. Ganglion cells depolarize. Spike frequency (rate) goes up. The On pathway

26. Building Receptive Fields • Center and surround organizations • On or Off responses (anatomical correlation with sublaminae of IPL) • Transient and sustained physiology • Color coding

27. Center Surround receptive fields require lateral interactions.

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29. Bipolar Cell morphology/physiolgy • Rod vs. Cone (Rod Bp output ??) • Midget vs. diffuse • On vs. Off • Unique Blue cone bipolar (non-midget) • Some of the anatomical subdivisions have no physiological correlates and recently vice versa (contrast sensitivity)

30. Several distinct morphological types have been identified. • Some match physiological types • Others remain unclassified

31. Ganglion Cell morphology predicts physiology

32. The Rod piggyback-pathway • Without a rod-specific ganglion cells, how does the brain receive rod signals? • Through the AII amacrine cells, rods piggy-back their signal through the cone pathway. • Rod  Rod Bp AII  Cone Bp  cone ganglion cells etc. etc.

33. Rod Pathway • No Direct Ganglion cell output. • Rod bipolar to amacrine cell • A combination of electric and chemical synapses • Output through cone G-cells

34. AII amacrine cell

35. How to spikes encode information? • Spatial information (by anatomical mapping to topographic cortex) • Temporal codes • Functional mapping (color signals to color cortex, motion signals to motion cortex etc.) • Cross correlation between neighboring cells or groups of cells (new horizons).

36. - Ganglion cell Receptive Fields • Center from on (+) bipolar • Surround from off (-) bipolars - + Antagonistic Center Vs. Surround

37. Center Surround receptive fields require lateral interactions.

38. - Ganglion cell Receptive Fields • What if the small spot of light illuminates the center of the cells’ receptive field ?

39. Ganglion cell Receptive Fields • What if a large spot of light illuminates the center of the cells’ receptive field ? -

40. - Ganglion cell Receptive Fields • What if the spot of light hits the surround?

41. Ganglion cell Receptive Fields • What if the surround is optimally stimulated? -

42. Ganglion cell Receptive Fields • What uniform illumination? - THE VISUAL SYSTEM CARES ABOUT CHANGE, CONTRAST, Not about uniform retinal illumination.

43. Ganglion cell Receptive Fields Figure 28-3 Summary of ganglion cell receptive fields, showing the spike trains generated by the stimulus.

44. How do spikes encode information? • Spatial information (by anatomical mapping to topographic cortex) • Temporal codes • Functional mapping (color signals to color cortex, motion signals to motion cortex etc.) • Cross correlation between neighboring cells or groups of cells (new horizons).

45. Building Receptive Fields • Center and surround organizations • On or Off responses (anatomical correlation with sublaminae of IPL) • Transient and sustained physiology • Color coding