ניורוביולוגיה ומדעי המח Introduction to Neurobiology

1. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems The retina “From Neuron to Brain” Chapter 19: 379-405 Transduction and signaling in the retina continued

2. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems The retina 1- Structure of the eye - ✔ 2- Muscles of the eye – fixation; convergence; accommodation - V 3- Structure of the retina - ✔ 4- The fovea: Comparing central and peripheral vision - ✔ 5-The blind spot - ✔ 6- Rods and Cones - ✔ 7- Structure-Function relationships 8- Color vision: Young & Helmholtz - ✔ 9- Visual Pigments 10- Photoreceptor sensitivity and signal amplification 11- Adaptation 12- lateral inhibition 13- Ribbon synapses

3. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems layers of neurons & connections in the retina.

4. The Retina and its layers

5. The Retina and its layers Cell bodies connections Cell bodies connections Cell bodies

6. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Rods and cones - sensitivity vs. acuity & color; - located in central vs. peripheral retina - night vs. day vision

9. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Depolarizing and Hyperpolarizing receptor, generator, and action potential signals

10. The photoreceptor response: Receptor potential & generator potential Graded potentials vs. Action potentials

12. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Transduction = the use of signal (optic) energy to trigger a neuronal response The absorption of light by photoreceptor cell Rhodopsin molecules is such a trigger, leading to a physiological response = an ion-specific change in cell conductance.

13. Rhodopsin is large, transverses the membrane, contains a small chromophore, Vitamin A aldehyde or retinal. Each rod contains many 100’s of disks; each disk 100’s of 1,000’s of molecules.

14. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems How do we know that Rhodopsin is responsible for vision?

15. because the absorption spectrum of Rhodopsin matches the sensitivity spectrum of vision.

16. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems When light is absorbed by Rhodopsin, a chain reaction occurs with 3 major steps: 1- 11-cis retinal is photo-isomerized to its all-trans configuration. 2- The opsin part of the molecule undergoes a number of conformational changes. 3- Rhodopsin is split into all-trans retinal and opsin.

17. Regeneration of Rhodopsin requires transfer of retinal to the pigment epithelium for return to its 11-cis conformation

18. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Metarhodopsin II exerts its effect on photoreceptor conductance by activating the G-protein transducin, which in turn increases the cellular concentration of cyclic GMP second messenger intermediary.

19. 1- Absorption of light leads to a conformational change from Rhodopsin to Metarhodopsin II. 2- In the presence of Metarhodopsin II, GDP, bound to the α-subunit of G-protein transducin, is exchanged for GTP. 3- the α transducin subunit, when bound to GTP, detaches, activating a phosphodiesterase, which hydrolyzes cGMP. 4- reduction of cGMP reduces Na+ channel opening leading to reduction in Na+ current.

20. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Openings of Na+ channels can be recorded by using specialized techniques.

24. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems There is a process of signal amplification – so that each photon absorbed leads to a large photoreceptor signal.

25. 1- Absorption of light leads to a conformational change from Rhodopsin to Metarhodopsin II. 2- In the presence of Metarhodopsin II, GDP, bound to the α-subunit of G-protein transducin, is exchanged for GTP. 3- the α transducin subunit, when bound to GTP, detaches, activating a phosphodiesterase, which hydrolyzes cGMP. 4- reduction of cGMP reduces Na+ channel opening leading to reduction in Na+ current.

26. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems There is a process of adaptation leading to a reduction of the effectiveness of each photon – reduced amplification – as there are many more photons being absorbed.

27. Part of the adaptation process depends on a fall in intracellular Calcium ion concentration In the light.

29. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Adaptation: 1- in transduction process 2- Rhodopsin available to absorb light 3- Calcium level in cell 4- neural interactions

30. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Adaptation: reduction in sensitivity of visual system in presence of high levels of ambient light.

33. ניורוביולוגיה ומדעי המחIntroduction to Sensory Systems Adaptation: 1- enable response over extremely large range of ambient light levels. 2- emphasize response to temporal change in light rather than steady state. 3- maintain same relative response irrespective of light (and response) level. 4- maintain constant appearance for same relative change in intensity.

34. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Lateral Inhibition

35. ניורוביולוגיה ומדעי המחIntroduction to Neurobiology Introduction to Sensory Systems Lateral Inhibition: 4 levels: 1- receptive field measurement 2- physiological mechanism 3- computational equivalent 4- psychophysical phenomenon 5- perceptual advantage

37. light ocelli eccentric cells A B C D E F

38. light ocelli eccentric cells each fiber excited by central light, inhibited by lateral light A B C D E F

39. light ocelli eccentric cells lateral inhibition A B C D E F

40. light ocelli eccentric cells lateral inhibition A B C C B A feedforward inhibition no disinhibition feedback inhibition with disinhibition

41. light ocelli eccentric cells lateral inhibition A B C C B A feedforward inhibition no disinhibition feedback inhibition with disinhibition X ✓

42. Lateral Inhibition: 4 levels of analysis: 1- receptive field measurement – center/surround 2- physiological mechanism – lateral inhibition 3- computational equivalent – smooth and take second derivative 4- psychophysical phenomenon – Mach bands 5- perceptual advantage – edge enhancement