The Visual system or Chapter 20 of Fundamental Neuroscience

1. The Visual systemor Chapter 20 of Fundamental Neuroscience Genevieve Legault AHD Wednesday April 1st 2008

2. Plan of presentation • Brief overview of eye anatomy • Retina anatomy • Different photoreceptors • Transduction of signal • Retinal cells • Visual pathway • Visual cortex and its organization • Summary

3. Objectives of presentation • Understand the mechanism of photoreceptors • Recognize the differences between rods and cones • Know the visual pathway, and their associated VF defects if interrupted • Learn the organization of the visual cortex

4. Brief overview of eye anatomy: Anatomy quizz → GUESS WHO ….

5. Anatomy quizz • I’m a transparent protective coating for the optic structures.

6. Brief overview of eye anatomy

7. Anatomy quizz • I’m a transparent protective coating for the optic structures. • Its lateral margin are continuous with which structure? • Conjunctiva!

8. Anatomy quizz • Circumferentially organized muscle

9. Brief overview of eye anatomy

10. Anatomy quizz • Iris sphincter is paraΣ: • Begins with preganglionic neurons with cell bodies in EW nucleus • Axons end in the ciliary ganglion • Postganglionic end as neuromuscular synapse (ACh)

11. Anatomy quizz • Iris sphincter is paraΣ: • Begins with preganglionic neurons with cell bodies in EW nucleus • Axons end in the ciliary ganglion • Postganglionic end as neuromuscular synapse (ACh)

12. Anatomy quizz • Epithelium covering which structure produces the fluid filling the anterior chamber?

13. Brief overview of eye anatomy

14. Anatomy quizz • Epithelium covering which structure produces the fluid filling the anterior chamber? • This fluid then drains into me.

15. Brief overview of eye anatomy

16. Anatomy quizz • Epithelium covering which structure produces the fluid filling the anterior chamber? • This fluid then drains into me. • If the outflow is obstructed, we then get me.

17. Glaucoma capsule • Damage from the periphery toward the center • ~ 90% open-angle or normal angle • Idiopathic ↑ in pressure • ~ 5% angle abnormally acute (closed-angle) • Obstruction of normal flow of fluid • Remaining: canals blocked by debris (infection, DM, hemorrhage into anterior chamber)

18. Glaucoma: which one is true? • Vision is typically: • 1) blurred and dimmed • 2) not blurred but dimmed • 3) blurred but not dimmed • 4) normal

19. Anatomy quizz • Radially arranged muscle

20. Brief overview of eye anatomy

21. Anatomy quizz • Iris dilator is Σ: • Preganglionic begins in the intermediolateral cell column of the spinal cord, in the upper thoracic region • Axons end in superior cervical ganglion • Postganglionic end in neuromuscular synapse (NE)

22. Iris is smart! • With light, ACh is released on both muscarinic sphincter muscles (contraction) and dilator muscles (presynaptic inhibition of NE release, therefore blocking dilator contraction)

23. Anatomy quizz • Uvea is formed by which 3 structures?

24. Anatomy quizz • Uvea is formed by which 3 structures? • Iris • Ciliary body • Choroid • Highly vascularized • Pigmented tissue layer between retina and sclera

25. Brief overview of eye anatomy

26. Retina = neural retina + retinal pigment epithelium → Continuous sheet of cuboidal cells bound by tight junctions Functions:-nutrition supply-protection of photoreceptors-phagocytosis

27. Neural retina = photoreceptors and associated neurons • Photoreceptors absorb quanta of light (photons) and convert it into electrical signal • Ganglion cells send axons as the optic nerve

28. 7 layers of neural retina • **** Pathway of light and neural outflow are inverted ****

29. Blood supply • Internal carotid artery↓ophthalmic artery → Posterior ciliary artery→Central retinal artery - external portion -inner retina of the optic nerve (neural retina) - choroidal -pial arteries to circulation optic nerve -outer retina

30. Detection and transduction of light in outer segment pointing toward the pigment epithelium Narrow stalk (cilium) connects to the inner segment (containing mitochondria) Then outer plexiform layer where synapse Photoreceptors

31. Pathway of light - Cilium

32. Rods Cones 2 types of photoreceptors

33. Transduction • Conformational change • ↓cGMP • Closing Na+ current • Hyperpolarization • Passive propagation

34. Transduction • Photoreceptors are the only sensory neurons that hyperpolarize in response to the relevant stimulus

35. At rest: ↑ cGMP level ↑ Na+ current Resting potential -40 mV Constant glutamate release Light: ↓ cGMP level Blocks Na+ current Hyperpolarize: -60 mV ↓ in tonic glutamate release Transduction

36. Cones • 3 types, each tuned to a different wavelength • L-cones = long wavelengths (red cones) • M-cones = medium wavelengths (green cones) • S-cones = short wavelengths (blue cones) • Each color = unique combination

37. Color confusion • Genetic defect in one of the opsin (one type of cone) • L and M opsins are located on Chromo X, therefore more frequent in ♂ • Inability to perceive red = protanopia • Inability to perceive green = deuteranopia

38. Fovea • Light reaches the macula • Center = fovea

39. Fovea • Light reaches the macula • Center = fovea • Thinner inner retina, to allow max of light (outer nuclear and photoreceptor outer segment only in the center) • Only cones

40. Receptive fields • Definition: • Sum of the areas in which the stimulus affects the activity of that neuron • Roughly circular • Center • Doughnut-shaped outer rim with usually the opposite response

41. Retinal synapses • Only ganglion cells have voltage-gated Na+ channels therefore only one using action potentials • Other cells use graded potentials

42. Retinal synapses • Outer plexiform layer • One photoreceptor • 1 centrally placed bipolar cell triad • 2 laterally placed horizontal cells • Inner plexiform layer • Bipolar cells with on or off ganglion cells • Amacrine cells with ganglion cells, other amacrine cells, and bipolar cells

43. Horizontal cells • Course parallel to the retina • Glutaminergic input from photoreceptor • GABAergic output to adjacent photoreceptors → inhibiting surround to sharpen receptive field

44. Bipolar cells • Between photoreceptor cells and ganglion cells • 1st cells to exhibit the center-surround receptive field organization • 2 types • On: depolarizing, sign-inverting • Off: hyperpolarizing, sign-conserving

45. Bipolar cells

46. Amacrine cells • May contain different neurotransmitters • Sense change in change (variation in speed)

47. Ganglion cells • Output cells of the retina: axons converge on the optic disc to form the optic nerve • Also have center-surround receptive fields (like bipolar cells)

48. Alpha Largest Periphery (input mainly from rods) Y cells Participate little in color, larger receptive field M cells = magnocellular layers in LGN Beta Medium-sized Central retina (input mainly from cones) X cells Color, small receptive fields P cells = parvocellular layers in LGN Ganglion cells: different types

49. Ganglion cells: different types • Other types: gamma, delta, epsilon • W cells • Smaller cell bodies • Variety of receptive field sizes and physiologic responses

50. Retinal projections