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Neuroscience: Exploring the Brain, 3e

Neuroscience: Exploring the Brain, 3e. Chapter 9: The Eye. Introduction. Significance of vision Relationship between human eye & camera Retina Photoreceptors: Converts light energy into neural activity Detects differences in intensity of light Lateral geniculate nucleus (LGN)

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Neuroscience: Exploring the Brain, 3e

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  1. Neuroscience: Exploring the Brain, 3e Chapter 9: The Eye

  2. Introduction • Significance of vision • Relationship between human eye & camera • Retina • Photoreceptors: Converts light energy into neural activity • Detects differences in intensity of light • Lateral geniculate nucleus (LGN) • First synaptic relay in the primary visual pathway • Visual information ascends to cortex interpreted and remembered

  3. Properties of Light • Light • Electromagnetic radiation • Wavelength, frequency, amplitude

  4. Properties of Light • Light • Energy is proportional to frequency • e.g., gamma radiation and cool colors - high energy • e.g., radio waves and hot colors - low energy

  5. Properties of Light • Optics • Study of light rays and their interactions • Reflection • Bouncing of light rays off a surface • Absorption • Transfer of light energy to a particle or surface • Refraction • Bending of light rays from one medium to another

  6. The Structure of the Eye • Gross Anatomy of the Eye • Pupil: Opening where light enters the eye • Sclera: White of the eye • Iris: Gives color to eyes • Cornea: Glassy transparent external surface of the eye • Optic nerve: Bundle of axons from the retina

  7. The Structure of the Eye • Ophthalmoscopic Appearance of the Eye

  8. The Structure of the Eye • Cross-Sectional Anatomy of the Eye

  9. Image Formation by the Eye • Refraction of light by the cornea • Eye collects light, focuses on retina, forms images

  10. Image Formation by the Eye • Accommodation by the Lens • Changing shape of lens allows extra focusing power

  11. Image Formation by the Eye • The Pupillary Light Reflex • Connections between retina and brain stem neurons that control muscle around pupil • Continuously adjusting to different ambient light levels • Consensual • Pupil similar to the aperture of a camera

  12. Image Formation by the Eye • The Visual Field • Amount of space viewed by the retina when the eye is fixated straight ahead

  13. Image Formation by the Eye • Visual Acuity • Ability to distinguish two nearby points • Visual Angle: Distances across the retina described in degrees

  14. Microscopic Anatomy of the Retina • Direct (vertical) pathway: • Ganglion cells  • Bipolar cells  • Photoreceptors

  15. Microscopic Anatomy of the Retina • Retinal processing also influenced lateral connections: • Horizontal cells • Receive input from photoreceptors and project to other photoreceptors and bipolar cells • Amacrine cells • Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells

  16. Microscopic Anatomy of the Retina • The Laminar Organization • Inside-out • Light passes through ganglion and bipolar cells before reaching photoreceptors

  17. Microscopic Anatomy of the Retina • Photoreceptor Structure • Converts electromagnetic radiation to neural signals • Four main regions • Outer segment • Inner segment • Cell body • Synaptic terminal • Types of photoreceptors • Rods and cones

  18. Microscopic Anatomy of the Retina • Regional Differences in Retinal Structure • Varies from fovea to retinal periphery • Peripheral retina • Higher ratio of rods to cones • Higher ratio of photoreceptors to ganglion cells • More sensitive to light

  19. Microscopic Anatomy of the Retina • Regional Differences in Retinal Structure (Cont’d) • Cross-section of fovea: Pit in retina where outer layers are pushed aside • Maximizes visual acuity • Central fovea: All cones (no rods) • 1:1 ratio with ganglion cells • Area of highest visual acuity

  20. Phototransduction • Phototransduction in Rods • Light energy interacts with photopigment to produce a change in membrane potential • Analogous to activity at G-protein coupled neurotransmitter receptor - but causes a decrease in second messenger

  21. Phototransduction • Phototransduction in Rods • Dark current: Rod outer segments are depolarized in the dark because of steady influx of Na+ • Photoreceptors hyperpolarize in response to light

  22. Phototransduction • Phototransduction in Rods • Light activated biochemical cascade in a photoreceptor • The consequence of this biochemical cascade is signal amplification

  23. Phototransduction • Phototransduction in Cones • Similar to rod phototransduction • Different opsins • Red, green, blue • Color detection • Contributions of blue, green, and red cones to retinal signal • Spectral sensitivity • Young-Helmholtz trichromacy theory of color vision

  24. Phototransduction • Dark and Light Adaptation • Dark adaptation—factors • Dilation of pupils • Regeneration of unbleached rhodopsin • Adjustment of functional circuitry 20–25 minutes All-cone daytime vision All-rod nighttime vision

  25. Phototransduction • Dark and Light Adaptation • Calcium’s Role in Light Adaptation • Calcium concentration changes in photoreceptors • Indirectly regulates levels of cGMP channels

  26. Retinal Processing • Transformations in the Outer Plexiform Layer • Photoreceptors release less neurotransmitter when stimulated by light • Influence horizontal cells and bipolar cells

  27. Retinal Processing • Receptive Field: “On” and “Off” Bipolar Cells • Receptive field: Stimulation in a small part of the visual field changes a cell’s membrane potential • Antagonistic center-surround receptive fields

  28. Retinal Processing • On-center Bipolar Cell • Light on (less glutamate); Light off -> more glutamate ‘Inverting’ synapse (inhib)

  29. Retinal Output • Ganglion Cell Receptive Fields • On-Center and Off-Center ganglion cells • Responsive to differences in illumination

  30. Retinal Output • Types of Ganglion Cells • Appearance, connectivity, and electrophysiological properties • M-type (Magno) and P-type (Parvo)ganglion cells in monkey and human retina

  31. Retinal Output • Color-Opponent Ganglion Cells

  32. Retinal Output • Parallel Processing • Simultaneous input from two eyes • Information from compared in cortex • Depth and the distance of object • Information about light and dark: ON-center and OFF-center ganglion cells • Different receptive fields and response properties of retinal ganglion cells: M- and P- cells, and nonM-nonP cells

  33. Concluding Remarks • Light emitted by or reflected off objects in space  imaged onto the retina • Transduction • Light energy converted into membrane potentials • Phototransduction parallels olfactory transduction • Electrical-to-chemical-electrical signal • Mapping of visual space onto retina cells not uniform

  34. End of Presentation

  35. Retinal Processing • Research in ganglion cell output by • Keffer Hartline, Stephen Kuffler, and Horace Barlow • Only ganglion cells produce action potentials • Research in how ganglion cell properties are generated by synaptic interactions in the retina • John Dowling and Frank Werblin • Other retinal neurons produce graded changes in membrane potential

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