Ch09

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Ch09

  1. 1. THE EYE
  2. 2. LIGHTVision is probably the most important sense in humans and animals. This system works bytransducing the property of light into a complex visual perceptLight is an electromagnetic radiation visible to the eye. It’s defined by 3 parameters:wavelength (distance btw two peaks or troughs)frequency (number of waves per second)amplitude (difference btw wave trough and peak)The energy content of a radiation isproportional to his frequency.Only a small part of theelectromagnetic spectrum is visibleto our eyes
  3. 3. LIGHTOptics is the study of light rays and their interactionsReflection: bouncing of light rays off a surfaceAbsorption: transfer of light energy to a particle or surfaceRefraction: changing of a direction due to change in speed of light rays, due to the passing from onemedium to another
  4. 4. ANATOMY OF THE EYEPupil: Opening where light entersthe eyeSclera: White of the eyeIris: Gives color to eyes. Contains 2muscles that give size to the pupilCornea: Glassy transparent externalsurface of the eyeExtraocular muscles: move theeyeball in the orbitOptic nerve: Bundle of axons fromthe retina
  5. 5. THE RETINAOptic disk: where blood vesselsoriginate and axons leave the retinaMacula: part of retina for centralvisionFovea: marks the center of the retina
  6. 6. CROSS SECTION OF THE EYECiliary muscles: Ligaments that suspend lensLens: Change shape to adjust focus. It divides eyes into two compartments:1) anterior chamber containing aqueous humor2) posterior chamber containing vitreous humor zonule fibers retina iris lens fovea light cornea aqueous humor optic nerve ciliary muscles vitreous humor sclera
  7. 7. IMAGE FORMATIONEye collects light, focuses on retina, forms images.The cornea is the site of most of the refractive power of the eye Focal distance: from refractive surface to the point where the rays converges. Depends on the curvature of the cornea
  8. 8. IMAGE FORMATIONAccommodation by the Lens Changing shape of lens allows for extra focusing power
  9. 9. IMAGE FORMATION
  10. 10. IMAGE FORMATIONThe Pupillary Light Reflex Depends on connections between retina and brain stem neurons that control muscle around pupil and aim to continuously adjust to different ambient light levels. It is consensual for both eyesThe Visual Field Amount of space viewed by the retina when the eye is fixated straight aheadVisual Acuity Ability to distinguish two nearby points Visual Angle: Distances across the retina described in degrees
  11. 11. MICROSCOPIC ANATOMY OF THE RETINAPhotoreceptors: cells that convert light energy into neural activityIn the Retina cells are organized in layers . Inside-out
  12. 12. MICROSCOPIC ANATOMY OF THE RETINAPhotoreceptor Structure Transduction of electromagnetic radiation to neural signals Four main regions 1) Outer segment 2) Inner segment 3) Cell body 4) Synaptic terminal Types of photoreceptors Rods (scotopic vision-dark) and cones (photopic vision-light)
  13. 13. MICROSCOPIC ANATOMY OF THE RETINARegional Differences in Retinal StructureVaries from fovea to retinal periphery In peripheral retina there is higher ratio of rods to cones, and higher ratio of photoreceptors to ganglion cells resulting in more sensitive to light In the fovea (pit in retina) visual acuity is maximal. In Central fovea there are only cones (no rods) and 1:1 ratio with ganglion cells
  14. 14. PHOTOTRANSDUCTIONPhototransduction in Rods Depolarization in the dark: “Dark current” and hyperpolarization in the light One opsin in rods: Rhodopsin Receptor protein that is activated by light G-protein receptor Photopigment
  15. 15. PHOTOTRANSDUCTIONDepolarization in the dark:“Dark current” andhyperpolarization in the light:Constant inward sodiumcurrentLight activate an enzime thatdestroy the cGMP, causingthe closing of Na+ channel
  16. 16. PHOTOTRANSDUCTION
  17. 17. PHOTOTRANSDUCTIONPhototransduction in Cons Similar to rod phototransduction Different opsins sensitive to different wavelengths: Red, green, blueColor detection is determined by the relativecontributions of blue, green, and red cones toretinal signal (Young-Helmholtz trichromacytheory of color vision)Dark and Light Adaptation is the transitionfrom photopic to scotopic vision (20-25minutes). It’s determined by: Dilation of pupils Regeneration of unbleached rhodopsin Adjustment of functional circuitry
  18. 18. RETINAL PROCESSINGPhotoreceptors release glutamate when depolarizedBipolar Cells. Can be categorized in 2 classes: OFF bipolar cells (they respond toglutamate by depolarizing) and ON bipolar cells (they respond to glutamate byhyperpolarizing) . Light off or on causes depolarization
  19. 19. RETINAL PROCESSINGGanglion Cell Receptive Fields On-Center and Off-Center cells Responsive to differences in illumination
  20. 20. RETINAL PROCESSINGTwo types of ganglion cells in monkey and human retina M-type (Magno) and P-type (Parvo) – 5 and 90 % of the ganglion cell population. The rest 5 % is non-P and non-M cells M-type: larger receptive field, faster conduction of AP, more sensitive to low contrast stimuliColor-Opponent Ganglion Cells
  21. 21. RETINAL PROCESSING

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