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Chapter 9: The Eye (Gross Anatomy of the Eye (Pupil: opening where light…

- - - - If light is shined in one eye, it should cause a pupillary response in both eyes
- - - - Respond to light
      - Rod and cone photoreceptors are the only light-sensitive cells in the retina
        
        Rods contain photopigment rhodopsin
        
        Many more rods than cones because their regions containing membranous disks are much large than those of cones
        
        Central retina: most cones are in the fovea - no rods in the central fovea
        
        Fovea: pit in the retina where the outer layers are pushed aside
        
        At the fovea, light strikes the photoreceptors directly, maximizing visual acuity
        
        Peripheral retina: higher ratio of rods to cones and of photoreceptors to ganglion cells
        
        Cones have three types of photopigments which are sensitive to different wavelengths, making them only responsible for color vision
      - "Duplex Retina"
        
        Scotopic conditions (night): mostly rods contribute to vision
        
        Photopic conditions (day): mostly cones contribute to vision
        
        Mesopic conditions (twilight): both contribute
    - - Only source of output from the retina - the only cells in this pathway which project axons through to the optic nerve
    - - Receive input from bipolar cells and project to ganglion cells, bipolar cells, and other amacrine cells
    - - Receive input from photoreceptors and project to other photoreceptors and bipolar cells
- - - - 11 cis-retinal: changes shape when it absorbs light - all-trans retinal
      - opsin: large membrane-bound protein which tunes the molecules' absorption of light to part of the visual spectrum
    - - Na+ channels are gated by cGMP, which is produced by guanylyl cyclase, and which keeps the channels open - light reduces cGMP, allowing the channels to close
      - Phototransduction: light energy interacts with rhodopsin in the stacked discs - rhodopsin is a receptor protein with a prebound retinal
        
        Activates transducin, whose alpha subunit activates phosphodiesterase (PDE)
        
        PDE hydrolyzes cGMP, Na+ channels close and the cell is hyperpolarized
  - - - Red (long wavelength)
      - Green (medium wavelength)
      - Blue (short wavelength)
- - - - The ciliary muscles are responsible for this action - contraction of the muscles causes lens to become rounder and thicker, whereas relaxation causes it to become flatter