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Visual System (Light & Dark Reactions (Visual Pigment Molecules (w/in…

- - - - approx. 50pA (inward current) keeping Vm @ -40 mV
- - - - Vertical Pathway
        
        photoreceptor–>bipolar cell –>ganglion cell
        
        forms center of RF
      - Lateral Pathway
        
        photoreceptor –>horizontal cell –>bipolar cell –>ganglion cell
        
        horizontal cells mediate inhibition
        
        forms inhibitory surround RF
    - - ON-center
        
        Vertical Pathway
        
        light in RF center –> photoreceptor hyperpolarized –> bipolar cell depolarized
        
        center = generated by vertical connections photoreceptors –> bipolar cell –> ganglion cells
        
        Lateral Pathway
        
        light in receptive field surround –> photoreceptor hyperpolarized –> bipolar cell hyperpolarized
        
        surround = lateral pathway between cones & horizontal cells
        
        inhibition
      - OFF-center
        
        Vertical Pathway
        
        light in RF center –> photoreceptor hyperpolarized –> horizontal cell hyperpolarized –> bipolar cell hyperpolarized
        
        Lateral Pathway
        
        light in RF surround –> photoreceptor hyperpolarized –> horizontal cell depolarized –> bipolar cell depolarized
      - RF of ganglion cell – 2 parts = center & surround = mutually antagonistic
        
        surround mediates inhibition of response to stimuli that is effective in center
        
        center = much more sensitive, but surround = much larger
      - center surround receptive fields cause signals to be contrast signal (difference between center & surround)
  - - - small receptive fields
      - selective to particular wavelengths of light (color)
      - connected w/ analysis of fine detail & color
      - terminate in P (parvocellular layers of LGN
    - - large receptive fields
      - not wavelength selective
      - responds well to large objects & movement
      - terminate in M (magnocellular) layers of LGN
- - - - cotains ONLY cones
      - no blood vessels @ center so photons hit it directly
    - - enter optic nerve
      - no receptor @ optic disk = blind spot
    - - retina converts optical image –> neural image
    - - 3 interconnected layers & 5 classes of neurons
        
        Lateral Info Flow
        
        each synaptic stage, laterally connecting neurons = horizontal cells & amacrine cells
        
        modify forward transmission across synaptic layer (largely by inhibition
        
        Vertical Info Flow
        
        1) transduction of image by photoreceptors
        
        2) photoreceptors synapse on bipolar cells
        
        3) bipolar cells synapse onto ganglion cells
    - - impossible to encode all info in optical image using single neural image
      - parallel pathways
        
        different cells used to create parallel circuits for simultaneous transmission of multiple neural images
    - - transduce light into electro chemical energy
        
        Rods
        
        more sensitive to light
        
        more disks
        
        more photopigment to absorb light
        
        only 1 rod pigment
        
        use only rods in starlight conditions (scotopic vision)
        
        achromatic: 1 channel responding only to intensity (grayscale)
        
        greatest density outside the fovea
        
        Cones
        
        less sensitive to light
        
        use only cones @ high light levels
        
        3 types of cones for short, medium & long wavelengths