From Retina to Cortex

From Retina to Cortex

1. Gross anatomy of eye

   • Stimuli
     • Wavelength
       • Hue (quality)
       • Saturation (mix w/ b/w)
     • Intensity
       • (energy/time/area)
   • range 400 (v)-700 (r)
   • Refraction
     • cornea
       • Focus
         • lens
           • fovea
             • lose= functionally blind

2. cellular organization

   • Rod*
     • Many rods 1 bipolar
       • horizontal cell
         • amacrine
           • bipolar to ganglion
       • low acuity
     • more photopig
       • can detect single photons
     • dim
       • saturate in daylight
     • everywhere but foveola
       • slower
       • achromatic
   • Cone*
     • 1:1 with bipolar
       • no convergence
         • sharp
         • high acuity
     • less photopig
       • require hundreds photons
     • Bright
       • Saturate only intense
     • [fovea]
       • faster
       • colors

3. Receptors

   • Rhodopsin
     • 7 TM
     • Residue 296
       • attachment site for retinal
         • Trans when activated
           • no longer fits binding pocket
     • floating disc
     • Gi
       • inact cGMP phosphodiesterase
   • releasing glutamate at resting*
     • only shifts when illumination changes
       • cGMP inact
         • Close channel
           • 0 pA
             • reduced glutamate release
     • -40
       • hyperpol
         • -70*
     • cGMP act
       • influx Na+
         • -50 pA
           • glutamate release
   • response to light
   • retinotopic map

4. Receptive fields

   • bipolar
     • convergence
     • antagonistic center-surround receptive field
     • on=depol
       • so TECHNICALLY every photoreceptor = "off" receptor
         • even illumination +/- balance
         • BUT depending on bipolar cell (mGluR), this signal may activate or inactive
     • sometimes color-specific
       • red/green
       • blue/yellow
   • ganglion
     • action potential!
     • Magno+parvo
     • baseline activation
       • on
         • increase
       • off
         • ZERO
       • contrast needed*
         • not all or nothing
           • balances on/off signals, winner wins
     • more convergence!
     • parvocellular
     • magnocellular

5. cortical cells

   • LGN
     • ventral 2
     • Dorsal 4
     • segregation by visual field
     • centrifugal (descending) inputs
     • ~50%=fovea
   • V1*
     • 4C-beta
     • 4C-alpha
     • neurons sensitive to lines, edges, movement
     • neurons with binocular input
     • caudal=center
       • ~50%=fovea
     • simple vs. complex
       • Simple
         • particular on/off strips
       • complex
         • specific ANGLE, not on/off region
         • SOME movement sensitive
       • hypercolumn
         • bipolar! communication!

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