Outer segments of rods and cones contain.........

- Rod discs contain rhodopsin (retinal + opsin)

  -Do not provide color sensitivity

   cone opsins

Photopigments of cones

  portion of the spectrum; basis of color vision

  -Cone opsin provides short, medium, or long wavelength (l) sensitivity 

   -Red, green, yellow overlap, so red and green cones respond to red,

    green, and yellow, but with different response magnitudes

  -Arrayed as opponents (opposites)

     in receptive fields of ganglion cells

    -Red/green (P ganglion cells)

   -Blue/yellow (red and/or green,

    also P ganglion cells)

   -Rod/cone opponents see

     black/white (M ganglion cells)

Characteristics of Cone Vision

-6X106 cones in retina

-Work best in bright light

 (photopic vision)

-Provide high acuity vision of

 fovea, which has only long

 slender cones

-Struck directly by light

  -0.5–1:1 output via midget

  ganglion cells with small

  receptive fields (one subtype

   of P ganglion cell)

  -Less scatter, no convergence,

  small receptive fields 

  >spatial resolution

  Þ sharp image

-Up to 55 Hz (good temporal

 resolution)

-Provide color sensitivity

midget ganglion cells 

The fovea also contains tightly-packed cones that are more slender, elongated, and therefore, more sensitive than other cones.

These cones also provide better spatial resolution of signals, because they synapse onto single cone bipolar cells, which in turn synapse onto single small "midget" ganglion cells.

The midget ganglion cells therefore have smaller receptive fields than other ganglion cells, so they provide better spatial resolution of signals

Characteristics of Rod Vision

-100X106 rods in retina

  -  Concentrated in periphery

  -  Collect and amplify best, so can work in dim light (scotopic vision), but  “bleach” in bright light (saturated unless sensitivity adjusted)

-  Convergence  greatly increases sensitivity and also the size of M  ganglion cell receptive fields, so scotopic vision has low acuity

  - Up to 12 Hz temporal resolution (summation increases sensitivity , but decreases  temporal resolution)  

-  Do not provide color sensitivity

Rod Based Vision is Grainy because.......

, because multiple rods synapse onto rod bipolar cells, and multiple rod bipolar cells synapse onto amacrine cells that synapse onto the M-type ganglion cells.

Because more synaptic convergence occurs in the synaptic connections of the peripheral retina than in the fovea, and M ganglion cells have large receptive fields, and they provide poor spatial resolution.  

Horizontal Processing 

- Horizontal processing: Lateral +

   feedback (recurrent) inhibition via:

1) Horizontal cells modulate

      cones/rods  ---> bipolar cells

-Gap junctions connect horizontal

         cells and increase size of

         the receptive field "surround“

 -Close in twilight (no surround,

         so image is grainy)

2) Amacrine cells connect  or

       modulate transmission from bipolar

       cells to ganglion cells

-AII connect rod bipolar cells to rod  ganglion cells

 -Starburst provide motion sensitivity   of rod ganglion cells

       -Some A cells are dopamergic

Off- Bipolar and ganglion cells

- Dark current ----> Glu release

  from cones (and   rods)

- "Off-center" bipolar cells are

  depolarized by Glu via

  ionotropic GluR

  -λ therefore hyperpolarizes

  them by causing disfacilitation

    (reduced excitation)

  -Off-center ganglion cell is

  also disfacilitated: ongoing AP 

     frequency is decreased when

     λ is in the center of its field

  center relative to surround

"ON"- Bipolar Ganglion and Cells

Glu inhibits On-center cells:

- "On-center" bipolar cells are

   hyperpolarized by Glu via

  metabotropic GluR6

  -λ therefore depolarizes them

  by causing disinhibition

  (reduced inhibition)

- On-center ganglion cell is also

  disinhibited: ongoing AP

  frequency is increased when

  λ is in the center of its field

  in center relative to surround

Center vs. surround light has opposite effect on P (but not midget) and M ganglion cells

- Cells are either “On-” or “Off-

  center”, defined by center

  response

- Diffuse illumination  is not

   effective (no contrast!)

 - Most ganglion cells are

  therefore contrast-detectors

  fields, response properties

Horizontal cells generate the antagonistic responses to surround illumination

- The bipolar cells respond oppositely when the surround is illuminated

- The ganglion cells therefore respond oppositely when the surround is illuminated

- The antagonistic surround responses are mediated by horizontal cells (which are large and also connected by gap junctions)

- Horizontal cells release GABA, providing lateral and feedback  inhibition of the photoreceptors in the center (that balances dark current)