Physiology of Photoreceptors

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Physiology of Photoreceptors

• Vertebrate photoreceptors hyperpolarize and
  produce graded potentials
• Photoreceptors use glutamate as transmitter
• Bipolar cells can both hyperpolarize and
  depolarize producing both ON and OFF responses
• ON bipolar glutamate is inhibitory
• OFF bipolar glutamate is excitatory

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Receptive Fields

• Record from a single ganglion cell in the retina
• Using small spots of light activate the portion
  of the visual field that activates the neuron

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Schematic Retina Showing a Receptive Field
Orange are excitatory inputs into the receptive
field. Blue are inhibitory inputs into the
receptive field.
Receptors
Horizontal Cells

Bipolar Cells
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Amacrine Cells
Ganglion Cells
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Light strikes a cone with an excitatory connection
to the ganglion cell
Number of Action Potentials 1
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Light now fillsthe excitatory region of the
receptive field.
Number of Action Potentials 12
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If light falls in the inhibitory region,
the firing rate of cell is reduced.
Number of Action Potentials 8
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Lateral Inhibition Variations in the On/Off
Structure

• Lights on the edge of the field cause a reduction
  in the background activity of the cell
• On and off neurons
• Center-surround structure need to examine in
  light of different channels of information direct
  to the cerebral cortex

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Receptive Fields in the Retina

• Two types of ganglion cells
• on and off dependent upon the bipolar neurons
• Center Surround structure of the receptive field
  described by Kuffler
• Best activated by central illumination
• Best inhibited by annular illumination

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Different View of Center-Surround Organization
Parallel Pathways

• Transformation of visual information is evident
  in the ganglion cells of the retina
• X cells sustained linear responses
• Y cells transient, excitatory non-linear
  responses

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P and M Projections to LGN Different Physiologic
Channels

• P cells in the retina (also known as midget
  ganglion cells) project to the parvocellular
  layers (3-6) of LGN
• M cells in the retina (also known as parasol
  cells) project to the magnocellular (ventral
  most) layers (1-2) of the LGN
• Intercalated layers are termed koniocellular
  (dustlike or tiny cells)

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Physiology of V1

• Originally studied by Hubel and Wiesel who
  demonstrated two types of neurons
• Simple cells constructed from LGN on/off cells
• Complex cells constructed from simple cells

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Cortical Simple Cell
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Cortical Complex Cells Example of Hierarchy of V1

• Strong orientation selectivity in cells
• Moving bars in a specific direction
• NO on/off areas like in simple cells
• Receptive fields were not elongated
• Located in layers 2,3, and 5 which receive input
  from layer 4 (from ? simple cells)

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