Lecture Notes: Bipolar Cells and Receptive Fields in the Retina

Photoreceptors

• Photoreceptors release glutamate in the dark.
• Light exposure stops the release of glutamate.
• Important to remember for physiology and MCAT questions.

Synapse Connections

• Photoreceptors synapse with:
  • Bipolar cells
  • Horizontal cells

Bipolar Cells

• General Function:

  • Release a small amount of glutamate without stimulation from rods.
  • Stimulate retinal ganglion cells by sending signals to the brain.

• Types of Bipolar Cells:

  • Off Bipolar Cells:
    • Depolarize with glutamate release from photoreceptors.
    • Increase glutamate release to retinal ganglion cells.
    • Cause faster firing of retinal ganglion cells.
  • On Bipolar Cells:
    • Hyperpolarize with glutamate release from photoreceptors.
    • Reduce glutamate release to retinal ganglion cells.
    • Cause slower or no firing of retinal ganglion cells.

Horizontal Cells

• Receive input from surround photoreceptors.
• Send inhibitory signals to center photoreceptors using GABA.
• Inhibitory interneurons reduce glutamate release from center photoreceptors.

Image Processing Scenarios

• Bright Center, Dark Surround:

  • Surround photoreceptors release glutamate, center does not.
  • Horizontal cell inhibits center, little glutamate to bipolar cell.
  • On Bipolar Cell:
    • Strong signal, rapid retinal ganglion cell firing.
  • Off Bipolar Cell:
    • Weak signal, slow retinal ganglion cell firing.

• Dark Center, Bright Surround:

  • Center photoreceptor releases glutamate, surround does not.
  • Minimal horizontal cell inhibition.
  • Off Bipolar Cell:
    • Strong signal, rapid retinal ganglion cell firing.

• Light Center, Light Surround:

  • No glutamate release from photoreceptors.
  • Minimal horizontal cell inhibition, low center stimulation.
  • Retinal Ganglion Cell Rate:
    • Intermediate firing rate.

Summary

• Different scenarios cause different firing rates in retinal ganglion cells.
• The brain interprets these rates as patterns.
• Real-life receptive fields contain more than three photoreceptors for more complexity.

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Note: This lecture provides a basic understanding of how receptive fields in the retina work. Further questions and comments are welcomed.