Receptive Fields and Responses in Bipolar and Ganglion Cells

Definition and Basics

• Receptive Field: Group of photoreceptors sending inputs to a bipolar cell or retinal ganglion cell, which then passes info to the brain.
• Photoreceptors: Cones and rods respond to light in the retina.
• On and Off Center Receptive Fields: Different responses to light based on bipolar cell types.

Photoreceptor Response to Light

• Hyperpolarization: Photoreceptors hyperpolarize in response to light due to opsins breaking down.
• Graded Potentials: Photoreceptors respond with graded potentials and release glutamate proportionally to depolarization level.
• Reduced Glutamate in Light: Results in less glutamate release when hyperpolarized by light.

Bipolar Cell Responses

• On-Center Bipolar Cells:
  • Have metabotropic glutamate receptors.
  • Light: Receives less glutamate, depolarizes, and releases more glutamate.
  • Dark: Receives more glutamate, hyperpolarizes, and releases less glutamate.
• Off-Center Bipolar Cells:
  • Have ionotropic glutamate receptors.
  • Light: Receives less glutamate, hyperpolarizes, and releases less glutamate.
  • Dark: Receives more glutamate, depolarizes, and releases more glutamate.

Ganglion Cell Responses

• Action Potential Firing: More glutamate from bipolar cells results in more action potentials in corresponding ganglion cells.
• On-Center vs. Off-Center:
  • On-Center Cells: Depolarize with less glutamate, respond more in light.
  • Off-Center Cells: Hyperpolarize with less glutamate, respond more in dark.

Influence of Surround on Receptive Field

• Dark Surround:
  • Surround photoreceptors depolarize → Release more glutamate onto horizontal cell.
  • Horizontal cell depolarizes and releases glycine, inhibiting central photoreceptor → Less glutamate released.
  • High response in on-center ganglion cells (lower glutamate preferred due to metabotropic receptors).
• Light Surround:
  • Surround photoreceptors hyperpolarize → Release less glutamate onto horizontal cell.
  • Horizontal cell releases less glycine, reducing inhibition on central photoreceptor → More glutamate released.
  • High response in off-center ganglion cells (more glutamate preferred).

Examples and Firing Responses

• Optimal Conditions:
  • Off-Center Bipolar Cell: Dark in the center, light in the surround → Highest depolarization rate → Most action potentials in ganglion cell.
  • Compare Examples:
    • Figure A: Light center (X), Light surround (✔) → Medial firing rate.
    • Figure B: Dark center (✔), Light surround (✔) → Maximal firing rate.
    • Figure C: Dark center (✔), Dark surround (X) → Medial firing rate.

Summary

• Different bipolar cells (on-center vs. off-center) and their receptors (metabotropic vs. ionotropic) determine specific responses to light or dark conditions.
• Surround conditions influence the central photoreceptor’s inhibition and overall response.

Questions

• Leave any questions about receptive fields and bipolar/ganglion cell responses in the comments for further clarification.