Which are the properties of ON-center and OFF-center bipolar cells and how does this influence ganglion cell responses to light?

• A. ON-center bipolar cells depolarize in response to light increments due to reduced glutamate acting on mGluR6 receptors.
• B. OFF-center bipolar cells depolarize in darkness.
• C. Photoreceptors release more glutamate in light, which excites OFF-center bipolar cells.
• D. ON-center bipolar cells depolarize in response to light increments due to reduced glutamate acting on mGluR6 receptors; OFF-center bipolar cells depolarize in darkness due to photoreceptor glutamate driving ionotropic receptors (AMPA/kainate).

Answer: (D)

The different glutamate receptor types on ON- and OFF-center bipolar cells determine how they respond to changes in photoreceptor output, shaping ganglion cell activity to light.

When light increases in the center, photoreceptors release less glutamate. ON-center bipolar cells have mGluR6 receptors that are activated by glutamate to inhibit the cell; so with less glutamate, this inhibition is lifted, and the ON-center bipolar cells depolarize and drive the ON-center ganglion cells to fire more. In the OFF pathway, bipolar cells use ionotropic glutamate receptors (AMPA/kainate) that are excitatory. In darkness, high glutamate from photoreceptors activates these receptors, depolarizing OFF-center bipolar cells and boosting OFF-center ganglion cell activity. With light, glutamate falls, the OFF-center bipolar cells hyperpolarize, reducing OFF-center ganglion cell firing.

In short, ON-center cells respond to light increments with depolarization and increased ganglion firing, while OFF-center cells respond in darkness (or to light offsets) due to their receptor types, producing opposite sign signals that complement each other in edge and contrast detection.