Which cells in the retina modify synapses between ganglion cells and bipolar cells via inhibitory feedback?

• A. Muller cells
• B. Astrocytes
• C. Horizontal cells
• D. Amacrine cells

Answer: (D)

Amacrine cells act as the inhibitory interneurons within the inner retina, connecting with bipolar cell terminals and ganglion cell dendrites in the inner plexiform layer. They release inhibitory neurotransmitters such as GABA or glycine, providing feedback (and feed-forward) inhibition that modulates the strength and timing of signals passing from bipolar cells to ganglion cells. This inhibitory control shapes receptive-field properties and helps with functions like motion detection and contrast adaptation. Muller and astrocyte glia mainly provide support and homeostasis, not targeted synaptic inhibition at this juncture. Horizontal cells mediate lateral inhibition at the outer plexiform layer, affecting photoreceptor-to-bipolar communication rather than the direct bipolar-to-ganglion synapse. So the cells that specifically modulate the bipolar-to-ganglion synapse through inhibitory feedback are the amacrine cells.