Retina Journey: Horizontal vs. Amacrine Cells

The eye-to-brain pipeline through the retina is photoreceptors –> bipolar cells –> ganglion cells. So who are these strange horizonal and amacrine cells, and what are they doing just floating around in the retina?

You can thinking of horizontal and amacrine cells as a type of “helper” cell, or perhaps like a “station master” sitting at the junction between each cell type as the signal comes through.

Both of these cell types are known as inhibitory interneurons. They listen to signals coming in from multiple cells types and send out modulating responses based on what they “hear.”

Horizontal Cells

Horizontal cells are named for their horizontal orientation within the retina. They sit at the junction between photoreceptors and bipolar cells, and make connections with multiple photoreceptors in a given area.

The horizontal cells receive input from all the cells they synapse onto. Based on the input of the collective, they will (or will not) send out inhibitory GABA neurotransmitters back to these cells. This in turn affects the strength of the signal that gets sent directly from the photoreceptor to its partner bipolar cell.

Horizontal cells are categorized as “A” and “B” type, with A types being axon-less, and B types having dendrites that synapse with nearby cones, while the axon sends signals to rods.

Amacrine Cells

Amacrine cells were so named because they were thought to lack an axon. Their name is derived from Greek, meaning “without long fiber.” This is true for most of the 60+ types of amacrine cells that we know of, but a few wide-field amacrine cells are known to have “axon-like” processes. However, like with horiziontal cells, all cell processes are internal to the retina.

Amacrine cells do traffic control for signals traveling from the bipolar cells to the ganglion cells. Most amacrine cells are found in the INL (Inner Nuclear Layer) thought a select few can also be found in the GCL (Ganglion Cell Layer) where they are known as “displaced” amacrine cells.

These cells send out information in multiple directions. They send inhibitory feedback to the bipolar cells they receive signals from, feedforward inhibition to ganglions, and also communicate with other nearby amacrine cells.

Amacrine cells are still a bit of a mystery to us! Though we know about many of their shapes and stratifications, we still don’t know the function of many of these cells. For certain cell types like the AII and starburst amacrine cells (SAC) we do have some knowledge of their functions. Starbursts are important to motion detection and direction, while the AII cell play a roll in speeding up or slowing down signals originating in rod cells.

Now that you’ve learned a little more about these important interneurons, it’s time to pick your team. Will the horizontal cells or amacrines come out on top? Choose your favorite and let the competition begin!

The vs starts at 11:00 AM ET on 6/23 and goes for 48 hours. Bonus information is detailed in your notifications.