Shedding light on the synaptic complexities of vision

An individual retinal cell can emit more than a single signal, according to a Northwestern Medicine study published in Natural communicationsa discovery that sheds new light on the complexity of how vision works in mammals.

Bipolar cells in the retina are responsible for transmitting signals between the eye’s photoreceptors and ganglion cells, a type of neuron that sends light-activated signals to the rest of the brain for processing.

Previously, it was thought that ganglion cells received different signals from different cell types in the retina, said Gregory Schwartz, Ph.D., Derrick T. Vail Professor of Ophthalmology and senior author of the study.

In the current study, Schwartz and colleagues used single-cell electrophysiology and serial scanning electron microscopy to track signals inside a mouse retina exposed to light. The team discovered that individual synapses in a retinal bipolar cell can transmit different signals to neurons from the same light stimuli.

“We thought it would end up being relatively simple: that ganglion cells receive inputs from different cells. That’s the classic way of describing how cells have a different response, because their wiring diagram is different,” Schwartz said . “But what was surprising was that this wasn’t the case. There are differences inside very small neurons at the level of the synapses, not at the level of the neurons they are actually connected to. was surprising.”

Given that neurons can have hundreds or even thousands of synapses, the findings not only upend conventional theories about visual processing, but also highlight how complex and complex visual signaling can be.

“Even some of the smallest neurons throughout the nervous system can respond differently at different synapses,” Schwartz said.

The findings also have broader implications for understanding neuronal connections in the brain, said David Swygart, Ph.D., a former doctoral student. student in the Schwartz lab and first author of the study.

“We knew that each neuron could be a computational unit, but now we think that each synapse should be considered a computational unit and that there are many more synapses in the brain than there are neurons in the brain,” he said. Swygart said. , who is now a researcher at Indiana University School of Medicine.

Schwartz and his collaborators will continue to study synapses in the retina, he said, but additional technological advances are needed to better understand these tiny connections.

“We’re at the edge of modern recording technology here, because we’re talking about very small things: two synapses that could be 10 microns apart, doing something different,” Schwartz said. “You can’t place electrodes on both simultaneously, and even imaging, particularly in the retina, has its own problems, obviously because you’re using light to study light-sensitive circuits. Even imaging techniques “The most advanced functional imaging doesn’t do that. I can’t really get the answer we want to get.”