Using tiny electrodes, researchers have discovered that retinal cells have distinct functions, producing various signals essential for how visual information is processed. Remarkably, these differences remain even in the retinas of blind animals, which is encouraging for the development of retinal implants.
The retina is often seen as an “extension of the brain.” This is because significant parts of visual signal processing occur not in the brain but within the nerve cells of the eye. When light reaches the retina, sensory cells activate and transmit electrical signals to the nerve cells located behind them. These signals are then sent to the brain.
Previously, it wasn’t clear how the retina’s signals were handled by the nerve cells. However, recent experiments at TU Wien (Vienna) have indicated that retinal ganglion cells may perform different functions for vision. Remarkably, they maintain these differences even when portions of the retina deteriorate, which is promising for restoring vision in blind individuals using electronic retinal implants.
Distinct cells with unique signals
“When light hits the photoreceptors in the retina, the nerve cells behind them generate electrical signals,” explains Paul Werginz from the Institute of Biomedical Electronics at TU Wien. “However, not all nerve cells produce the same signaling patterns.” Some types of nerve cells consistently activate when light is turned on or off. In certain cells, the frequency of their signals quickly drops, while others sustain a relatively high level of activity and continue generating strong electrical signals.
The reasons for these varying activity patterns were previously unknown. One might assume that cells of the same type would behave similarly. Werginz poses an interesting question: do the different behaviors of retinal ganglion cells result from their integration into diverse biological circuits with differing input signals, or is there an inherent difference guided by biophysical principles making these cells produce distinct signals despite receiving the same inputs? Essentially, each ganglion cell type could have a unique identity.
Electrical signals versus light
To explore this, the researchers utilized retinas from mice, maintaining their entire neuronal network functional for several hours. They could stimulate the retinal ganglion cells in two ways: by exposing the retina to light and observing the response, or by directly applying electric current to the ganglion cells. The latter method allows them to examine neuron properties without relying on the typical input sources.
Werginz notes, “We discovered that when we electrically stimulated the cells, their signaling patterns closely resembled those produced in response to light. Ganglion cells that displayed prolonged activity patterns when exposed to light behaved similarly when electrically stimulated.”
This indicates that the variations in signaling between these cells aren’t solely down to receiving different inputs in the retina’s circuitry; the tendency to produce longer or shorter signal patterns is an intrinsic characteristic of the cells.
“This is surprising but may play a crucial role in visual signal processing,” remarks Werginz. “These cell-type distinctions likely emerge early in the retina’s developmental process.”
Consistent characteristics, even in blindness
A vital question arises: are these intrinsic traits stable even if the cells lose their primary function, such as when the retina’s photoreceptors fail to function? One might expect their behavior to change. It’s often noted that unused nerve cells within the brain can be reorganized. For instance, if someone loses a finger, the nerve cells that handled signals from that finger do not just become inactive; they are repurposed for other functions.
However, retinal ganglion cells are different: “We investigated the cells from mice that had been blind for 200 days, and their retinal ganglion cells still exhibited precisely the same characteristics: some could be activated briefly by electric input, while others continued to show prolonged reactions,” says Werginz. Thus, these cells keep their fundamental capacity to transmit certain signals.
This is reassuring news for developing retinal implants, which aim to replace lost photoreceptors with electrical stimulation using thousands of electrodes in blind patients, according to Werginz: “If differences between cell types are stable, then the remaining ganglion cells can still be utilized post-blindness, allowing for the development of improved stimulation strategies in the future.”
