We know that at least three types of cells in the retina allow us to see images or distinguish between night and day, but now, researchers at the Johns Hopkins School of Medicine have discovered yet another type of cell that can sense light and contribute to vision … in fish.
Reported recently in the magazine Nature, the team of neuroscientists says that retinal horizontal cells (nerve cells), once thought only to communicate with neighbouring nerve cells and not even to the brain, are light-sensitive themselves.
“This is mind-boggling,” said King-Wai Yau, PhD., a Professor of Neuroscience at the Solomon H. Snyder Department of Neuroscience at Johns Hopkins.
“For more than 100 years, it’s been known that rod cells and cone cells are responsible for sensing light, and therefore, vision. Then, about seven years ago, another light sensor was discovered in the retina, revealing a third type of light-sensitive cells in mammals,” he said.
Focusing their efforts on the melanopsin light sensor, which is responsible for sensing day and night but barely involved in mammals, Yau’s team looked for melanopsin containing cells in other vertebrates, and found some in the retinal horizontal cells in goldfish and catfish.
Catfish contain two flavours of retinal horizontal cells: those that connect to cone cells, which respond to bright light, and those that connect to rod cells, which respond to dim light. The team took electrical readings from single isolated retinal horizontal cells. They found that light caused a change in electrical current in cone horizontal cells but not in rod horizontal cells.
Horizontal cells, said Yau, allow cross-talk between neighbouring photoreceptor cells, allowing these cells to compare the light they sense, a process necessary for the brain to see images. The team found that these fish horizontal cells are thousands of times less light sensitive than their partner cone cells.
“The bottom line is that the light effect on the horizontal cells is subtle, perhaps to allow the eyes of these animals to fine-tune their functions to different ambient light conditions. That these horizontal cells are light sensitive at all is a very surprising finding and changes how we think about retinas as a whole,” said Yau.