First, the J cell is one of the first types of retinal neuron to be genetically controlled. In 2008, a team of Harvard researchers genetically engineered a mouse so that its J cells became fluorescent. Check out this spectacular image of J cells glowing when illuminated by ultraviolet light (like stuff that glows under black light when you go clubbing). Here’s another one. With this kind of genetic control, scientists can do yet other tricks, which makes experiments on the J cell easier.
Second, the J cell has a beautiful and remarkable shape. In the images above, you can see that J dendrites point downwards on the retina. This is different from the typical retinal neuron, which extends dendrites symmetrically in all directions. If you look at the overview of the J cell on EyeWire, you’ll see its beauty up close. Note that the dendrites curve to one side of the cell body.
Third, the J cell is activated by visual stimuli that move downward on the retina. The Harvard researchers recorded the electrical activity of the J cell while stimulating it with moving spots of light. They found that downward motion activated the J cell the most, and upward motion the least. Detecting motion is an important visual function for many animals, used for example to escape predators and capture prey. The J cell illustrates that the retina is not just for sensing light, but also starts the process of visual perception.
Is it a coincidence that J cell dendrites point downward, and the J cell is activated by downward motion? Or are the structure and function of the J cell related in some unknown way? Let’s figure it out!
You can find out more about the retina and the J cell in this interview with Prof. Josh Sanes, Director of the Harvard Center for Brain Science. There is also a more detailed article on the EyeWire wiki.