Scientists have believed that neurons need a long period of fine-tuning and training with other neurons before they take on their adult role. But after using new technology for the first time to watch these cells develop, a team of researchers at the Stanford University School of Medicine found that neurons come into this world with a good idea about what they’ll become as adults. The work, which is detailed in a paper in the March 24 issue of Neuron, took place in the brain of a small see-through fish called a zebra fish. Stephen Smith, PhD, professor of molecular and cellular physiology, and graduate student Christopher Niell immobilized a young fish at an age when the nerves first grow from the eye to reach the brain. Then, with the aid of a 6-foot-long laser and some fancy microscopy, the researchers were able to watch individual neurons as they matured in real time. The pair specifically monitored hundreds of neurons in the region of the brain that respond to images. Niell set up a tiny LCD screen showing squares the size of the fish’s favorite planktonic food moving up and down or left and right. They expected to find that young neurons fire in response to a variety of different images, then refine their role over time so that in the adult fish the neurons only respond to images moving in a certain direction or near the left or right side of the visual field. What they found was a surprise. As soon as the neurons were old enough to respond to the LCD screen, they specifically fired when they sensed only one type of movement. When the tiny square moved left to right, a distinct population of neurons turned fluorescent colors to indicate their activity. Moving the square the reverse direction triggered a different population of neurons to light up.
