The Role of Neuronal Identity in Synaptic Competition

Are babies’ brains less complicated than those of adults? What methods are used in contemporary neuroscience for studying central nervous system? Jeremy R. Knowles Professor of Molecular and Cellular Biology at Harvard University Jeff Lichtman speaks on how axons are fighting each other for the right to dominate a target cell.

My laboratory and others have looked at young brains to see what does the wiring diagram look like in young brains. And the place where we can look easiest actually is not the brain at all, but it’s the connections between cells in the spinal cord called motor neurons and muscle fibers. It’s part of the nervous system, because the muscle fibers are caused to contract by signals that come out the axons of neurons that send processes to muscle. But it’s a very easy place to look, because there’s nothing else out there, there’s not a lot of wires, there’s only the wires coming from the spinal cord. And when we began looking at this we were surprised to see exactly the opposite of what you would have expected. At the time a young mouse is not yet a good walker, in its first week of life, when it’s just kind of crawling around but not walking yet, the wiring diagram of the muscle is not less complicated than the adult wiring diagram, it’s much more complicated.

Even though I think I’m an open-minded person compared to my children, I’ve become narrow-minded. There’s a well-known expression ‘you can’t teach old dogs new tricks’. Certainly we talk about old people as being set in their ways as if they’ve become less flexible about what they think. I think this is part of that learning process, that is what learning may be — choosing a way to think about the world. And so these mechanisms, these developmental mechanisms are important because they probably have a lot to do with what we become.

We’ve been working on a wide range of tools that allow us to see more clearly what’s going on in the central nervous system. One is to use more than two colors, to come up with techniques that generate maybe even thousands of colors. These are mice we call ‘brainbow mice’, because there’s a certain rainbow of colors in individual cells which allow us to disambiguate many different axons and dendrites at the same time. And the other is tools to slice brains so thin that in any one section it’s unambiguous what we’re seeing, and then look at slice after slice, after slice, after slice, after slice…