One of the coolest things about science – and probably one of the most annoying things to the general public – is that it is always changing. For example, when eggs are good for you, then they’re bad for you, then just the egg whites, and then the whole thing is good for you again. The reason for this, of course, is that science can never prove anything; it can only disprove. And if you disprove something over and over again, eventually people come to believe it is a general rule.
But every now and then, accepted general rules get disproven.
This is exactly what happened recently at Northwestern University, in part due to the fresh eyes of a graduate student named Mark Sheffield. While studying the functioning of neurons taken from the brains of mice, Sheffield noticed that the neuron continued firing now and then up to minutes after the neuron was stimulated. The researchers believe others had noticed this in the laboratory before but dismissed it as an error in the equipment. But thanks to the persistence and fresh look of Sheffield, the team discovered brand new attributes of neurons that fly in the face of everything currently written in textbooks. The results are published in a new paper in Nature.
Typical scientific thinking says that signals come in through small tree limb-like tentacles called dendrites, which transmit the signal to the body of the neuron called the soma before being carried off and rebroadcast by the tail-like axon. And that was the end of the story. That’s how neurons worked.
However, Sheffield and his teacher Nelson Spruston discovered two brand new modes of operation. First, it appears that the axon can work in reverse by sending signals into the cell’s body. Secondly, the axons can completely bypass the dendrites and cell body and talk directly to each other.
For the first discovery, the team noticed that the nerve cells could “store” their information for up to minutes before firing off signals. This contradicts the typical “signal in, immediate signal out” model that neuroscientists adhere to. What’s more, the signals generated minutes after the initial stimulation. This “persistent firing” takes place completely in the axon, without any electrical activity in the body or dendrites.
The second discovery was even more surprising. When they stimulated one neuron, they sometimes detected this persistent firing in the axon of a completely separate, unstimulated neuron, with no dendrites or cell bodies involved in the communication.
Both of these discoveries are a complete mystery. Since it was before thought to be impossible, the researchers don’t really know why these processes happen. But they do have theories.
One is that this slow-acting process – axons responding minutes after stimulation are thousands of times slower than normal neuron functions – is part of our working memory, but it could also be relevant to disease. The persistent firing of these inhibitory neurons might counteract hyperactive states in the brain, such as preventing the runaway excitation that happens during epileptic seizures.
The only thing that is for sure is that much more research is needed before anyone can really venture a guess.
“The next big question is: how widespread is this behavior?” said Spruston. “Is this an oddity or does it happen in lots of neurons? We don’t think it’s rare, so it’s important for us to understand under what conditions it occurs and how this happens.”