How the Brain Balances Excitation and Inhibition

Summary

Inhibitory neurons have “often been ascribed support roles,” said Annabelle Singer, a neuroscientist and neuroengineer at the Georgia Institute of Technology and Emory University. That’s likely because it’s simply easier to study excitatory neurons. For example, an excitatory place cell in the hippocampus can fire when an animal is in a particular location. When this happens, its excitation of other cells can be observed. “It’s very clear-cut,” she said. But an inhibitory neuron “fires a lot everywhere, and it’s much harder to say what is it responding to,” she said. We don’t know what signal it is inhibiting, and the cells connected to it don’t respond with firing of their own. Still, studies are starting to illuminate how and when inhibitory neurons fire. In a recent study published in Nature, Singer and her colleagues found that inhibitory neurons help mice learn rapidly and remember where to find food by selectively decreasing how much they fire when the animal is near a location where food can be found. By firing less frequently as the mouse approaches the location, inhibitory neurons enhance the desired signals, thereby “enabling this learning about the important location,” Singer said. This suggests that they play a much more active role in memory than previously thought. Neurotransmitters work really fast, but a lot of the behavioral and cognitive components that we need are really slow. Mark Cembrowski, University of British Columbia What’s more, the prevalent view of inhibitory neurons once cast them as more generalist in their activity, doing this kind of “blanket-y inhibition, inhibiting everything that is around their axons,” said Nuno Maçarico da Costa, a neuroscientist at the Allen Institute. But da Costa and his team, as part of the Microns project, a large-scale effort to fully map out a 1-cubic-millimeter portion of a mouse’s visual cortex, discovered that inhibitory neurons are very specific in choosing what cells to inhibit. The brain’s circuits ...