Unlike a computer, there’s no powering down for your brain — whether you’re quietly daydreaming, anesthetized, or in the deepest sleep, your "operating system" continues to run. A new study from Brown University probed mice to learn exactly how the brain maintains the necessary baseline of activity while idle. The researchers discovered the brain cycles up and down between a state of excitement and inhibition, with each of five separate neurons contributing in a unique way to these balanced modulations.

“Too much excitation relative to inhibition you get a seizure, too little you become comatose,” said Dr. Barry Connors, professor and chair of neuroscience at Brown and senior author of the new study. “So whether you are awake and active and processing information or whether you are in some kind of idling state of the brain, you need to maintain that balance.”

To begin their study, the researchers concentrated on the barrel cortex, a part of the outer layer of the mouse brain that is responsible for processing sensations on the animal’s face, including its all-important whiskers. Specifically, the researchers looked at the activity of excitatory pyramidal cells and four kinds of inhibitory interneurons (PV, SOM, VIP, and NPY) located in the barrel cortex.

Pyramidal cells receive excitatory impulses, essentially a chemical current caused by neurotransmitters. In mammals, these cells are the most numerous of all excitatory cell type in the cortical structures, suggesting they must play important roles in various cognitive functions. By comparison, the job description of an interneuron, a specialized type of neuron, is to form a connection between other types of neurons.

For the experiment, then, Neske and his team extracted slices of tissue from the barrel cortex of mice, and then they induced up and down cycles within these tissues. All the while they recorded each cell’s electrical properties and behaviors — firing rates, for example, and responses to excitation. The overall activity, they discovered, formed a continuous dance of interneuron activity.

Head-to-Head

Activity among the neurons included the fast spikes of PV cells and the slow spikes of the SOM, VIP, and NPY cells. Surprisingly, the latter cells were active at levels similar to or even higher than neighboring excitatory pyramidal cells. Apparently, when going head to head with their excited neighbors, these inhibitory cells need to match the general commotion with slowing down effects of their own. This balance of power between interneurons and pyramidal cells maintains an idle but always ready state of mind, just like a car waiting at a red light.

The study “calls for more comparative work to be done among cortical areas,” Neske said. Going forward, he plans to more extensively explore these cells to gain an even clearer picture of the brain's methods for remaining balanced while waiting.

Source: Neske GT, Patrick SL, Connors BW. Contributions of Diverse Excitatory and Inhibitory Neurons to Recurrent Network Activity in Cerebral Cortex. Journal of Neuroscience. 2015.