A healthy brain maintains a harmony of neurons that excite or inhibit other neurons, but the lines between different types of cells are blurrier than researchers once thought.

From Santiago Ramón y Cajal’s hand came branches and whorls, spines and webs. Now-famous drawings by the neuroanatomist in the late 19th and early 20th centuries showed, for the first time, the distinctiveness and diversity of the fundamental building blocks of the mammalian brain that we call neurons.

In the century or so since, his successors have painstakingly worked to count, track, identify, label and categorize these cells. There is now a dizzying number of ways to put neurons in buckets, often presented in colorful, complex brain cell atlases. With such catalogs, you might organize neurons based on function by separating motor neurons that help you move from sensory neurons that help you see or number neurons that help you estimate quantities. You might distinguish them based on whether they have long axons or short ones, or whether they’re located in the hippocampus or the olfactory bulb. But the vast majority of neurons, regardless of function, form or location, fall into one of two fundamental categories: excitatory neurons that trigger other neurons to fire and inhibitory neurons that stop others from firing.

Maintaining the correct proportion of excitation to inhibition is critical for keeping the brain healthy and harmonious. “Imbalances in either direction can be really catastrophic,” said Mark Cembrowski(opens a new tab), a neuroscientist at the University of British Columbia, or lead to neurological conditions. Too much excitation and the brain can produce epileptic seizures. Too little excitation can be associated with conditions such as autism.

Neuroscientists are working to uncover how these two classes of cells work — and specifically, how they interact with a rarer third category of cells that influence their behavior. These insights could eventually help reveal how to restabilize networks that get out of balance, which can even occur as a result of normal aging.

...read more at quantamagazine.org