Inhibition is key.

Two sentences* from the third chapter of Prof. György Buzsáki's 2006 book "Rhythms of the Brain":

"In the absence of inhibition, any external input, weak or strong, would generate more or less the same one-way pattern, an avalanche of excitation involving the whole population."

"Without inhibition and dedicated interneurons, excitatory circuits cannot accomplish anything useful."

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*from pages 61 & 78, respectively, of the 2011 paperback edition.