A neuron that has just calmed down is like a pianist lifting their hands from the keys. But inside, silent work continues: tiny pumps push chloride ions outward, returning the instrument to a state of readiness. If the pumps fall behind, chloride builds up, and then an inhibitory signal meant to quiet the cell suddenly starts exciting it. This paradox, as a new study shows, drives the progression of an epileptic seizure.

The authors built a computer model of a neural network where the balance of excitation and inhibition depended precisely on this chloride 'recharge.' They turned one key parameter, the fraction of inhibitory signals that open channels for chloride entry, and watched the seizure pass through different stages. When this fraction is small, convulsions quickly die out or never start. Increase it, and the network enters the tonic phase: muscles stiffen like a taut string. Then comes the clonic phase with rhythmic twitching. At high values, spiral waves of activity sweep across the cortex, and inhibition can barely stop them.

A curious twist occurred at intermediate parameter values: the tonic stage dropped out, and the seizure jumped straight to clonic twitches. Moreover, when the authors simultaneously varied the strength of excitatory and inhibitory connections, the picture grew more complex. Stronger excitation widened the seizure zone, while stronger inhibition, on the contrary, lengthened the quiet period before the attack and dampened the clonic phase. So the simple chemistry of one ion turns into the conductor of a whole orchestra of symptoms. And the pianist at the keys regains control, or loses it, if the chloride balance is disturbed.