Why does a rise of potassium outside the cells make every neuron nearby easier to fire? · Grey Matter

The resting voltage is set mainly by the potassium gradient, so it depends on the ratio of potassium inside to potassium outside.

Why does a rise of potassium outside the cells make every neuron nearby easier to fire?

The resting voltage is set mainly by the potassium gradient, so it depends on the ratio of potassium inside to potassium outside. Outside, potassium is only a few millimolar, so adding a few more millimolar changes that ratio a lot and moves every neuron's resting voltage upward, closer to threshold, all at once. Intense firing releases potassium into the narrow extracellular space faster than pumps can take it back, and astrocytes normally soak it up and spread it through their coupled network. When they fall behind, the extra potassium depolarises the whole neighbourhood, which raises excitability and, at high levels, can start a spreading depolarisation.