excitatory postsynaptic potential (EPSP)

An excitatory postsynaptic potential, or EPSP, is a small nudge of electrical change in a receiving neuron that pushes it a little closer to firing off its own signal. Picture a neuron as resting in a slightly negative electrical mood, like a quiet battery waiting. When the cell before it releases a chemical messenger across the tiny gap between them (the synapse), that messenger can open little doorways on the receiving cell that let positively charged particles flow in. This brief inflow makes the inside of the cell a touch less negative — a shift called depolarization — and that nudge toward zero is the EPSP. The word excitatory simply means it encourages, rather than discourages, the neuron from firing.

On its own, a single EPSP is usually too gentle to make anything happen — it is one small push, and it quickly fades. The magic is that a neuron listens to thousands of these inputs at once and adds them up. Each neuron has a tipping point called the firing threshold, and if enough EPSPs arrive close together in time or in space, their combined push can lift the cell past that threshold and trigger a full electrical spike (an action potential) that races down to the next cell. So EPSPs are like many hands giving a stuck door a series of shoves: any one shove does little, but enough of them at the right moment swing the door open. They also work against opposing inputs — inhibitory signals that push the cell the other way — and the constant tug-of-war between excitation and inhibition is how neurons decide, moment by moment, whether to speak up or stay silent.