postsynaptic potential

A postsynaptic potential is a small, brief change in the voltage across the membrane of a neuron that is receiving a message. Neurons talk to each other across tiny gaps called synapses: the sending cell squirts out chemical messengers called neurotransmitters, and these drift across the gap and stick to matching catcher molecules, called receptors, on the receiving cell. That sticking pops open or closes tiny gated pores in the membrane, letting charged particles (ions) flow in or out. The flow nudges the receiving cell's voltage up or down a little, and that nudge is the postsynaptic potential. Think of it as the soft knock on the door that one neuron gives another, rather than the full shout of a nerve impulse.

These nudges come in two flavors. An excitatory postsynaptic potential pushes the cell's voltage toward firing, making it a bit more likely to send its own signal; an inhibitory one pulls it the other way, making firing less likely. Each single knock is usually too weak on its own to do much, so the receiving neuron acts like a tiny accountant, adding up the pushes and pulls arriving from thousands of synapses moment by moment. If the running total tips the voltage past a threshold, the cell fires off an all-or-nothing spike of its own. In this way, postsynaptic potentials are the votes that decide whether a neuron speaks up, and they sit at the heart of how brains compute, learn, and store memories.

Two features make them special. Unlike the fixed, full-sized spike that travels down a neuron, postsynaptic potentials are graded — they come in many sizes, depending on how much neurotransmitter landed — and they are local, fading out as they spread from the synapse instead of marching the whole length of the cell. This graded, fading quality is exactly what lets a neuron weigh and blend its many inputs rather than simply passing along the first message it receives.