electrotonic conduction

Electrotonic conduction is the quiet, passive way a voltage change spreads along a neuron's membrane — like ripples fading out across a pond. When a small electrical disturbance appears at one spot on the membrane (say, a gentle push of charge), that voltage doesn't just stay put: it leaks sideways through the neighboring stretch of membrane and tugs the voltage there a little too. But nothing actively pumps the signal forward. The membrane simply lets the change flow outward on its own, and as it travels, it shrinks. A few fractions of a millimeter away, the original bump in voltage is already much smaller; a bit farther still, it has all but vanished. Because the signal weakens steadily with distance, this kind of spread is called passive and decremental — decremental meaning it decays as it goes.

The reason it fades is that a neuron is a leaky cable, not a perfect wire. Its thin membrane holds charge like a tiny battery but also lets some of it dribble out, so current escapes through the walls as it tries to travel down the length. Engineers describe this with cable theory, and a single number — the length constant — captures how far the signal gets before it drops to about a third of its starting strength; fatter axons and well-insulated ones carry it farther. The key contrast is with the action potential, the neuron's all-or-nothing electrical spike, which regenerates itself at every step and so travels long distances without shrinking. Electrotonic conduction does no such regeneration: it is the fast, local, fading spread that, for example, carries small graded signals across a neuron's body and dendrites and helps decide whether an action potential will be triggered in the first place.