To send a signal, neurons disrupt this status quo in one of two main ways. The figure demonstrates that distant EPSPs start off stronger. Rather, it propagates along the neuron as a self-sustaining pulse of voltage known as an action potential. In effect, the original trigger's role is taken by the pulse itself. Because the signals regenerate themselves, active propagation is the only effective way to transmit signals down axons, which, in humans, can be as long as a meter.
Passive propagation is much simpler.
Ion channels, open at one end of the neuron, let in a packet of charge that simply pushes itself along the neuron by its own Coulomb repulsion. The signal gets no help from voltage-sensitive ion channels because the voltage never climbs high enough to open them. Consequently, passive propagation is unsuitable for axons, but it plays the major role in dendrites. The basic principles behind active propagation were worked out in the 1940s by Alan Hodgkin and Andrew Huxley. Studying signal propagation in dendrites is more challenging because the diameter of a typical dendrite in the mammalian brain is at most 25 m m.