Why can a neuron not fire again immediately after a spike, and why does the spike travel only one way? · Grey Matter

About a millisecond after opening, each sodium channel is plugged by its inactivation gate and stays shut until the membrane has been back near rest for a while, so for that time no input can fire a second spike (the absolute refractory period).

Why can a neuron not fire again immediately after a spike, and why does the spike travel only one way?

About a millisecond after opening, each sodium channel is plugged by its inactivation gate and stays shut until the membrane has been back near rest for a while, so for that time no input can fire a second spike (the absolute refractory period). Meanwhile potassium channels are still open, holding the voltage low, so for a few milliseconds more only a stronger input succeeds (the relative refractory period). Along the axon, the patch the spike has just left is refractory, so the current can only excite the patch ahead. Together these effects cap the firing rate and make conduction one-way.