A neuron receives input on its dendrites, adds it up in the cell body, carries the result along its axon, and hands it to the next cell at the terminal. This figure animates one neuron doing all of that, from the stimulus to the potential born in the next cell.
Potential arriving on a dendrite spreads passively and collects in the cell body. One weak stimulus does not reach threshold, but fire it twice in quick succession and the second arrives before the first has faded, so the two add up. That is temporal summation. Inputs arriving on different branches at the same moment add in the same way, which is spatial summation.
Whether the sum crosses threshold is decided at the axon hillock, where sodium channels are densest. Below threshold nothing leaves the cell body. Above it, an action potential of full size is produced. A stronger stimulus does not make a bigger spike, only an earlier one. That is the all-or-none law.
The action potential rises because sodium flows in, and comes back down because potassium flows out a little later. The gap between those two events is the width of the spike.
Conduction along the axon happens because the depolarized patch drives current into the patch ahead, which then reaches threshold and opens its own channels. An unmyelinated axon repeats this patch by patch, so the impulse only crawls.
Myelin is membrane wrapped many times over, and a sheathed stretch holds almost no charge and leaks almost nothing. Current that would have gone into charging the membrane runs down the axon instead. Channels sit only at the nodes of Ranvier, so current is only generated there. That is why the glow in the figure appears at the nodes and nowhere else, and it is what saltatory conduction means. Turn the sheath off in the controls and the same axon conducts several times slower.
The impulse does not run backwards, because the membrane it has just passed is refractory. While the inactivation gates are shut, no amount of stimulus produces another action potential.
When the impulse reaches the terminal, calcium channels open, and the calcium that enters is the trigger for vesicles to fuse with the membrane and empty transmitter into the cleft. Transmitter that crosses opens receptors, and a potential is born in the next cell. The synaptic delay in the readouts is the time this chemical step costs.
That potential stays far below threshold. A real neuron receives thousands of synapses and fires only when many of them line up in time and place. One synapse is not enough, and the figure shows exactly that.
Pick a part in the parts panel, or press the figure directly, and the frame and the explanation move there. The controls panel fires the stimulus and takes the myelin sheath on and off. The readouts panel shows the conduction velocity and the synaptic delay. Panels can be dragged by their header and resized from any edge or corner, and double-clicking a title pins it to the top right.