Neural impulses reaching the presynaptic terminals have to communicate their information to the next neuron in line. The mechanism for doing so is the synapse which consists of a presynaptic terminal, a gap called the synaptic cleft, and the postsynaptic receptor site on the next neuron.
Synaptic transmission takes time.
Spatial summation - Different synaptic inputs firing together will have a combined effect on whether or not the next neuron fires. The close the input is to the axon hillock, the greater its effect. The further away from the axon hillock the input occurs, the harder it is for it to move the cell to the firing threshold. Another way to look at this is that the nervous system encodes inputs in terms of importance by putting the more important synapses closer to the axon hillock. Then again, if enough neurons get together, regardless of their location, they can also produce the next action potential.