The neurons lie end to end in a chain to transmit the impules in an animal body. The neurons are not connected. there occur a very minute gap between terminal portion of axon, of one neuron and dendron of the other neuron. The gap is called synapse. At the end of the axon, the electrical impulse set offs the release of some chemicals. These chemicals cross the gap (synapes), and start a similar electrical impulse in dendrite of the next neuron.
Synapes actually act like one way valves. This is because the chemical substance is present at only one side of the gap. In this way synapes ensures that nerve impulses travels only in one direction.