A neuron is made up of a main cell body called cyton and cell processes called axon and dendrites. The dendrites receive the electrical impulse and send it towrds the cell body. The cyton contains neurofibrils which transmit the current downwards towards the axon.
The axon if covered by a myelin sheath, will have certain gaps on its outer surface called the nodes of Ranvier. The electrical impulse jumps from one Node of Ranvier to the next. This kind of jumping conduction is called Saltatory conduction. Once the impulse reaches at the terminal of the axon, it stimulates certain vesicles which are there at the end called synaptic vesicles which contain a certain chemical called the neurotransmitter. The electrial stimulus bursts these vesicles and the neurotransmitter is then released into a gap called the synaptic cleft. This neurotrasmitter will now stimulate the next neuron. Hence the direction of the nerve impulse is always unidirectional.