In what form do nerve fibres carry electrical impulses?

Neurons or nerve cells generate chemical or electrical impulses that pass over the cell's membrane as depolarization waves. 

Every neuron has a nerve cell body, as well as dendrites and axons, which carry the impulses. Signal transmission is always unidirectional, flowing either towards or away from the nerve cell body along a dendritic or an axon. 

Thus, the impulse always goes in one direction.