Unidirectional transmission of a nerve impulse through nerve fibre is due to the fact that

A synapse occurs where the axon of one neuron (the pre-synaptic neuron) meets either the dendrite or the cell body of another (the post-synaptic neuron). At the tip of the pre-synaptic axon is a button shaped swelling called a synaptic knob, inside which are numerous mitochondria and vesicles packed with a substance called a neurotransmitter is present. This neurotransmitters at the time of nerve impulse diffuse across the space and bind to receptors in the membrane of the post-synaptic cell. This opens ion channels in the post-synaptic membrane of the postsynaptic cell resulting in a change in the post-synaptic cell's membrane potential and (if the cell is sufficiently excited) in the consequent generation of a nerve impulse.