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 the dendrite 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 which at the time of nerve impulse conduction 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 resulting in a change in its membrane potential and in the consequent generation of a nerve impulse.