Blockade of nerve conduction by a local anaesthetic is characterized by:

Local anesthetics cross the neuronal membrane in un-ionized state and become ionized again in the neuron to block sodium channels. Due to this reason, sodium bicarbonate increases the rapidity of onset of action (LA are weak bases and in an alkaline medium easily cross the membrane). Un-myelinated and weakly myelinated fibres are blocked first and then the myelinated ones. These do not affect the resting membrane potential rather inhibit the depolarization. Resting nerves are less sensitive to block by LA than the stimulated nerves.