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.
Greater potential to block a resting nerve as compared to a stimulated nerve
Need to cross the cell membrane to produce the block
Large myelinated fibers are blocked before the unmyelinated fibers
Cause consistent change of resting membrane potential