ABSTRACT
Local anaesthetic agents reversibly block the conduction of nerves by impairing the propagation of action potentials. They exist in two forms: a base (fat soluble) and a cation (water soluble). The base form is able to diffuse into the axon where it dissociates, enabling the cationic form to block the sodium channel. The equilibrium of the two forms is determined by the pKa of the local anaesthetic and the pH of the surrounding tissues. The susceptibility of nerves depends on several factors including nerve size and myelination, thus small nerve fibres (C and Aβ fibres) tend to be more easily blocked with higher doses being required to block other sensory modalities and motor fibres.
