The existence of a differential block is still part of the theory of regional anaesthesia. In 1980 it was described in detail by Cousins and Bridenbough in their standard work Neural Blockade. The theory of differential sensitivity of fibres in the peripheral nervous system essentially goes back to Gasser and Erlanger, who in 1929 established that when isolated peripheral nerves are electri cally stimulated in the presence of increasing concentrations of co caine, the compound action potentials of slow-conducting fibres are blocked before those of fast-conducting fibres show any measur able...