Developmental neurophysiology of mammalian peripheral nerves and age-related differential sensitivity to local anaesthetic

Using an in vitro nerve preparation, we have studied the relative electrophysiological properties of myelinated and unmyelinated nerve fibres in the vagus nerve of 1-, 9- and 36-month-old rabbits and their sensitivity to local anaesthetic. The baseline (values before infusion of local anaesthetic) mean amplitude and conduction velocity (CV) of the compound action potential (APc) were recorded and the nerve was exposed to a range of concentrations (0.5-4.0 mmol litre^-1) of lignocaine for periods sufficient to attain equilibrium block. There was an increase in the amplitude of the A fibre elevation from the 1-month to the 9- and 36-month-old rabbits. The CV of the A and B fibres increased significantly with age, while the CV of the C fibres did not change. The ED₅₀ values of lignocaine for reduction of the A fibre elevation in the 1-, 9-and 36-month-old rabbits were 0.66, 0.94 and 0.85 mmol litre^-1, respectively. The respective values for the B fibres were 0.74, 1.21 and 0.82 mmol litre^-1, while those of the C fibres were 1.50, 2.44 and 2.07 mmol litre^-1. In general, nerves from young and old rabbits were more sensitive to local anaesthetic-induced conduction blockade, suggesting that smaller doses of local anaesthetic are required clinically for anaesthesia in paediatric and older age groups.