Summary

Abstract

The fibres of the peripheral and autonomic nervous system are divided into well-defined groups according to their conduction speed and physiological function. The groups are differentiated by axonal diameter and degree of myelination. According to a theory first formulated over 50 years ago, they are also distinguished from one another by differential sensitivity to local anaesthetics. The theory states that sensitivity decreases with increasing axonal diameter and myelination. The use of a sufficiently low concentration of local anaesthetic leads to the so-called differential block, not only in the recording of summation potentials in vitro, but also in clinical regional anaesthesia of peripheral nerves. The differential block consists in isolated interruption of conduction in non-myelinated fibres of the autonomic nervous system and in only slightly myelinated afferent fibres of temperature and pain receptors while voluntary motoricity and the sensation of touch remain unaffected. In regional anaesthesia using clinical doses, the differential sensitivity leads to a marked dissociation of latency times in these fibre systems. The theory of differential block has become a fundamental principle of neurophysiology, pharmacology and anesthesiology and has found expression in all the textbooks in these fields. Clinically, the differential block is used for differentiation, localization and sometimes therapy of vegetative pain syndromes, and also in obstetric peridural anaesthesia, in order to achieve analgesia of labour pains without affecting voluntary motoricity [6, 56, 58, 63].