The electrophysiological identification of single nerve fibres, with particular reference to the slowest‐conducting vagal afferent fibres in the cat

Although there are many unmyelinated afferent axons in the peripheral nerves of mammals there is very little known either about their behaviour or about the receptors with which they are connected. Myelinated fibres, on the other hand, have frequently been studied as single units. This situation is due to the difficulty of isolating the unmyelinated fibres as single units for electrophysiological recording. Activity in unmyelinated afferent fibres has been recorded in multifibre preparations, notably by Zotterman (1939) and Maruhashi, Mizuguchi & Tasaki (1952). Only occasionally did these authors succeed in analysing activity in individual fibres. Douglas & Ritchie (1957a, b) have recently succeeded in detecting C fibre activity in whole nerves. Because of this lack of information about individual unmyelinated fibres it was decided to follow up the observation (Iggo, 1956b) that afferent fibres with conduction velocities as low as 1-3 m/sec could be isolated as single units from the cervical vagus of the goat. In addition to measurements of conduction velocity other tests were used in an attempt to decide whether the fibres were unmyelinated. New methods for identifying single fibres in a multifibre strand are described. With the aid of these methods and by refinements of the usual dissection methods very slowly conducting fibres have now been isolated in the cat. Evidence is presented for the conclusion that most of these fibres were probably unmyelinated. Preliminary reports have already appeared (Iggo, 1956 b, 1957 c). The gastric and intestinal receptors for which these figures were the centripetal axons are described elsewhere (Iggo, 1957a, b).