21. C Fibre action potentials

Much of the electrophysiological work on C-fibres has been carried out using microelectrodes to record single fibre responses in animal studies due to the problems associated with recording of C-fibre compound action potentials. These problems are largely due to the very wide range of conduction velocities, producing large temporal dispersion of signals over typical stimulation to recording site distances, coupled with their low amplitude. However, the recording and identification of such potentials in humans is an important tool in the study of nociciption and pain. This paper will look at the factors which affect recorded compound action pote ntials and their inter-relationships. Computer simulation will be used to demonstrate how parameters under the control of the investigator such as electrode position and configuration can produce changes in the recorded wave shape. The same computer programme will model physiological parameter changes such as nerve depth, inter-nodal distances and conduction velocity and it will be shown how simulation techniques can help to identify the origin of morphological change. In particular, the effect of effective depolarisation length and conduction velocity distribution will be applied to a model of C-fibre stimulation. The resulting computer-simulated compound action potentials will be used to illustrate the problems associated with in-vivo recordings and how simulation may be used in conjunction with pattern-recognition techniques to improve their analysis.