Influence of variable nerve fibre geometry on the excitation and blocking threshold. A simulation study A. Vuckovic J. J. Struijk N. J. M. Rijkhoff Center for Sensory-Motor Interaction (SMI), Department of Health Science & Technology, Aalborg University, Aalborg, Denmark Abstract —The aim of the study was to investigate how variable fibre geometry influ- ences the excitation and blocking threshold of an undulating peripheral nerve fibre. The sensitivity of the excitation and blocking thresholds of the nerve fibres to various geometric and stimulation parameters was examined. The nerve fibres had a spiral shape (defined by the undulation wavelength, undulation amplitude and phase), and the internodal length varied. Diameter-selective stimulation of nerve fibres was obtained using anodal block. Simulation was performed using a two-part simulation model: a volume conductor model to calculate the electrical potential distribution inside a tripolar cuff electrode and a model of a peripheral undulating human nerve fibre to simulate the fibre response to stimulation. The excitation threshold of the undulating fibres was up to 100% higher than the excitation threshold of the straight fibres. When a nerve was stimulated with long pulses, which are typically applied for anodal block (>400\JLS), the blocking threshold of the undulating fibres was up to four times higher than the blocking threshold of the straight fibres. Dependencies of the excitation threshold on geometric and stimulation parameters were the same as for a straight fibre. Dependencies of the blocking threshold on geometric and stimulation parameters were different compared with a straight fibre. Owing to the fibre undulation and variable internodal length, the blocking threshold and the minimum pulse duration to obtain anodal block were generally different in the proximal and distal directions. Owing to variable fibre geometry, the excitation threshold varied by up to -f- 40% of the mean value, and the blocking threshold varied by up to ±60 % of the mean value. Owing to undulation, the blocking threshold of large fibres could be higher than the blocking threshold of smalt-diameter fibres, even if they had the same geometry. The results indi- cate that, during skeletal muscle stretching and contracting or during variation in joint angle, the excitation and blocking thresholds of the nerve fibres change owing to vari- ations in fibre geometry. A straight fibre model could be too simple for modelling the response of peripheral nerve fibres to electrical stimulation. Keywords—Anodal block. Selective fibre activation. Peripheral nerve model. Undulat- ing nerve fibre. Variable fibre geometry Med. Biol. Eng. Comput.. 2005, 43, 365-374 1 Introduction To OUTAIN a gradual increase in force during activation of skeletal muscles, a natural recruitment order, which includes acti- vation of small before large nerve fibres, is needed (HENNEMAN. 1981). Such an aclivation order of nerve fibres can be achieved by electrical stimulation using the technique of anodal bktck (RUKHOFF et al.. 1994). In this technique, both large and small nerve fibres are excited, whereas propagation of action potentials is selectively blocked in large nerve fibres only. Correspondence should be addressed to Dr Aleksandra Vuckovic; email; av@smi.auc.dk Paper received 26 July 2004 and in final form 20 January 2005 MSEC online number: 20054010 • IFMBE:2005 The influence of various electrical and geometric parameters on the excitation and blocking thresholds of the nerves in sacra! roots was examined in a simulation study (RlJKHOFP et al.. 1994). In that study, a myelinaled nerve fibre model describing a straight nerve fibre, with constant morphological parameters along the fibre, was used. However, peripheral nerve fibres follow an undulating course of variable wavelength and ampli- tude (HANINKC, 1986), and morphological parameters vary as well (BEHSE, 1990). The undulating course of peripheral nerve fibres is related to the quantity and arrangement of the endoneurial collagen fibres in the nerve (HANINEC, 1986). The nerve fibre's undulation represents a physiological reserve length for nerve stretching (CLARKH and Br;ARN, 1972; HANINHC. 1986). Undulation of nerve fibres has been reported for nerves in rats (CLARKE and BEARN, 1972; HANINEC. 1986) and humans (ZVERINA and SPRINCL. 1979). Medical & Biological Engineering & Computing 2005, Vol. 43 365