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A model is presented that calculates the single-fiber extracellular field and action potential (ap) of an active myelinated nerve fiber placed centrally or eccentrically inside a nerve with a cylindrical geometry, representing essentially a one-fascicle nerve. This one-fascicle nerve has the dimensions and conductivities of the rat peroneal nerve branch. The results show a wide variety of wave shapes to be measured, depending on the position of the intraneural electrode with respect to the fiber axis and to the nodes of Ranvier and depending on the presence of an isolating cuff around the nerve. Action potential shapes may range from the 'classical' quasi-biphasic one, to more triphasic, or even more complicated in the case of a short insulating cuff being present around the nerve. In the latter case, when measured bipolarly, ap-wave shapes become almost monophasic. Index Terms - Action potential (ap) recording, extracellular ap modeling, functional electric stimulation and recording, nerve cuff, selective recording.

In epidural spinal cord stimulation it is likely that not only dorsal column fibers are activated, but that dorsal root fibers will be involved as well. In this investigation a volume conductor model of the spinal cord was used and dorsal root fibers were modeled by an electrical network including fiber excitation. The effects of varying some geometrical fiber characteristics, as well as the influence of the dorsal cerebrospinal fluid layer and the electrode configuration on the threshold stimulus for their excitation, were assessed. The threshold values were compared with those of dorsal column fibers. The results of this modeling study predict that, besides the well known influence of fiber diameter, the curvature of the dorsal root fibers and the angle between these fibers and the spinal cord axis were of major influence on their threshold values. Because of these effects, threshold stimuli of dorsal root fibers were relatively low as compared to dorsal column fibers. Excitation of the dorsal root fibers occurred near the entry point of the fibers.