EXTRACELLULAR POTENTIALS FROM ACTIVE MYELINATED FIBERS INSIDE INSULATED AND NONINSULATED PERIPHERAL-NERVE

A model is presented that calculates the single-fiber extracellular fi eld and action potential (ap) of an active myelinated nerve fiber plac ed centrally or eccentrically inside a nerve with a cylindrical geomet ry, representing essentially a one-fascicle nerve. This one-fascicle n erve has the dimensions and conductivities of the rat peroneal nerve b ranch. 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 prese nce of an isolating cuff around the nerve. Action potential shapes may range from the ''classical'' quasi-biphasic one, to more triphasic, o r even more complicated in the case of a short insulating cuff being p resent around the nerve. In the latter case, when measured bipolarly, ap-wave shapes become almost monophasic.