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Electrical stimulation of unmyelinated nerve fibers is analyzed, using point sources in a simple volume conductor model and a dynamic Hodgkin-Huxley model of nerve fiber. The excitation and blocking threshold of single cathode stimulation with an indifference anode at infinity are calculated by solving difference equations with axons of different diameters. The relation between the blocking threshold and the pulsewidth of single cathode stimulation is also calculated. The results suggest a method of selectively stimulating the smaller fibers in a compound nerve trunk. Two kinds of stimulation electrodes are designed to test this method. Both of them are proven to yield results in accordance with the authors' model by the animal experiments on a toad's sciatic nerve trunk. It is possible to excite the smaller fibers without exciting the larger ones in a compound nerve trunk by properly controlling the stimulus intensity. The method is likely to be used in both physiological experiments and neural prostheses.
Date of Publication: March 1994