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A method to activate electrically smaller nerve fibers without activating larger fibers in the same nerve trunk is proposed. The method takes advantage of the fact that action potentials are blocked with less membrane hyperpolarization in larger fibers than in smaller fibers. In this nerve stimulation system, quasitrapezoidal current pulses are delivered through a tripolar cuff electrode to effect differential blocking membrane hyperpolarization. The quasitrapezoidal pulses with a square leading edge, a 350- mu s plateau, and an exponential trailing phase ensure the blockage of propagating action potentials and prevent the occurrence of anodal break excitation. The tripolar cuff electrode design restricts current flow inside the cuff and thus eliminates the undesired nerve stimulation due to a virtual cathode. Experiments were performed on 13 cats. The cuff electrode was placed on the medial gastrocnemius nerve. Both compound and single fiber action potentials were recorded from L7 ventral root filaments. The results demonstrate that larger alpha motor axons could be blocked at lower current levels than smaller alpha motor axons, and that all alpha fibers can be blocked at lower current levels than gamma fibers.