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Abstract

Cuff electrodes (40a, 40b) are surgically implanted around S.sub.3 sacral ventral root nerve trunks (16a, 16b). The sacral ventral roots have smaller diameter nerve fibers (20a, 20b) which convey action potentials to cause detrusor activation to contract the bladder (10) and larger diameter nerve fibers (18a, 18 b) which carry action potentials for causing contraction of a urethral sphincter (12) to block the flow of urine from the bladder. A current source (50) causes current pulses (52) between such electrical contacts (46, 48) and a central electrical contact (44). The current pulses have an appropriate amplitude and waveform to initiate action potentials adjacent the central contact and to block the propagation of action potentials adjacent the end electrodes along the larger diameter nerve fibers (which have fewer nodes between the contacts) but not the smaller diameter nerve fibers (which have more nodes between the electrodes). In this manner, action potentials are electrically excited to propagate at least downstream on the smaller diameter nerve fibers causing contraction of the bladder. Concurrently, blocking action potentials are allowed to propagate upstream on at least the larger diameter nerve fibers, collision blocking naturally occurring action potentials propagating downstream. The transmission of action potentials downstream on the larger diameter nerve fibers is blocked by the current pulses allowing the urinary sphincter to relax.

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