Modulation of axonal excitability by high-frequency biphasic electrical current

Impact of Bioelectronic Medicine on the Neural Regulation of Pelvic Visceral Function

Neuromodulation elicited by electrical stimulation of peripheral or spinal nerves is a U.S. Food and Drug Administered (FDA)-approved therapy for treating disorders of the pelvic viscera, including urinary urgency, urgency-frequency, nonobstructive urinary retention and fecal incontinence. The technique is also being tested experimentally for its efficacy in treating interstitial cystitis, chronic constipation and pelvic pain. The goal of neuromodulation is to suppress abnormal visceral sensations and involuntary reflexes and restore voluntary control. Although detailed mechanisms underlying the effects of neuromodulation are still to be elucidated, it is generally believed that effects are due to stimulation of action potentials in somatic afferent nerves. Afferent nerves project to the lumbosacral spinal cord, where they release excitatory neurotransmitters that activate ascending pathways to the brain or spinal circuits that modulate visceral sensory and involuntary motor mechanisms. Studies in animals revealed that different types of neuromodulation (for example, stimulation of a sacral spinal root, pudendal nerve or posterior tibial nerve) act by releasing different inhibitory and excitatory neurotransmitters in the central nervous system. In addition, certain types of neuromodulation inhibit visceral smooth muscle by initiating reflex firing in peripheral autonomic nerves or excite striated sphincter muscles by initiating reflex firing in somatic efferent nerves. This report will provide a brief summary of (a) neural control of the lower urinary tract and distal bowel, (b) clinical use of neuromodulation in the treatment of bladder and bowel dysfunctions,

Post stimulus effects of high frequency biphasic electrical current on a fibre's conductibility in isolated frog nerves

OBJECTIVE

High frequency biphasic (HFB) electrical currents are widely used in nerve blocking studies. Their safety margins largely remain unknown and need to be investigated.

APPROACH

This study, exploring the post stimulus effects of HFB electrical currents on a nerve's conductibility, was performed on bullfrog sciatic nerves. Both compound action potentials (CAPs) and differential CAPs (DCAPs, i.e. control CAPs subtracted by CAPs following HFB currents) were obtained, and N1 and N2 components, which were the first and second upward components of DCAPs, were used for analyses of the effects introduced by HFB electrical stimulation.

MAIN RESULTS

First, HFB currents of 10 kHz at a completely blocking threshold were applied for 5 s. The maximum amplitudes and conducting velocities of the CAPs were significantly (P < 0.02) decreased within the observed period (60 s) following HFB currents. The DCAPs displayed clear N1 and N2 components, demonstrating respectively the losses of the fibres' normal conductibility and the appearances of new delayed conductions. Decreases of N1 amplitudes along time, regarded as the recovery of the nerve's conductibility, exhibited two distinct phases: a fast one lasting several seconds and a slow one lasting longer than 5 min. Further tests showed a linear relationship between the HFB stimulation durations and recovering periods of N1 amplitudes. Supra-threshold blocking did not cause higher N1 amplitudes.

SIGNIFICANCE

This study indicates that HFB electrical currents lead to long lasting post stimulus reduction of a nerve's conductibility, which might relate to potential nerve injuries. A possible mechanism, focusing on changes in intracellular and periaxonal ionic concentrations, was proposed to underlie the reduction of the nerve's conductibility and potential nerve injuries. Greater caution and stimulation protocols with greater safety margins should be explored when utilizing HFB electrical current to block nerve conductions.