Ion-selective electrodes for manipulating cation availability in nerve stimulation and for neural conduction block

During stimulation, the applied electrical charge induces similar flows of multiple extracellular cations (K+,Na+ and Ca2+) in the electrode vicinity. This is rather counter-productive as these cations play varying roles in the initiation and propagation of action potentials. As a result, a significant percentage of applied electric charge is being wasted. Now, scientists at MIT and Harvard Medical School have reported a way to alter the cation concentrations using commercially available cation-selective resin solutions deposited on planar electrodes. In one experiment, they applied small positive DC current (≤1µA, 10 to 100 times below the nerve activation threshold)  to a centrally-located calcium-selective electrode for 1 min to deplete Ca2+ concentration from the fluid surrounding the nerve. Immediately thereafter, they applied the supra-threshold electrical pulses between two lateral uncoated electrodes, while the central electrode was off. The researchers achieved a 70% decrease in the amount of current required for reaching the nerve activation threshold. In another experiment, the K+– and Na+-selective electrodes were used to deplete the concentrations of these ions at some distance from the stimulating electrode. Such cation depletion caused a complete conduction block for 10 min after applying a cation-depleting DC current of 1µA for 5 min. Both K+– and Na+-selective electrodes were equally effective in blocking the action potential propagation. This finding could have important applications in shutting off the nociceptive neural activity in relieving chronic pain. Finally, the developed cation-selective electrodes have two important features making them attractive for neuroprosthetic applications: 1) they can be microfabricated and 2) they do not require a chemical reservoir for their operation.