Selective activation of small-diameter motor fibres using exponentially rising waveforms: a theoretical study

The present study investigated the possibility of using exponentially rising waveforms for selectively activating small motor fibres in a nerve bundle enclosed by a cuff electrode. Exponentially rising waveforms were studied using models of motor fibres and a volume conductor model. With an exponentially rising waveform (duration: 2 ms, time constant. 1 ms) large (15.5 mu m) and small (8 mu m) nerve fibres located at the edge of the nerve bundle had a current threshold of 125 mu A and 53 mu A, respectively. These reversals in the recruitment order of large and small nerve fibres located at the edge of the nerve bundle were observed for exponentially rising waveforms of 2, 4, and 6 ms in duration with time constants of 0.9, 0.6 and 0.6 ms, respectively. Reversals of the same nerve fibres located at the centre of the nerve bundle were observed for exponentially rising waveforms of 4 and 6 ms in duration, with a time constant of 0.6 ms for both waveforms. The underlying mechanism for selective activation of small nerve fibres with exponentially rising waveforms was found to be a combination of a decrease in the size of the local excitations in the centre node due to sodium channel inactivation and blocking of action potentials in large nerve fibres due to their larger difference in the membrane potential of adjacent nodes. The exponentially rising waveforms were compared with both rectangular prepulses and ramp prepulses. The rectangular prepulses were found to be unable selectively to activate small nerve fibres with the volume conductor model and criteria used in the present study, whereas the ramp prepulses performed as well as the exponentially rising waveforms. In conclusion, a novel stimulation paradigm has been proposed that may provide smooth, gradual control of muscle force with minimum fatigue.