Selective Activation of peripheral nerve fibre groups of different diameter by triangular shaped stimulus pulses.

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Abstract
The differential block of cutaneous nerve fibers [cat] was achieved with a simple method of electrical stimulation, employing a single pair of active electrodes. The method allowed the selective activation of 95% of small myelinated (.delta.) axons, without activation of the larger (.beta.) ones; and activation of unmyelinated (C) fibers, without A fiber activation. Asynchronous firing of myelinated axons was absent in the majority of the experiments. The method employed triangularly shaped electrical pulses, with a steep rise front and a slow, exponential decay. The outward flow of current at the cathode fired conducted impulses in larger and smaller axons, and the inward flow inactivated differentially the conduction in the smaller ones. The differential effect of anodal currents depended upon the greater internal conductance and greater conduction velocity of larger fibers. The method has the advantage over the conventional polarization block of simpler surgical preparation, longer nerve survival and minimal latency distortion. However, it cannot be applied in experiments requiring physiological stimulation of peripheral receptors.