Dipole distance for minimum threshold current to stimulate unmyelinated axons with microelectrodes

Excitation thresholds for long nerve or muscle fibers with two point sources parallel to the fiber axis depend on the dipole length. The aim of this study was to find the optimal interelectrode distance for the minimum stimulation current. For a specific electrode-fiber distance (z_el) dipole length is constrained by the energy efficacy of the electrodes requiring small interelectrode distances, and by rather low stimulation currents requiring large dipole distances. Far-field values for optimal dipole distance (approximately 1.4 *z_el) can be explained by the superposition of the positive parts of the activating functions for the monopolar elements of the dipole. A current redistribution effect in a target fiber close to the electrodes shifts the dipole length for threshold stimulation from the theoretical optimal activating function approach value towards greater dipole distances. Spike initiations in straight fibers and retinal ganglion cell axons are investigated.