In nerve electrophysiology, it is widely believed that two colliding nerve pulses annihilate and cannot pass through each other. However, there is little evidence in the literature that this process actually occurs. We show that action potentials in the nerve fibers of earthworms and lobsters in fact pass through each other without inducing major perturbations.

We compare two different approaches for explaining the nerve pulse: the Hodgkin-Huxley model and a more recent electromechanical soliton model. The former does not allow for pulse penetration; the latter treats the nerve pulse as a nondissipative electromechanical pulse and predicts pulse penetration. Therefore, penetration experiments are critical for discriminating between the two approaches. We use the ventral cords of earthworms and lobsters, specifically the median and lateral giant axons. We stimulate the axons with two pairs of electrodes, one at each end of the nerve. Our measurements reveal that the myelinated fibers (from earthworms) and nonmyelinated fibers (from lobsters) yield no change in the shape or velocity of the colliding pulses, implying that action potentials propagate as electromechanical pulses. The presence of electromechanical pulses is consistent with the experimental finding that changes in nerve thickness are found and no heat is dissipated during the nerve pulse.

Our findings are not consistent with the Hodgkin-Huxley model, which has long been accepted as a description of nerve-pulse propagation.