Comment on “Penetration of Action Potentials During Collision in the Median and Lateral Giant Axons of Invertebrates”

The action potential (AP) is an electrical impulse elicited by depolarization of the neuronal membrane from the resting membrane potential (around $-70\mathrm{mV}$). It propagates along the axon, allowing for rapid and distant communication. Recently, it was claimed that two APs traveling in opposite direction will pass unhindered through each other (penetrate) upon collision [Gonzalez-Perez et al.Phys. Rev. X 4, 031047 (2014)]. We tested this claim under carefully controlled conditions and found that we cannot reproduce penetration. Instead, APs consistently annihilated upon collision. This is consistent with a vast body of literature.

Figure 1

Action potentials annihilate upon collision. (a) Experimental setup: Pairs of stimulation electrodes for generating APs in both ends, and pairs of recording electrodes are also placed towards both ends. (b) Stimulation of the anterior end of increasing strength and recording at the posterior end (consecutive traces, stimulation strength indicated) evokes APs to illustrate single same-fiber activation by (1) the threshold phenomenon and (2) the amplitude independence of stimulation strength (collectively known as the all-or-none phenomenon). (c) Similar to (b) with stimulation in the posterior end and single APs traveling in opposite directions (posterior recording). Note that the threshold is higher in the posterior than in the anterior end, and the latency is shorter when stimulating posteriorly. (d) Simultaneous stimulation in both ends with increasing strengths first evokes anterior AP until the strength is suprathreshold for generating APs in both ends (1.4 V). Here, the APs propagate from both the anterior and posterior ends and collide in the middle. Since the anterior and posterior APs never appeared together in the same recording, annihilation occurred. This was observed for all earthworms ($n=14$). (e) Anterior (black lines)/posterior (blue lines, time-reversed) overlaid APs stem from the same nerve fiber as verified by their similarity in peak-to-peak amplitude (ordinate location of red dots, $\text{ratio}=96\%$). (f) Amplitude ratios for the population (PA/AP) are close to 100%. (g) Velocities based on the accurate method (using two sets of electrodes, i.e., dual recording) in the anterior-posterior (A–P) direction plotted against velocities in the reverse direction, i.e., posterioranterior (P–A) ($n=10$ earthworms). A–P and P–A velocity are the same according to a Kolmogorov-Smirnov test. (h) Apparent velocities using only one set of recording electrodes, as in Ref. [14], in the anterior end (i.e., mono recording). The velocity was estimated as the distance divided by time from stimulation to arrival, thus ignoring the impulse width and AP-generation delay, resulting in a disparity in velocities. (i) Velocities using the reverse configuration (recording in the posterior end), giving opposite disparity in velocities, which illustrates the methodological flaw of using mono recording.