This study uses a high-density multielectrode array (MEA) to analyze the propagation of an action potential in the axon of cultured neurons. The device has 11011 electrodes, and can record 126 simultaneously. The authors trigger a spike extracellularly, then record the spike-triggered response many times with different electrode configurations to get the entire response of the MEA, which makes it a very interesting set of data. Normally a single electrode signal is not sufficient to record axonal spikes in a single trial, but the trick is to increase the signal-to-noise ratio by using several electrodes to detect spikes (the SNR increases as the square root of the number of electrodes). This is done using template matching. This allows the authors to measure propagation velocity and jitter not just between two points, as was previously done, but all along the axon. Although it’s not commented, it is interesting to see for example that velocity is apparently not constant, it looks as if there are sorts of jumps (plateaus in Fig. 4f). As expected, the jitter in spike time increases with distance from initiation site. A simple model, used by the authors, predicts indeed that variance increases linearly with distance (by assuming that each axonal compartment introduces an independent noise). However, it is hard to say whether the data follow this model, because no alternative model is tested. Here is one: as the authors later show, conduction velocity depends on previous history (slowing down at high firing rate); if there is jitter in conduction velocity, then variance should grow quadratically with distance. By the way there is a small error in the reporting of jitter: it should be in s/m^(1/2) (because variance is in s^2/m, according to the authors’ model), not s/m. Finally, the authors show that spikes slow down at high rate; something which was known before but not with this level of detail. The authors mention a few possible mechanisms; I would add inactivation of Nav channels.