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Background. It is a common incident in nature, that two waves or pulses run into each other head-on.The outcome of such an event is of special interest, because it allows conclusions about the underlying physical nature of the pulses. The present experimental study dealt with the head-on meeting of two action potentials (AP) in a single excitable plant cell (Chara braunii internode).Methods. The membrane potential was monitored at the two extremal regions of an excitable cell. In control experiments, an AP was excited electrically at either end of the cell cylinder. Subsequently, stimuli were applied simultaneously at both ends of the cell in order to generate two APs that met each other head-on.Results. When two action potentials propagated into each other, the pulses did not penetrate but annihilated (N=26 experiments in n=10 cells). Conclusions Graphical abstract Highlights-When two pulses meet, they reveal information about their physical nature.-Upon running into each other, two action potentials in an excitable plant cell annihilate -Action potentials in plant cells and nerves are similar nonlinear phenomena.

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Single cell measurements with living specimen like, for example, the ciliated protozoan Paramecium caudatum can be a challenging task. We present here a microfluidic trapping mechanism for measurements with these micro-organisms that can be used, e.g., for optical measurements to correlate cellular functions with the phase state of the lipid membrane. Here, we reversibly trap single cells in small compartments. Furthermore, we track and analyze the swimming behavior of single cells over several minutes. Before and after reversible trapping the swimming speed is comparable, suggesting that trapping does not have a large effect on cell behavior. Last, we demonstrate the feasibility of membrane order measurements on living cells using the fluorescent dye 6-lauryl-2-dimethylaminonaphthalene (Laurdan).