  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). 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. When
two action potentials propagated into each other, the pulses did not penetrate
but annihilated (N=26 experiments in n=10 cells). APs in excitable plant cells
did not penetrate upon meeting head-on. In the classical electrical model, this
behavior is specifically attributed to relaxation of ion channel proteins. From
an acoustic point of view, annihilation can be viewed as a result of nonlinear
material properties the entire system. The present results suggest that APs in
excitable animal and plant cells belong to a similar class of nonlinear
phenomena. Intriguingly, other excitation waves in biology (intracellular
waves, cortical spreading depression, etc.) also annihilate upon collision and
are thus expected to follow the same underlying principles as the observed
action potentials.
