  The excitation of many cells and tissues is associated with cell mechanical
changes. The evidence presented herein corroborates that single cells deform
during an action potential (AP). It is demonstrated that excitation of plant
cells (Chara braunii internodes) is accompanied by out-of-plane displacements
of the cell surface in the micrometer range (1-10 micron). The onset of
cellular deformation coincides with the depolarization phase of the AP. The
mechanical pulse (i) propagates with the same velocity as the electrical pulse
(within experimental accuracy; 10 mm/s), (ii) is reversible, (iii) in most
cases of biphasic nature (109 out of 152 experiments) and (iv) presumably
independent of actin-myosin-motility. The existence of transient mechanical
changes in the cell cortex is confirmed by micropipette aspiration experiments.
A theoretical analysis demonstrates that this observation can be explained by a
reversible change in the mechanical properties of the cell surface
(transmembrane pressure, surface tension and bending rigidity). Taken together,
these findings contribute to the ongoing debate about the physical nature of
cellular excitability.
