Thesis defense of Anne Paeger

The origin of cellular excitability has not yet been clearly elucidated. It has been proposed that the nonlinear stimulus-response curve of excitable cells, manifesting in all-or-none pulses (action potentials), is based on a phase transition in the cell membrane and is not a purely molecule-based phenomenon. Indeed, typical traces of transitions have already been found in a small number of studies with excitable cells. Further investigations are needed to show whether these findings are of a general nature. In this work, state diagrams of the cell membrane of intact, motile specimens of the ”swimming neuron” Paramecium are measured. Therefore, individual cells were trapped in a microfluidic channel and investigated by fluorescence spectroscopy. The thermo-optical state diagrams exhibited reversible sigmoidal and break-like regimes, which are clear indications for a transition in the cell cortical membranes. This transition had a width of ∼ 10 − 15 ∘C and a midpoint that was located ∼ 4 ∘C below the growth temperature. It can be shifted due to changes in growth temperature or by the addition of an anesthetic (hexanol). These results suggested that the cortical membrane(s) of Paramecia reside in a phase transition regime under physiological growth conditions.

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