One of the most striking phenomena in biology is the action potential (AP), a nonlinear pulse with threshold and amplitude saturation (all-or-none-behavior) that propagates along neurons and other cells. In the classical interpretation the AP is considered to be an electrical phenomenon – a regenerating current flowing in a “biological cable”. In contrast, the thermodynamic interpretation has emphasized that conservation laws necessitate pulses and that pulses must manifest as transient changes of all observables of the system (electrical, mechanical, thermal, etc.). It is a key prediction of the latter approach that the cell membrane must undergo thermodynamic state changes during an AP.

In order to characterize the thermodynamic state of an excitable membrane, plant cells (Chara australis) were stained with Di-4-ANEPPDHQ. The location of the dye in the cell membrane was confirmed by confocal microscopy and changes of fluorescence emission were investigated as a function of temperature and extracellular pH. In parallel, emission of the dye was studied in artificial lipid vesicles (DMPC, DMPS) in the vicinity of the main transition temperature. In all these systems, the emission spectrum shifted as a function of membrane state. This shift became nonlinear and was maximal when the membrane underwent a transition $\left(\frac{\partial \lambda }{\partial T}\sim (6-10)nm°C^{-1}\right)$. In the excitable cell Di-4-ANEPPDHQ exhibited a transient blueshift by ∼7 nm during an AP. A blueshift also occurred upon cooling and extracellular acidification. These results provided evidence for a sequence of state changes during an AP in which the cellular membrane condenses followed by expansion. This finding is in line with the thermodynamic interpretation of cellular excitability. Future studies should confirm/falsify these findings with other fluorescent dyes or state-sensitive techniques.

生物学中最引人注目的现象之一是动作电位 (AP)，它是一种具有阈值和振幅饱和度（全或无行为）的非线性脉冲，它沿着神经元和其他细胞传播。在经典解释中，AP 被认为是一种电现象——一种在“生物电缆”中流动的再生电流。相比之下，热力学解释强调守恒定律需要脉冲，并且脉冲必须表现为系统所有可观察量（电、机械、热等）的瞬态变化。后一种方法的关键预测是细胞膜在 AP 期间必须经历热力学状态变化。

为了表征可兴奋膜的热力学状态，植物细胞（Chara australis）用 Di-4-ANEPPDHQ 染色。通过共聚焦显微镜确认染料在细胞膜中的位置，并研究了荧光发射随温度和细胞外 pH 值的变化。同时，在主要转变温度附近的人造脂质囊泡 (DMPC、DMPS) 中研究了染料的发射。在所有这些系统中，发射光谱随膜状态而变化。这种转变变成非线性，并且在膜经历转变时达到最大$\left(\frac{\partial \lambda }{\partial 吨}～(6-10)n米°C^{-1}\right)$. 在可兴奋细胞中，Di-4-ANEPPDHQ 在 AP 期间表现出约 7 nm 的瞬态蓝移。冷却和细胞外酸化也发生蓝移。这些结果为 AP 期间的一系列状态变化提供了证据，其中细胞膜在膨胀后凝结。这一发现与细胞兴奋性的热力学解释一致。未来的研究应该用其他荧光染料或状态敏感技术来证实/证伪这些发现。