A Thermodynamic Interpretation of the Stimulated Raman Spectroscopic Signature to Action Potential in Single Neurons

It has previously been suggested that the plasma membrane condenses and melts reversibly during an action potential in a neuron, analogous to an acoustic wave travelling in the compressive membrane region. If true it has fundamental consequences for our understanding of the regulation of biological functions during an action potential. It has long been known that the electrical dipoles in the neuronal membrane reorient during an action potential, observed through a variety of optical methods. However, this information has been insufficient to confirm if and how the collective thermodynamic state of the neuronal membrane changes during an action potential. Here, we show that hyperspectral stimulated Raman spectroscopy (SRS) can resolve the thermodynamic state of the neuronal membranes in a single neuron during an action potential. These measurements indicate that the system becomes ordered and compressed during the de-polarisation phase and disordered and expanded during hyper polarisation Therefore, the observation is consistent with the acoustic hypothesis and SRS provides a powerful tool to not only further validate the hypothesis in future, but also explore the role of membrane thermodynamics during an action potential.