Surface Waves and Axoplasmic Pressure Waves in Action Potential Propagation: Fundamentally Different Physics or Two Sides of the Same Coin?

In this commentary, we argue that El Hady and Machta’s “surface wave” model 1 for mechanical waves accompanying action potential (AP) propagation describes the same underlying process as the “axoplasmic pressure wave” model introduced earlier by Rvachev. 2 Both models describe mechanical modes that store potential energy in the elastic components of the axon (axonal membrane, cytoskeleton, bulk axoplasmic deformation), with kinetic energy carried by the axoplasmic fluid and axoplasmic viscosity playing a significant role. The “surface wave” model quantitatively considers driving by the traveling electrical depolarization wave of the AP, whereas the “axoplasmic pressure wave” model qualitatively considers driving not only by the AP’s electrical depolarization but also by other mechanisms, such as cytoskeletal actomyosin contractility. In addition, the “axoplasmic pressure wave” model considers mechanisms for synchronizing the depolarization wave and the pressure wave. Although derived using different approaches, the two models yield identical dependencies for the mechanical modes in key limits. The confusion in the literature, which treats these models as describing distinct processes, needs to be resolved to improve comprehensive understanding of the AP phenomenon and to guide future research.