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

TL;DR

This paper compares two models of mechanical waves in action potential propagation, showing they describe the same underlying physics despite different approaches, and emphasizes the need to unify these perspectives for better understanding.

Contribution

It demonstrates that surface wave and axoplasmic pressure wave models are fundamentally equivalent, clarifying misconceptions and guiding future research in neural electrophysiology.

Findings

Both models describe elastic and viscous properties of axons.

The models yield identical dependencies in key limits.

Clarifies that the models are different perspectives of the same process.

Abstract

In this commentary, we argue that El Hady and Machta's "surface wave" model for mechanical waves accompanying action potential (AP) propagation describes the same underlying process as the "axoplasmic pressure wave" model introduced earlier by Rvachev. 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.