Cartoon depicts the vision of a sound-wave propagating in a neuron as an action potential.

Sound waves in lipid films can annihilate each other upon collision, just like action potentials in neurons

Nerve impulses, also known as action potentials, are believed to propagate in a manner similar to the conduction of current in an electrical cable. However, for as long as the electrical theory has been around, scientists have also been measuring various other physical signals that are equally characteristic of a nerve impulse, such as changes in the mechanical and optical properties that propagate in sync with the electrical signal. Furthermore, several studies have reported reversible temperature changes that accompany a nerve impulse, which is inconsistent with the electrical model from a thermodynamic standpoint. To address these inconsistencies, researchers had previously proposed that nerve pulse propagation results from the same fundamental principles that cause the propagation of sound in a material and not the flow of ions or current. In this framework, the electro-mechanical nature of the nerve impulse emerges naturally from the collective properties of the plasma membrane, in which the sound or the compression wave propagates. Thus the characteristics of the wave are derived from the principles of condensed matter physics and thermodynamics, unlike the emphasis on molecular biology in the electrical theory.

The suggestion has been highly controversial because of the well-accepted and widely successful nature of the electrical basis of nerve pulse propagation. As a wave phenomenon nerve pulse propagation has remarkable properties, such as a threshold for excitation, non-dispersive (solitary) and all-or-none propagation, and annihilation of two pulses that undergo head-on collision. On the other hand, sound waves are generally not associated with such characteristics, rather they are known to spread-out, disperse, dissipate, superimpose and interfere, which is counter-intuitive given the properties of nerve impulses. Therefore, experimental evidence for such a phenomenon was crucial, which was provided by us in 2014. We showed that sound or compression waves can indeed propagate within a molecular thin film of lipid molecules mimicking action potentials in the plasma membrane. Remarkably, even in such minimalist system that is devoid of any proteins and macromolecules other than lipids, these waves behave strikingly similar to nerve impulses in a neuron, including the solitary electromechanical pulse propagation, the velocity of propagation and all-or-none excitation [1,2]. These characteristics were shown to be a consequence of the conformation change or a phase transition in the lipid molecules that accompany the sound wave. Thus only when sufficient energy is provided to cause a phase change in the lipids (fluid to gel like) the entire pulse propagates otherwise nothing propagates, the so-called all-or-none propagation.

Now in a research published today in the Journal of Royal Society Interface, we have shown that these waves can even annihilate each other upon collision, just like nerve impulses. Even from a purely acoustic physics perspective, this is a remarkable finding. The amplitudes of two sound pulses colliding head-on typically superimpose linearly before passing each other unaffected. Even nonlinear sound pulses, such as solitons, typically remain unaffected upon collision, which was a major criticism of the proposed acoustic theory of nerve pulse propagation. With the observation of annihilation of colliding sound pulses in the model lipid system, we have shown that qualitative characteristics of the entire phenomena of nerve pulse propagation can be derived solely from the principles of condensed matter physics and thermodynamics, without the need for molecular models or fit parameters of the electrical theory. We have demonstrated a very unique acoustic phenomenon that combines all the observable characteristics that define the propagation of nerve impulses. The study therefore has given major boost to the hypothesis that the underlying physics of propagation of sound and nerve impulses is one and the same.

Following references provide the order in which the story has developed over past 4 years

  1. Evidence for two-dimensional solitary sound waves in a lipid controlled interface and its implications for biological signalling
  2. Solitary shock waves and adiabatic phase transition in lipid interfaces and nerves
  3. Nonlinear fractional waves at elastic interfaces
  4. Collision and annihilation of nonlinear sound waves and action potentials in interfaces

The story has also been published on my linked in account

A version of this article also appeared on the Oxford Science blog on 27th June