Soliton model
The soliton model, originally proposed by Heimburg & Jackson (2005) and later extended by Andersen et al., offers a radically different account of nerve impulse propagation — and by extension, of how ultrasound might interact with it.
Rather than treating the action potential as a purely electrical phenomenon governed by voltage-gated ion channels (as in the Hodgkin-Huxley framework), the soliton model proposes that nerve impulses are electromechanical density pulses propagating along the lipid bilayer as thermodynamic solitons — solitary waves that arise near a lipid phase transition.
The key physical premise is that biological membranes operate close to a liquid-to-gel phase transition at physiological temperature. Near this transition, the membrane exhibits anomalously high compressibility, allowing a localized mechanical compression wave to propagate without dispersion. This wave is accompanied by transient changes in membrane thickness, capacitance, and electrical potential — producing what is observed electrophysiologically as an action potential.
In this framework, ultrasound could interact with neuronal activity by:
- Perturbing the lipid phase state, shifting the membrane closer to or further from the transition point
- Directly exciting or suppressing soliton propagation through acoustic coupling to the density wave
- Altering the thermodynamic conditions (temperature, pressure) that govern phase transition dynamics
Limitations and relevance to this project. The soliton model remains highly controversial. It has difficulty accounting for the pharmacological specificity of anesthetics at the single-channel level, the directional propagation of action potentials, and the well-established role of specific ion channel subtypes in shaping AP waveforms. It has not been incorporated into any of the major computational FUS frameworks (NICE, SONIC, MorphoSONIC).
For the purposes of this project, the soliton model is noted as a theoretical alternative but is not integrated into the unified model, which remains grounded in the Hodgkin-Huxley formalism.