## 4 Mechanisms of Peripheral US Neuromodulation
Thus, it is likely that other mechanically gated ion channels may participate to collectively enhance the neural activity, which include, but are not limited to, voltage-sensitive sodium [92; 93; 94], two-pore-domain potassium (K2P) [95], ASIC [95], TRP [96], and Piezo channels [97; 98]. Virtually no study has been conducted to assess US neuromodulation on intact DRG, the clustering of sensory afferent somata. A recent study indicates that action potentials can be evoked by focused US in dissociated DRG neurons [69], opening new avenues of research on US DRG neuromodulation in future preclinical and clinical studies. The underlying mechanisms of US activation of DRG neurons are unclear and likely involve sodium, potassium, and non-selective cation channels [69]. The non-thermal and non-cavitation bio-effect of the focused US at low and intermediate intensity is of central interests for reversible US neuromodulation; it likely induces local acoustic forces below the harmful range. Potential mechanisms of neuromodulation include the mechanical gating of transmembrane ion channels, as discussed earlier. In addition, several theories have attributed the US neuromodulatory effect to altered properties of the lipid-bilayer membrane at the nerve endings, axons, and somata, including the soliton model [99], the flexoelectricity hypothesis [100], the neuronal intramembrane cavitation excitation (NICE) model [101], and, more recently, the theory of direct transmembrane pore formation [102]. The main assumption of the soliton model is that the transmission of signal through a nerve occurs as an electromechanical soliton wave packet rather than a complete electrical phenomenon. However, this model could not mathematically explain the role of voltage-gated ion channels in action potential generation. The flexoelectric effect hypothesizes that the mechanical energy of the curved lipid-bilayer membrane leads to electrical membrane polarization to depolarize the neural membrane, but the relevant mathematical formulation to account for action potential generation has yet to be established. In the NICE model, it is hypothesized that US stimulation with sufficient intensity (\(>\)0.10 W/cm\({}^{2}\)\(I_{SPTA}\)) causes nanobubble formation in the intramembrane space, which subsequently changes the transmembrane capacitance. Hence, the NICE model suggests the membrane capacitive current caused by the change of transmembrane capacitance as the source of US neuromodulation and appears to explain the increased conduction velocities by US modulation from our recent study with single-unit recordings [43]. The recent experimental finding on an expression system indicates that US as low as 0.4 W/cm\({}^{2}\) can form pores in the lipid-bilayer membrane large enough to allow the passage of the large dye molecule calcein [102], indicating that the pores are sufficiently large for the passage of sodium and potassium ions to excite neurons. Both the NICE model and the transmembrane pore formation theory can explain the enhanced excitability of peripheral nerve axons by US, consistent with a recent ex vivo study with single-unit recordings from individual axons [43]. Further experimental and theoretical studies are required to advance our mechanistic understanding of peripheral US neuromodulation.