Feasibility and main mechanisms underlying in vivo ultrasound neurostimulation of the ventral nerve cord's giant axons of lumbricus terrestris

Biomedical focused ultrasound (US) could represent a new modality of neurostimulation overcoming the limitations of current techniques in terms of invasiveness and spatial resolution. While it has already been proven that low energy US can stimulate nervous activity at the systemic level, little is known regarding the biophysical mechanisms underlying this phenomenon. In this in vivo study, we intended firstly to prove the feasibility of generating US-induced nervous responses in a model of earthworm giant axons. Secondly, this nervous model was used to investigate the biological mechanisms responsible for US neurostimulation. Following a hypothesis of a cavitation-induced phenomenon proposed in previous studies, we used a confocal US device driven with US sequences specifically designed to induce cavitation (f = 1.1 MHz, Ncycles/pulse = 44 - 88, PRF = 32 - 250 Hz, Npulses/train = 76, TRF = 0.25 Hz). In vivo neural responses, constitued of trains of action potentials (APs) could be triggered by US along the lateral giant fibers (LGF) using acoustic pressures ranging from 5.8 to 14.3 MPa. These levels of pressure and the associated radiation forces are repeatable effects, while the observed phenomenon of US neurostimulation was not. Cavitation is a complex and random phenomenon, and the acoustic signatures recorded by hydrophone confirmed that identical ultrasound burst could induce very different bubbles cloud in the vicinity of the targeted nerve. These first results suggest that, in these experimental conditions, cavitation rather than radiation force is the main biophysical mechanism underlying ultrasound neurostimulation. Moreover, according to our observations, there is no direct causal link between a single US pulse and a single AP. APs should not be studied individually, but as a group, or a serial of subgroups, triggered by a train of US pulses.