A causal study of the phenomenon of ultrasound neurostimulation applied to an in vivo invertebrate nervous model
Jérémy Vion-Bailly, W. Apoutou N’Djin, Ivan Mauricio Suarez Castellanos, Jean-Louis Mestas, Alexandre Carpentier, Jean-Yves Chapelon
Scientific Reports 2019 · 10.1038/s41598-019-50147-7
Abstract
Focused ultrasound are considered to be a promising tool for the treatment of neurological conditions, overcoming the limitations of current neurostimulation techniques in terms of spatial resolution and invasiveness. Much evidence to support the feasibility of ultrasound activation of neurons at the systemic level has already been provided, but to this day, the biophysical mechanisms underlying ultrasound neurostimulation are still widely unknown. In order to be able to establish a clear and robust causality between acoustic parameters of the excitation and neurobiological characteristics of the response, it is necessary to work at the cellular level, or alternatively on very simple animal models. The study reported here responds to three objectives. Firstly, to propose a simple nervous model for the study of the ultrasound neurostimulation phenomenon, associated with a clear and simple experimental protocol. Secondly, to compare the characteristics of this model's nervous response to ultrasound neurostimulation with its nervous response to mechanical and electrical stimulation. Thirdly, to study the role played by certain acoustic parameters in the success rate of the phenomenon of ultrasound stimulation. The feasibility of generating action potentials (APs) in the giant axons of an earthworm's ventral nerve cord, using pulsed ultrasound stimuli (f = 1.1 MHz, N cycles = 175-1150, PRF = 25-125 Hz, N pulses = 20, P A = 2.5-7.3 MPa), was demonstrated. The time of generation (TOG) of APs associated with ultrasound stimulation was found to be significantly shorter and more stable than the TOG associated with mechanical stimulation (p < 0.001). By applying a causal approach to interpret the results of this study, it was concluded that, in this model, the nervous response to focused ultrasound is initiated along the afferent neurons, in between the mechanosensors and the synaptic connections with the giant axons. Additionally, early results are provided, highlighting a trend for the success rate of ultrasound neurostimulation and number of APs triggered per response to increase with increasing pulse repetition frequency (p < 0.05 and p < 0.001, respectively), increasing pulse duration and increasing pulse amplitude.
Abstract via europepmc.
Exposures
Exposure 1: Ventral nerve cord ultrasound neurostimulation (LEUS)
Target: earthworm neurons — “ventral nerve cord (medial and lateral giant fibers) of the earthworm, exposed at the anterior, medial, or posterior third of the body”
Device: custom-built · Ferroperm · PZ28 ✓
| Waveform | pulsed | |
|---|---|---|
| Fundamental frequency (kHz) | 1,100 | ✓✓✓ |
| Pulse duration (ms) | 0.16, 1.05swept | ✓✓✓ |
| Pulse repetition frequency (Hz) | 25, 125swept | ✓✓✓ |
| Duty cycle (%) | not reported | ⚑ |
| Sonication duration (s) | not reported | ⚑ |
| Free-field pressure (kPa) | 2,500, 7,300swept | ✓✓✓ |
|---|---|---|
| Free-field Isppa (W/cm²) | not reported | |
| Free-field Ispta (W/cm²) | not reported | |
| In-situ estimate | not reported | |
| In-situ pressure (kPa) | not reported | |
| In-situ Isppa (W/cm²) | not reported | |
| In-situ Ispta (W/cm²) | not reported |
The main ultrasound sequences used in this study consisted of a series of pulsed ultrasound bursts (f=1.1 MHz, Ncycles=175, PRF=125 Hz, Npulses=20). PRF was compared between 25 Hz and 125 Hz across trials (n=6) to assess success rate, and pulse duration (Ncycles=175-1150, i.e. ~0.16-1.05 ms) and pressure amplitude (2.5-7.3 MPa) were swept to identify response thresholds.
Flags from extraction
exposures[0].target.terms— Earthworm ventral nerve cord / giant fibers have no dedicated target id in the vocabulary; classified as 'other'.exposures[0].timing.duty_cycle_pct— Paper reports N_cycles, PRF and N_pulses per burst but never states a duty cycle percentage; not computed here.exposures[0].timing.sonication_duration_s— Paper describes bursts of N_pulses=20 at a given PRF but does not state a total burst/train duration in seconds; not computed here.n_sessions_per_subject— Each earthworm underwent a variable, undefined number of LEUS bursts/trials over a single extended session (30 min to several hours); no discrete session count is stated.exposures[0].in_situ— Ultrasound was delivered through a thin water layer directly onto the earthworm body (in vivo tissue path) but no derated/measured in-situ pressure or intensity at the nerve is reported; only the free-field water pressure is given.