Effects of Pulsed Acoustic and Mechanical Stimuli on the Excitability of Isolated Neuronal and Cardiac Cells

C. elegans: Sensing the low-frequency profile of amplitude-modulated ultrasound

Several research groups have demonstrated that C. elegans can respond to pulsed ultrasound stimuli, and elucidating the underlying mechanisms is necessary to develop ultrasound neuromodulation. Here, amplitude-modulated (AM) ultrasound is applied to C. elegans, and its behavioral responses are investigated in detail. By loading surface acoustic waves (SAWs) onto free-moving worms on an agar surface, a carrier wave with a frequency of 8.80 MHz is selected. The signal is modulated by a rectangular or sinusoidal profile. It is demonstrated that sinusoidal modulation can produce similar responses in worms to rectangular modulation, with the strongest responses occurring at modulation frequencies of around 1.00 kHz. Meanwhile, the behavioral response is relatively weak when the ultrasonic signal is unmodulated, that is, when only the carrier wave is applied. At modulation frequencies other than 100.00 Hz to 10.00 kHz, the worms respond weakly, but when a second modulation frequency of 1.00 kHz is introduced, an improvement in response can be observed. These results suggest that C. elegans may sense the low-frequency envelope and respond to amplitude-modulated ultrasonic stimuli like an amplitude demodulator. MEC-4, an ion channel for touch sensing, is involved in the behavioral response of C. elegans to ultrasound in the present setup.

A review of the bioeffects of low-intensity focused ultrasound and the benefits of a cellular approach

This review article highlights the historical developments and current state of knowledge of an important neuromodulation technology: low-intensity focused ultrasound. Because compelling studies have shown that focused ultrasound can modulate neuronal activity non-invasively, especially in deep brain structures with high spatial specificity, there has been a renewed interest in attempting to understand the specific bioeffects of focused ultrasound at the cellular level. Such information is needed to facilitate the safe and effective use of focused ultrasound to treat a number of brain and nervous system disorders in humans. Unfortunately, to date, there appears to be no singular biological mechanism to account for the actions of focused ultrasound, and it is becoming increasingly clear that different types of nerve cells will respond to focused ultrasound differentially based on the complement of their ion channels, other membrane biophysical properties, and arrangement of synaptic connections. Furthermore, neurons are apparently not equally susceptible to the mechanical, thermal and cavitation-related consequences of focused ultrasound application-to complicate matters further, many studies often use distinctly different focused ultrasound stimulus parameters to achieve a reliable response in neural activity. In this review, we consider the benefits of studying more experimentally tractable invertebrate preparations, with an emphasis on the medicinal leech, where neurons can be studied as unique individual cells and be synaptically isolated from the indirect effects of focused ultrasound stimulation on mechanosensitive afferents. In the leech, we have concluded that heat is the primary effector of focused ultrasound neuromodulation, especially on motoneurons in which we observed a focused ultrasound-mediated blockade of action potentials. We discuss that the mechanical bioeffects of focused ultrasound, which are frequently described in the literature, are less reliably achieved as compared to thermal ones, and that observations ascribed to mechanical responses may be confounded by activation of synaptically-coupled sensory structures or artifacts associated with electrode resonance. Ultimately, both the mechanical and thermal components of focused ultrasound have significant potential to contribute to the sculpting of specific neural outcomes. Because focused ultrasound can generate significant modulation at a temperature <5°C, which is believed to be safe for moderate durations, we support the idea that focused ultrasound should be considered as a thermal neuromodulation technology for clinical use, especially targeting neural pathways in the peripheral nervous system.

Review of Noninvasive or Minimally Invasive Deep Brain Stimulation

Brain stimulation is a critical technique in neuroscience research and clinical application. Traditional transcranial brain stimulation techniques, such as transcranial magnetic stimulation (TMS), transcranial direct current stimulation (tDCS), and deep brain stimulation (DBS) have been widely investigated in neuroscience for decades. However, TMS and tDCS have poor spatial resolution and penetration depth, and DBS requires electrode implantation in deep brain structures. These disadvantages have limited the clinical applications of these techniques. Owing to developments in science and technology, substantial advances in noninvasive and precise deep stimulation have been achieved by neuromodulation studies. Second-generation brain stimulation techniques that mainly rely on acoustic, electronic, optical, and magnetic signals, such as focused ultrasound, temporal interference, near-infrared optogenetic, and nanomaterial-enabled magnetic stimulation, offer great prospects for neuromodulation. This review summarized the mechanisms, development, applications, and strengths of these techniques and the prospects and challenges in their development. We believe that these second-generation brain stimulation techniques pave the way for brain disorder therapy.

Investigating the effect of thermal stress on nerve action potential using the soliton model

The thermal mechanism of acoustic modulation of the reversible electrical activities of peripheral nerves is investigated using the soliton model, and a numerical solution is presented for its non-homogenous version. Our results indicate that heating a small segment of the nerve will increase the action potential conduction velocity and decrease its amplitude. Moreover, cooling the nerve will have the reverse effects, and cooling to temperatures below the nerve melting point can reflect back a significant portion of the action potentials. These results are consistent with the theory of the soliton model, as well as with the experimental findings. Although there exists a discrepancy between the results of the soliton model and experimental pulse amplitude data, from the free energy point of view, the experiments are compatible with Heimburg and Jackson theory. We conclude that the presented model accompanied by the free energy view is capable of simulating the effects of thermal energy on nerve function. One potential application of the developed theoretical model will be investigation of the reversible and irreversible effects of thermal energy induced by various energy modalities, including therapeutic ultrasound, on nerve function.

Ultrasound induced increase in excitability of single neurons

The aim of this study was to carefully assess the level of modulation in electrical excitability of single neurons with the application of high frequency ultrasound. High frequency tone bursts of ultrasound have been shown to dramatically increase the spike frequency of primary hippocampal neurons in culture. In addition, these ultrasonic bursts also induce silent or still developing neurons to fire. Results indicate that the increase in excitability is largely mediated by mechanical effects and not thermal effects of ultrasound. Future studies on culture models exposed to varying ultrasound protocols may provide insight into the feasibility of using ultrasound as a means for neurostimulation studies conducted on brain slice and in vivo models.

Ultrasonically-assisted intracortical microstimulation of the rat

High frequency tone bursts of ultrasound are capable of increasing the sensitivity of rat motor cortex to electrical stimulation. In this study, 11.75 MHz ultrasound pre-stimuli were delivered to the forelimb motor region of the rat cortex followed by an electrical pulse train to assess changes in cortical activation. The temporal peak intensity of the ultrasound delivered to the brain ranged from 100 to 150 W/cm(2). Tone bursts of 10 to 50 ms in duration were delivered once per second over periods of 30 to 240 seconds. The intracortical microstimulation (ICMS) current needed for forepaw motor response decreased by as much as 40% when applying 50 ms ultrasound pulses. Brain excitability changes were seen with a thermal index (TI) as low as 2.0. Ultrasound application alone was not able to induce motor responses.

In vitro effects of ultrasound with different energies on the conduction properties of neural tissue

The effect of ultrasound at various energy levels on the conduction properties of neural tissue is explored in this in vitro study. Excised sciatic nerves from the bullfrog were used for experiments. The nerves were stimulated by 3.5 MHz continuous wave ultrasound at 1, 2, and 3 W for 5 min. The peak-to-peak amplitude of the electrically evoked compound action potential (CAP) and the conduction velocity (CV) were measured in the nerves before and during ultrasound stimulation. The CV of the nerves increased by 5-20% for ultrasound stimulations at 1-3 W. The CAP amplitude increased by 8% during stimulation with 1 W ultrasound, and progressively decreased for 2 and 3 W ultrasound. This indicates that the effect of lower energy ultrasound increases both the CV and the CAP amplitude and that the reduction in the CAP amplitude for higher energy ultrasound is associated largely with ultrasonic thermal effects.

Biological effects of ultrasound: development of safety guidelines. Part II: general review

In the 1920s, the availability of piezoelectric materials and electronic devices made it possible to produce ultrasound (US) in water at high amplitudes, so that it could be detected after propagation through large distances. Laboratory experiments with this new mechanical form of radiation showed that it was capable of producing an astonishing variety of physical, chemical and biologic effects. In this review, the early findings on bioeffects are discussed, especially those from experiments done in the first few decades, as well as the concepts employed in explaining them. Some recent findings are discussed also, noting how the old and the new are related. In the first few decades, bioeffects research was motivated partly by curiosity, and partly by the wish to increase the effectiveness and ensure the safety of therapeutic US. Beginning in the 1970s, the motivation has come also from the need for safety guidelines relevant to diagnostic US. Instrumentation was developed for measuring acoustic pressure in the fields of pulsed and focused US employed, and standards were established for specifying the fields of commercial equipment. Critical levels of US quantities were determined from laboratory experiments, together with biophysical analysis, for bioeffects produced by thermal and nonthermal mechanisms. These are the basis for safety advice and guidelines recommended or being considered by national, international, professional and governmental organizations.

Thermoacoustic energy effects in electrical arcs

Electrical arcs commonly occur in electrical injury incidents. Historically, safe work distances from an energized surface along with personal barrier protection have been employee safety strategies used to minimize electrical arc hazard exposures. Here, the two-dimensional computational simulation of an electrical arc explosion is reported using color graphics to depict the temperature and acoustic force propagation across the geometry of a hypothetical workroom during a time from 0 to 50 ms after the arc initiation. The theoretical results are compared to the experimental findings of staged tests involving a mannequin worker monitored for electrical current flow, temperature, and pressure, and reported data regarding neurologic injury thresholds. This report demonstrates a credible link between electrical explosions and the risk for pressure (acoustic) wave trauma. Our ultimate goal is to protect workers through the design and implementation of preventive strategies that properly account for all electrical arc-induced hazards, including electrical, thermal, and acoustic effects.