Optical read-out and modulation of peripheral nerve activity

Neurogenic Flare Response following Image-Guided Focused Ultrasound in the Mouse Peripheral Nervous System in Vivo

Focused ultrasound (FUS) has been used to non-invasively elicit or inhibit motor neuronal activity in the mouse peripheral nervous system in vivo. However, less is known about whether FUS elicits immune system responses associated with peripheral sensory neuronal activity. In this study, we sought to determine that non-invasive ultrasound image-guided FUS can elicit the neurogenic axon reflex of peripheral nerves in the mouse sciatic nerve. The local vasodilation in the plantar view of the hind paw detected with a high-resolution laser Doppler imager indicated neurogenic flare responses after FUS stimulation. The effects of FUS were compared with control groups, where a distinct pattern of blood flow changes was observed only in FUS-elicited neurogenic flare responses. The findings indicate that image-guided FUS elicits local axon reflexes in vivo with a high degree of specificity and penetration depth.

Sural Nerve Perfusion in Mice

Peripheral nerve function is metabolically demanding and nerve energy failure has been implicated in the onset and development of diabetic peripheral neuropathy and neuropathic pain conditions. Distal peripheral nerve oxygen supply relies on the distribution of red blood cells (RBCs) in just a few, nearby capillary-sized vessels and is therefore technically challenging to characterize. We developed an approach to characterize distal sural nerve hemodynamics in anesthetized, adult male mice using in vivo two-photon laser scanning microscopy. Our results show that RBC velocities in mouse sural nerve vessels are higher than those previously measured in mouse brain, and are sensitive to hindlimb temperatures. Nerve blood flow, measured as RBC flux, however, was similar to that of mouse brain and unaffected by local temperature. Power spectral density analysis of fluctuations in RBC velocities over short time intervals suggest that the technique is sufficiently sensitive and robust to detect subtle flow oscillations over time scales from 0.1 to tens of seconds. We conclude that in vivo two-photon laser scanning microscopy provides a suitable approach to study peripheral nerve hemodynamics in mice, and that local temperature control is important during such measurements.

Image-guided focused ultrasound modulates electrically evoked motor neuronal activity in the mouse peripheral nervous system in vivo

OBJECTIVE

Focused ultrasound (FUS) has recently been demonstrated capable of exciting motor neuronal activity. However, comprehensive understanding of elucidated excitatory and inhibitory effects is required to better assess FUS-mediated modulation. In this study, we demonstrate that image-guided FUS can selectively modulate motor neuron activity in the mouse sciatic nerve in vivo and attribute motor responses to thermal effects.

APPROACH

FUS was applied on the sciatic nerve of anesthetized mice in vivo through the intact skin and muscle using ultrasound imaging for targeting. Both excitatory and inhibitory effects were recorded using electromyography (EMG) along with muscle response of the hind limb. The effects of FUS modulation versus heating by invasive alternative heating source (AHS) on electrically evoked EMG responses in the sciatic nerve in vivo were also investigated. The safety and reversibility of the technique were validated using histology and EMG recovery.

MAIN RESULTS

The FUS was capable of eliciting motor neuronal activity comparable to electrical stimulation ES, and facilitating motor neuronal response on electrically evoked potentials with temperature elevation up to 11.5 °C  ±  0.3 °C (PRF  ⩽  40 Hz). On the other hand, FUS-induced temperature elevations above 15.1 °C  ±  1.6 °C (PRF  ⩾  100 Hz) resulted in the suppression of electrically-evoked motor neuronal activity along with a decrease in EMG latency and area under the curve (AUC), which was validated against the invasive AHS with temperature elevation of 18.1 °C  ±  8.5 °C. Histological findings along with EMG responses after FUS modulation demonstrated a reversible or irreversible modulation.

SIGNIFICANCE

The findings reported herein indicate that image-guided FUS (PRF  ⩽  100 Hz) induces safe and controllable modulation of involuntarily evoked motor neuron activity in vivo.

Real-Time Prosthetic Digit Actuation by Optical Read-out of Activity-Dependent Calcium Signals in an Ex Vivo Peripheral Nerve

Improved neural interfacing strategies are needed for the full articulation of advanced prostheses. To address limitations of existing control interface designs, the work of our laboratory has presented an optical approach to reading activity from individual nerve fibers using activity-dependent calcium transients. Here, we demonstrate the feasibility of such signals to control prosthesis actuation by using the axonal fluorescence signal in an ex vivo mouse nerve to drive a prosthetic digit in real-time. Additionally, signals of varying action potential frequency are streamed post hoc to the prosthesis, showing graded motor output and the potential for proportional neural control. This proof-of-concept work is a novel demonstration of the functional use of activity-dependent optical read-out in the nerve.