Modelling the impact of altered axonal morphometry on the response of regenerative nervous tissue to electrical stimulation through macro-sieve electrodes

A method for quantitative spatial analysis of immunolabeled fibers at regenerative electrode interfaces

BACKGROUND

Regenerative electrodes are being explored as robust peripheral nerve interfaces for neuro-prosthetic control and sensory feedback. Current designs differ in electrode number, spatial arrangement, and porosity which impacts the regeneration, activation, and spatial distribution of fibers at the device interface. Knowledge of sensory and motor fiber distributions are important in optimizing selective fiber activation and recording.

NEW METHOD

We use confocal microscopy and immunofluorescence methods to conduct spatial analysis of immunolabeled fibers across whole nerve cross sections.

RESULTS

This protocol was implemented to characterize motor fiber distribution within 3 macro-sieve electrode regenerated (MSE), 3 silicone-conduit regenerated, and 3 unmanipulated control rodent sciatic nerves. Total motor fiber counts were 1485 [SD: +/- 50.11], 1899 [SD: +/- 359], and 5732 [SD: +/- 1410] for control, MSE, and conduit nerves respectively. MSE motor fiber distributions exhibited evidence of deviation from complete spatial randomness and evidence of dispersion and clustering tendencies at varying scales. Notably, MSE motor fibers exhibited clustering within the central portion of the cross section, whereas conduit regenerated motor fibers exhibited clustering along the periphery.

COMPARISON WITH EXISTING METHODS

Prior exploration of fiber distributions at regenerative interfaces was limited to either quadrant-based density analysis of randomly sampled subregions or qualitative description. This method extends existing sample preparation and microscopy techniques to quantitatively assess immunolabeled fiber distributions within whole nerve cross-sections.

CONCLUSIONS

This approach is an effective way to examine the spatial organization of fiber subsets at regenerative electrode interfaces, enabling robust assessment of fiber distributions relative to electrode arrangement.

Nonbinary 2D Distribution Tool Maps Autonomic Nerve Fiber Clustering in Lumbosacral Ventral Roots of Rhesus Macaques

Neuromodulation of the peripheral nervous system (PNS) by electrical stimulation may augment autonomic function after injury or in neurodegenerative disorders. Nerve fiber size, myelination, and distance between individual fibers and the stimulation electrode may influence response thresholds to electrical stimulation. However, information on the spatial distribution of nerve fibers within the PNS is sparse. We developed a new two-dimensional (2D) morphological mapping tool to assess spatial heterogeneity and clustering of nerve fibers. The L6-S3 ventral roots (VRs) in rhesus macaques were used as a model system to map preganglionic parasympathetic, γ-motor, and α-motor fibers. Random and ground truth distributions of nerve fiber centroids were determined for each VR by light microscopy. The proposed tool allows for nonbinary determinations of fiber heterogeneity by defining the minimum distance between nerve fibers for cluster inclusion and comparisons with random fiber distributions for each VR. There was extensive variability in the relative composition of nerve fiber types and degree of 2D fiber heterogeneity between different L6-S3 VR levels within and across different animals. There was a positive correlation between the proportion of autonomic fibers and the degree of nerve fiber clustering. Nerve fiber cluster heterogeneity between VRs may contribute to varied functional outcomes from neuromodulation.

A Custom Interface for the Joint Operation of Med Associates and Tucker-Davis Technologies Hardware in a Rodent Behavioral Model

BACKGROUND

Rodent behavioral models with an electrophysiological component may require the joint operation of hardware from Med Associates, Inc. (St. Albans, VT) and Tucker-Davis Technologies (TDT; Alachua, FL). Although these manufacturers do produce supplemental hardware for interfacing with each other, investing in such hardware may be untenable for research groups with limited funds who wish to use equipment already in their possession.

NEW METHOD

We designed a printed circuit board (PCB) in KiCad and had it fabricated by Advanced Circuits (Aurora, CO), with components sourced from Digi-Key (Thief River Falls, MN). The PCB provided 8 channels of bidirectional communication for the transmission of signals between Med Asssociates' SG-716B SmartCtrl connection panel and TDT's RZ5D base station. This setup enabled the coordinated operation of programs running separately on each set of hardware.

RESULTS

The custom-built PCB facilitated the joint operation of Med Associates and TDT hardware in a go/no-go detection task involving rats with electrical implants in their sciatic nerves.

COMPARISON WITH EXISTING METHODS

Conventional methods for interfacing Med Associates and Tucker-Davis Technologies rely on the purchase of pre-built hardware whose costs can add up to thousands of dollars. The present method offers a viable alternative that is easily implemented and considerably less expensive (below $200).

CONCLUSION

The present approach provides an inexpensive yet effective alternative to far more costly interfacing solutions offered by Med Associates and Tucker-Davis Technologies.

Sensory Percepts Elicited by Chronic Macro-Sieve Electrode Stimulation of the Rat Sciatic Nerve

Objective: Intuitive control of conventional prostheses is hampered by their inability to provide the real-time tactile and proprioceptive feedback of natural sensory pathways. The macro-sieve electrode (MSE) is a candidate interface to amputees’ truncated peripheral nerves for introducing sensory feedback from external sensors to facilitate prosthetic control. Its unique geometry enables selective control of the complete nerve cross-section by current steering. Unlike previously studied interfaces that target intact nerve, the MSE’s implantation requires transection and subsequent regeneration of the target nerve. Therefore, a key determinant of the MSE’s suitability for this task is whether it can elicit sensory percepts at low current levels in the face of altered morphology and caliber distribution inherent to axon regeneration. The present in vivo study describes a combined rat sciatic nerve and behavioral model developed to answer this question.

Approach: Rats learned a go/no-go detection task using auditory stimuli and then underwent surgery to implant the MSE in the sciatic nerve. After healing, they were trained with monopolar electrical stimuli with one multi-channel and eight single-channel stimulus configurations. Psychometric curves derived by the method of constant stimuli (MCS) were used to calculate 50% detection thresholds and associated psychometric slopes. Thresholds and slopes were calculated at two time points 3 weeks apart.

Main Results: For the multi-channel stimulus configuration, the average current required for stimulus detection was 19.37 μA (3.87 nC) per channel. Single-channel thresholds for leads located near the nerve’s center were, on average, half those of leads located near the periphery (54.92 μA vs. 110.71 μA, or 10.98 nC vs. 22.14 nC). Longitudinally, 3 of 5 leads’ thresholds decreased or remained stable over the 3-week span. The remaining two leads’ thresholds increased by 70–74%, possibly due to scarring or device failure.

Significance: This work represents an important first step in establishing the MSE’s viability as a sensory feedback interface. It further lays the groundwork for future experiments that will extend this model to the study of other devices, stimulus parameters, and task paradigms.

High-Frequency Alternating Current Block Using Macro-Sieve Electrodes: A Pilot Study

Background and objective

High-frequency alternating current (HFAC) can yield a rapid-acting and reversible nerve conduction block. The present study aimed to demonstrate the successful implementation of HFAC block delivery via regenerative macro-sieve electrodes (MSEs).

Methods

Dual-electrode assemblies in two configurations [dual macro-sieve electrode-1 (DMSE-I), DMSE-II] were fabricated from pairs of MSEs and implanted in the transected and subsequently repaired sciatic nerves of two male Lewis rats. After four months of postoperative nerve regeneration through the MSEs' transit zones, the efficacy of acute HFAC block was tested for both configurations. Frequencies ranging from 10 kHz to 42 kHz, and stimulus amplitudes with peak-to-peak voltages ranging from 2 V to 20 V were tested. Evoked muscle force measurement was used to quantify the nerve conduction block.

Results

HFAC stimulation delivered via DMSE assemblies obtained a complete block at frequencies of 14 to 26 kHz and stimulus amplitudes of 12 to 20 V p-p. The threshold voltage for the complete block showed an approximately linear dependence on frequency. The threshold voltage for the partial conduction block was also approximately linear. For those frequencies that displayed both partial and complete block, the partial block thresholds were consistently lower.

Conclusion

This study provides a proof of concept that regenerative MSEs can achieve complete and reversible conduction block via HFAC stimulation of regenerated nerve tissue. A chronically interfaced DMSE assembly may thereby facilitate the inactivation of targeted nerves in cases wherein pathologic neuronal hyperactivity is involved.

A Pre-clinical Study of the Response Threshold of Intact and Transected Nerves to Stimulation by Transcutaneous Intense Focused Ultrasound

We used diagnostic ultrasound imaging to guide individual bursts (0.1 s) of 2 MHz intense focused ultrasound (iFU) to determine the sensitivity of intact and transected nerves. We found that all nerves had greater sensitivity to iFU stimulation than surrounding muscle. Intact nerves from healthy volunteers had less sensitivity to iFU stimulation (272 ± 35 W/cm² [median ± standard error]) than transected nerves (19 ± 37 W/cm²). Intact, contralateral nerves of amputees dichotomized naturally into two groups-one very sensitive to iFU stimulation (6 ± 2 W/cm²) and one relatively insensitive (539 ± 19 W/cm²), compared with the intact nerves of healthy volunteers. Our study demonstrates the ability of iFU under ultrasound image guidance to stimulate deep, intact and transected peripheral nerves. It also highlights differences in the receptivity to ultrasound stimulation of the peripheral nerves of amputees versus healthy volunteers.