Relationship between fascicle size and perineurial collagen IV content in diabetic and control human peripheral nerve

Computational modelling of nerve stimulation and recording with peripheral visceral neural interfaces

Objective.Neuromodulation of visceral nerves is being intensively studied for treating a wide range of conditions, but effective translation requires increasing the efficacy and predictability of neural interface performance. Here we use computational models of rat visceral nerve to predict how neuroanatomical variability could affect both electrical stimulation and recording with an experimental planar neural interface.Approach.We developed a hybrid computational pipeline,VisceralNerveEnsembleRecording andStimulation (ViNERS), to couple finite-element modelling of extracellular electrical fields with biophysical simulations of individual axons. Anatomical properties of fascicles and axons in rat pelvic and vagus nerves were measured or obtained from public datasets. To validate ViNERS, we simulated pelvic nerve stimulation and recording with an experimental four-electrode planar array.Main results.Axon diameters measured from pelvic nerve were used to model a population of myelinated and unmyelinated axons and simulate recordings of electrically evoked single-unit field potentials (SUFPs). Across visceral nerve fascicles of increasing size, our simulations predicted an increase in stimulation threshold and a decrease in SUFP amplitude. Simulated threshold changes were dominated by changes in perineurium thickness, which correlates with fascicle diameter. We also demonstrated that ViNERS could simulate recordings of electrically-evoked compound action potentials (ECAPs) that were qualitatively similar to pelvic nerve recording made with the array used for simulation.Significance.We introduce ViNERS as a new open-source computational tool for modelling large-scale stimulation and recording from visceral nerves. ViNERS predicts how neuroanatomical variation in rat pelvic nerve affects stimulation and recording with an experimental planar electrode array. We show ViNERS can simulate ECAPS that capture features of our recordings, but our results suggest the underlying NEURON models need to be further refined and specifically adapted to accurately simulate visceral nerve axons.

Ultra-high-frequency ultrasound imaging of sural nerve: A comparative study with nerve biopsy in progressive neuropathies

INTRODUCTION

Nerve ultrasound has been used increasingly in clinical practice as a complementary test for diagnostic assessment of neuropathies, but nerve biopsy remains invaluable in certain cases. The aim of this study was to compare ultra-high-frequency ultrasound (UHF-US) to histologic findings in progressive polyneuropathies.

METHODS

Ten patients with severe, progressive neuropathies underwent ultrasound evaluation of the sural nerve before nerve biopsy. Ultrasound data were compared with histologic results in a retrospective manner.

RESULTS

Sural nerves were easily identified on UHF-US. Nerve hyperechogenicity correlated with inflammatory infiltrates on biopsy. Nerve fascicles could be identified and measured on ultrasound in the majority of patients.

DISCUSSION

Hyperechogenicity on UHF-US may be a marker of nerve inflammation in neuropathies. Furthermore, the UHF-US probe allows for evaluation of sensory nerves in spite of their small size, providing valuable information on their size and on their internal structure.

On the parameters used in finite element modeling of compound peripheral nerves

OBJECTIVE

Computational modeling is an important tool for developing and optimizing implantable neural stimulation devices, but requires accurate electrical and geometrical parameter values to improve predictive value. We quantified the effects of perineurial (resistive sheath around each fascicle) and endoneurial (within each fascicle) parameter values for modeling peripheral nerve stimulation.

APPROACH

We implemented 3D finite element models of compound peripheral nerves and cuff electrodes to quantify activation and block thresholds of model axons. We also implemented a 2D finite element model of a bundle of axons to estimate the bulk transverse endoneurial resistivity; we compared numerical estimates to an analytical solution.

MAIN RESULTS

Since the perineurium is highly resistive, potentials were approximately constant over the cross section of a fascicle, and the perineurium resistivity, longitudinal endoneurial resistivity, and fascicle diameter had important effects on thresholds. Activation thresholds increased up to ~130% for higher perineurium resistivity (~400 versus 2200 Ω m) and by ~35%-250% for lower longitudinal endoneurial resistivity (3.5 versus 0.75 Ω m), with larger increases for smaller diameter axons and for axons in larger fascicles. Further, thresholds increased by ~30%-180% for larger fascicle radii, yielding a larger increase with higher perineurium resistivity. Thresholds were largely insensitive to the transverse endoneurial resistivity, but estimates of the bulk resistivity increased with extracellular resistivity and axonal area fraction; the numerical and analytical estimates were in strong agreement except at high axonal area fractions, where structured axon placements that achieved tighter packing produced electric field inhomogeneities.

SIGNIFICANCE

We performed a systematic investigation of the effects of values and methods for modeling the perineurium and endoneurium on thresholds for neural stimulation and block. These results provide guidance for future modeling studies, including parameter selection, data interpretation, and comparison to experimental results.

Automatic Fascicle Length Estimation on Muscle Ultrasound Images With an Orientation-Sensitive Segmentation

GOAL

The fascicle length obtained by ultrasound imaging is one of the crucial muscle architecture parameters for understanding the contraction mechanics and pathological conditions of muscles. However, the lack of a reliable automatic measurement method restricts the application of the fascicle length for the analysis of the muscle function, as frame-by-frame manual measurement is time-consuming. In this study, we propose an automatic measurement method to preclude the influence of nonfascicle components on the estimation of the fascicle length by using motion estimation of fascicle structures.

METHODS

The method starts with image segmentation using the cohesiveness of fascicle orientation as a feature, obtaining the fascicle change by tracking manually marked points on the fascicular path with the Lucas-Kanade optical flow algorithm applied on the segmented image.

RESULTS

The performance of this method was evaluated on ultrasound images of the gastrocnemius obtained from seven healthy subjects (34.4 ± 5.0 years). Waveform similarity between the manual and dynamic measurements was assessed by calculating the overall similarity with the coefficient of multiple correlations (CMC). In vivo experiments demonstrated that fascicle tracking with the orientation-sensitive segmentation (CMC = 0.97 ± 0.01) was more consistent with the manual measurements than existing automatic methods (CMC = 0.87 ± 0.10).

CONCLUSION

Our method was robust to the interference of nonfascicle components, resulting in a more reliable measurement of the fascicle length.

SIGNIFICANCE

The proposed method may facilitate further research and applications related to real-time architectural change of muscles.

Internal-specific morphological analysis of sciatic nerve fibers in a radiofrequency-induced animal neuropathic pain model

This study investigated the reversible effects of pulsed radiofrequency (PRF) treatment at 42 °C on the ultrastructural and biological changes in nerve and collagen fibers in the progression of neuropathic pain after rat sciatic nerve injury. Assessments of morphological changes in the extracellular matrices by atomic force microscopy and hematoxylin-eosin, Masson's trichrome and picrosirius-red staining as well as the expressions of two fibril-forming collagens, types-I and -III, and two inflammatory cytokines, TNF-α and IL-6, were evaluated on day 30 after RF exposure. There were four groups for different RF thermal treatments: no treatment, no current, PRF, and continuous RF (CRF). An RF procedure similar to that used in human clinical trials was used in this study. The CRF treatment at 82 °C led to neural and collagen damage by the permanent blockage of sensory nociceptors. The PRF treatment led to excellent performance and high expandability compared to CRF, with effects including slight damage and swelling of myelinated axons, a slightly decreased amount of collagen fibers, swelling of collagen fibril diameters, decreased immunoreactivity of collagen types-I and -III, presence of newly synthesized collagen, and recovery of inflammatory protein immunoreactivity. These evidence-based findings suggest that PRF-based pain relief is responsible for the temporary blockage of nerve signals as well as the preferential destruction of pain-related principal sensory fibers like the Aδ and C fibers. This suggestion can be supported by the interaction between the PRF-induced electromagnetic field and cell membranes; therefore, PRF treatment provides pain relief while allowing retention of some tactile sensation.

Morphometric analysis of human sciatic nerve perineurial collagen type IV content

OBJECTIVES

Aging is the process which unavoidably alters structure and function of the basal membranes in humans. Though, collagen type IV presents the most prominent component of the basal membranes, we estimated its presence in the perineurium of the human sciatic nerve samples during the aging process.

MATERIALS AND METHODS

Material was 12 sciatic nerve samples, obtained from cadavers whose age ranged from 36 to 84 years. Cadavers were classified into three age groups: first which age ranged from 35 to 54 years, second which age ranged from 55 to 74 years and third which included cases older than 75 years. Tissue slices were further stained by labeled streptavidin-biotin method with collagen type IV monoclonal antibody and analyzed with light microscope under 100× lens magnification with oil immersion. Digital images of sciatic nerve perineurium were further processed and analyzed with ImageJ software.

RESULTS

Our results showed that there is statistically significant increase of perineurial area, perimeter, collagen type IV area, and collagen type IV area per perineurial perimeter unit in the third age group. These parameters also increased in the second age group, but this increase was not significant. Multiple regression analysis showed that beside fascicular size, age more significantly predict perineurial collagen type IV content.

CONCLUSIONS

Results of morphometric and statistical analysis pointed to the conclusion that there is significant increase of sciatic nerve perineurial thickness during the aging process. This increase might represent the consequence of perineurial collagen type IV deposition with aging.

A mechanical model for collagen fibril load sharing in peripheral nerve of diabetic and nondiabetic rats

Peripheral neuropathy affects approximately 50% of the 15 million Americans with diabetes. It has been suggested that mechanical effects related to collagen glycation are related to the permanence of neuropathy. In the present paper, we develop a model for load transfer in a whole nerve, using a simple pressure vessel approximation, in order to assess the significant of stiffening of the collagenous nerve sheath on endoneurial fluid pressure. We also develop a fibril-scale mechanics model for the nerve, to model the straightening of wavy fibrils, producing the toe region observed in nerve tissue, and also to interrogate the effects of interfibrillar crosslinks on the overall properties of the tissue. Such collagen crosslinking has been implicated in complications in diabetic tissues. Our fibril-scale model uses a two-parameter Weibull model for fibril strength, in combination with statistical parameters describing fibril modulus, angle, wave-amplitude, and volume fraction to capture both toe region and failure region behavior of whole rat sciatic nerve. The extrema of equal and local load-sharing assumptions are used to map potential differences in diabetic and nondiabetic tissues. This work may ultimately be useful in differentiating between the responses of normal and heavily crosslinked tissue.

Extracellular matrix and matrix metalloproteinases in sciatic nerve

Although matrix metalloproteinases (MMPs) are increasingly being implicated in several pathologies of the nervous system, it is not yet clear what role they play in normal neurobiological processes. We review the expression of extracellular matrix (ECM) components as well as MMPs and tissue inhibitors of metalloproteinases (TIMPs) in the peripheral nervous system. We explore the expression of certain MMPs and the four TIMPs at the mRNA level in the postnatal mouse sciatic nerve. In addition, we have used substrate gel and in situ zymography to determine levels of MMP-2 and -9 and TIMP activity in rat sciatic nerve after crush and during regeneration. A rapid and transient increase in MMP-9 localised at and immediately distal to the site of injury was observed, whereas an increase in MMP-2 activity was delayed, prolonged, and extended proximal and distal to the injury site. This activity coincides with periods of axonal elongation, suggesting that it could act to facilitate axonal extension along the nerve matrix. We also detected multiple species of gelatinolytic inhibitory activity, including TIMP-1 and -3 in control and injured nerve. These activities probably act to prevent uncontrolled gelatinolytic activity, maintaining nerve integrity at the level essential for axonal regrowth.

A quantitative analysis of perineurial cell basement membrane collagen IV, laminin and fibronectin in diabetic and non-diabetic human sural nerve

The thickness of the perineurial cell basement membrane was examined in diabetic and non-diabetic human sural nerve. A significant increase in thickness was found in the diabetic group. The nature of this thickening was investigated using immunohistochemistry and image analysis in order to semi-quantify three of the major intrinsic components of the perineurial cell basement membrane: collagen IV, laminin and fibronectin. Amounts of all three components were shown to be increased in the diabetic group, but not significantly so. However, significant linear correlations between fascicle size and perineurial collagen IV, laminin and fibronectin were identified in both diabetic and non-diabetic nerve.