It is necessary to understand the morphology of the vagus nerve (VN) to design and deliver effective and selective vagus nerve stimulation (VNS) because nerve morphology influences fiber responses to electrical stimulation. Specifically, nerve diameter (and thus, electrode-fiber distance), fascicle diameter, fascicular organization, and perineurium thickness all significantly affect the responses of nerve fibers to electrical signals delivered through a cuff electrode. We quantified the morphology of cervical and subdiaphragmatic VNs in humans, pigs, and rats: effective nerve diameter, number of fascicles, effective fascicle diameters, proportions of endoneurial, perineurial, and epineurial tissues, and perineurium thickness. The human and pig VNs were comparable sizes (∼2 mm cervically; ∼1.6 mm subdiaphragmatically), while the rat nerves were ten times smaller. The pig nerves had ten times more fascicles—and the fascicles were smaller—than in human nerves (47 vs. 7 fascicles cervically; 38 vs. 5 fascicles subdiaphragmatically). Comparing the cervical to the subdiaphragmatic VNs, the nerves and fascicles were larger at the cervical level for all species and there were more fascicles for pigs. Human morphology generally exhibited greater variability across samples than pigs and rats. A prior study of human somatic nerves indicated that the ratio of perineurium thickness to fascicle diameter was approximately constant across fascicle diameters. However, our data found thicker human and pig VN perineurium than those prior data: the VNs had thicker perineurium for larger fascicles and thicker perineurium normalized by fascicle diameter for smaller fascicles. Understanding these differences in VN morphology between preclinical models and the clinical target, as well as the variability across individuals of a species, is essential for designing suitable cuff electrodes and stimulation parameters and for informing translation of preclinical results to clinical application to advance the therapeutic efficacy of VNS.

Electrical stimulation and block of peripheral nerves hold great promise for treatment of a range of disease and disorders, but promising results from preclinical studies often fail to translate to successful clinical therapies. Differences in neural anatomy across species require different electrodes and stimulation parameters to achieve equivalent nerve responses, and accounting for the consequences of these factors is difficult. We describe the implementation, validation, and application of a standardized, modular, and scalable computational modeling pipeline for biophysical simulations of electrical activation and block of nerve fibers within peripheral nerves. The ASCENT (Automated Simulations to Characterize Electrical Nerve Thresholds) pipeline provides a suite of built-in capabilities for user control over the entire workflow, including libraries for parts to assemble electrodes, electrical properties of biological materials, previously published fiber models, and common stimulation waveforms. We validated the accuracy of ASCENT calculations, verified usability in beta release, and provide several compelling examples of ASCENT-implemented models. ASCENT will enable the reproducibility of simulation data, and it will be used as a component of integrated simulations with other models (e.g., organ system models), to interpret experimental results, and to design experimental and clinical interventions for the advancement of peripheral nerve stimulation therapies.

Vagus nerve stimulation (VNS) is used clinically to treat epilepsy, depression, and obesity, and is under investigation for other applications. In many cases, the stimulation parameters used in preclinical VNS studies did not translate to successful clinical outcomes. We propose that differences in nerve morphology across preclinical models (e.g., rats and pigs) and humans, as well as variance within those populations, contribute to variance in therapeutic response due to variable levels of neural activation or block. Therefore, we quantified vagal morphology across species and simulated excitation and block thresholds using computational models. We quantified the morphology of the cervical and subdiaphragmatic vagus nerves (VN) in rats, pigs, and humans. The human and pig VNs had similar effective diameters (~3.7 mm at the cervical level and ~2.4 mm at the subdiaphragmatic level), although the pig nerves had almost 10x more fascicles (~41 fascicles for pig VN vs. ~5.8 fascicles for human VN). Conversely, the rat nerves were ~10x smaller (0.265 mm diameter at the cervical level and 0.147 mm at the subdiaphragmatic level) and had only one or a couple of fascicles. For our rat samples, the ratio of perineurium thickness to fascicle diameter corresponded well to the previous estimate of 3% of the fascicle diameter (Grinberg et al., 2008). Conversely, for our human samples, the perineurium was substantially thicker (~8.0% and ~21% at the cervical and subdiaphragmatic levels, respectively), and the ratio of perineurium thickness to fascicle diameter decreased with increased fascicle diameter. We designed and implemented a computational pipeline to construct volume conductor finite element models (FEMs) in COMSOL for multiple vagus nerve samples from each species, instrumented with a cuff electrode as used preclinically or clinically. We coupled the FEMs to models of mammalian myelinated and unmyelinated fibers in NEURON to quantify excitation and block thresholds. The intensity of VNS stimulation required to achieve activation of 50% of the 5.7 to 10 μm model myelinated fibers in human models were ~5–10x greater than in rat nerve samples, and intensities to activate 0.8 μm model unmyelinated fibers were ~20x greater. Further, thresholds of excitation and block varied across individuals within a species and were governed by species‐ and individual‐specific factors such as fascicle diameter and fascicle organization within the nerve. Human and pig cervical models show comparable within‐species variability (the highest thresholds to activate 50% of model nerve fibers were ~2x larger than the lowest) and greater variability than across the rat cervical models (the highest thresholds to activate 50% of model nerve fibers were ~1.25x larger than lowest). Morphologically‐based computational models of the cervical and subdiaphragmatic VN in rats, pigs, and humans indicate that cross‐ and within‐species differences in nerve morphology are important considerations for understanding differences in VNS responses between individua...