Comparison between the cervical and abdominal vagus nerves in mice, pigs, and humans

Abstract

Background Vagus nerve (VN) stimulation is currently evaluated as a novel approach to treat immune‐mediated disorders. The optimal stimulation parameters, however, largely depend on the VN composition potentially impacting on its clinical translation. Hence, we evaluated whether morphological differences exist between the cervical and abdominal VNs across different species. Materials and methods The cervical and abdominal VNs of mouse, pig, and humans were stained for major basic protein and neurofilament F to identify the percentage and size of myelinated and non‐myelinated fibers. Results The percentage of myelinated fibers was comparable between species, but was higher in the cervical VN compared with the abdominal VN. The cervical VN contained 54 ± 4%, 47 ± 7%, and 54 ± 7% myelinated fibers in mouse, pig, and humans, respectively. The myelinated fibers consisted of small‐diameter (mouse: 71%, pig: 80%, and humans: 63%), medium‐diameter (mouse: 21%, pig: 18%, and humans: 33%), and large‐diameter fibers (mouse: 7%, pig: 2%, and humans: 4%). The abdominal VN predominantly contained unmyelinated fibers (mouse: 93%, pig: 90%, and humans: 94%). The myelinated fibers mainly consisted of small‐diameter fibers (mouse: 99%, pig: 85%, and humans: 74%) and fewer medium‐diameter (mouse: 1%, pig: 13%, and humans: 23%) and large‐diameter fibers (mouse: 0%, pig: 2%, and humans: 3%). Conclusion The VN composition was largely similar with respect to myelinated and unmyelinated fibers in the species studied. Human and porcine VNs had a comparable diameter and similar amounts of fibrous tissue and contained multiple fascicles, implying that the porcine VN may be suitable to optimize stimulation parameters for clinical trials.

Quantified Morphology of the Cervical and Subdiaphragmatic Vagus Nerves of Human, Pig, and Rat

“…It should also be noted that in nerve morphology and ultrastructure studies, it is common to quantify the largest or smallest diameter, rather than effective circular diameter; we only compared to data that used the latter metric, which is more clearly defined. Our human cervical nerve diameters (1.9 ± 0.5 mm) were smaller than most prior published data, which reported mean diameters of 3.0 mm for 71 left cervical and 3.0 mm for 68 right cervical VNs across three studies ( Verlinden et al, 2016 ; Hammer et al, 2018 , 2015 ), but 2.0 mm for 1 left and 5 right VNs in one study ( Stakenborg et al, 2020 ). All values are nonetheless consistent with the use of 2 mm or 3 mm inner-diameter cuff electrodes for clinical implanted cervical VNS ( Santos, 2004 ), although VNS can alter nerve morphology with increased intra- and peri-neural connective tissue ( Martín-Portugués et al, 2005 ).…”

“…A nerve : total cross-sectional area of the nerve; d nerve : effective circular diameter of the nerve; # fasc: number of fascicles; d fasc : effective circular diameter of each fascicle; A endo : cross-sectional area covered by the endoneurium, i.e., area within the inner perineurial traces; A peri : cross-sectional area covered by the perineurium; A epi : cross-sectional area covered by the epineurium, where A epi = A nerve – A endo – A peri . From top to bottom, the human cervical data from literature for A nerve and d nerve are from Hammer et al (2015) (left VN), ( Hammer et al, 2015 ) (right VN), ( Verlinden et al, 2016 ) (left VN), ( Verlinden et al, 2016 ) (right VN), ( Hammer et al, 2018 ) (left VN), ( Hammer et al, 2018 ) (right VN), ( Stakenborg et al, 2020 ) (right and left VN), and the subdiaphragmatic data are from Tailai et al (1980) (anterior VN), ( Tailai et al, 1980 ) (posterior VN), ( Gravgaard, 1968 ) (anterior VN), ( Gravgaard, 1968 ) (posterior VN), ( Stakenborg et al, 2020 ) (posterior VN). The human # fasc data are from Verlinden et al (2016) (left VN), ( Verlinden et al, 2016 ) (right VN), ( Seki et al, 2014 ) (left VN), ( Seki et al, 2014 ) (right VN), ( Hammer et al, 2018 ) (left VN), ( Hammer et al, 2018 ) (right VN), ( Stakenborg et al, 2020 ) (right and left VN) at the cervical level, and ( Stakenborg et al, 2020 ) (posterior VN) at the subdiaphragmatic level.…”

“…The human A endo data are from the same first four references as the # fasc data where plotted in mm 2 and from the same first two references where plotted as a percentage. All pig data from literature were from Stakenborg et al (2020) . For the rat data, top to bottom, the cervical data for A nerve and d nerve are from Fazan and Lachat (1997) (right VN), ( Licursi de Alcântara et al, 2008 ) (left VN), ( Licursi de Alcântara et al, 2008 ) (right VN), and the subdiaphragmatic data are all from Prechtl and Powley (1985) (anterior VN at diaphragm, posterior VN at diaphragm, anterior gastric branch, posterior gastric branch), where they analyzed individual fascicles.…”

“…We compared our measurements to published data ( Figure 7 ), although many metrics were not previously quantified. In particular, no studies quantified the fascicle diameters for human or pig VNs, and only one recent paper included nerve diameter and fascicle number for pig VN ( Stakenborg et al, 2020 ), although their methods for quantifying nerve diameter were not provided. It should also be noted that in nerve morphology and ultrastructure studies, it is common to quantify the largest or smallest diameter, rather than effective circular diameter; we only compared to data that used the latter metric, which is more clearly defined.…”

Quantitative estimation of nerve fiber engagement by vagus nerve stimulation using physiological markers

“…Our analysis is based on the assumption that fiber engagement and eCAP amplitude are linearly related. However, studies have shown that the distribution of nerve fibers is non-uniform within the cervical vagus nerve, even in the relatively simple rodent nerves [70,71]. Fibers that lie closer to the stimulating or recording electrode are easier to be excited or contribute more heavily to the eCAP measurements; both effects are more prominent with larger fibers.…”