On the Relationship Between Fascicle Diameter and Perineurium Thickness in the Ulnar Nerve of Pigs

The anatomical structures of peripheral nerves significantly influence the performance and selectivity of neural interfaces. Among these structures, the perineurium thickness, because of its high resistivity, is important in shaping the electric field distribution within a nerve. However, data on the perineurium thickness of somatic nerves in animals is sparse. As animal models are the first step towards developing novel implantable nerve interfaces, this study characterises the perineurium thickness in the ulnar nerve of pigs. Distal and proximal sections of the ulnar nerve
$(\mathbf{n}=\boldsymbol{6})$
were extracted, stained with haematoxylin and eosin, and the perineurium thickness and fascicular diameters were measured. In total, 129 fascicles were quantified, and the average fascicle diameter was
${269\pm 73.3}\ \upmu \mathrm{m}$
and
$\boldsymbol{277\pm 81.1}\ \upmu \mathrm{m}$
for the distal and proximal nerves, respectively
$(\boldsymbol{p}=\boldsymbol{0.59})$
. A linear relationship was observed between fascicle diameter and perineurium thickness
$(\mathbf{R}^{2}=\boldsymbol{0.69})$
. This relationship was affected by the location of the nerve section, with distal sections having a greater perineurium thickness than proximal segments. Finally, equations were provided to estimate the perineurium thickness based on fascicle diameter. This information can be used to develop realistic peripheral nerve models, which can reduce variability and improve the selectivity of neural interfaces.