Table 3.
Model parameters to estimate the bulk transverse endoneurial resistivity for a bundle of unmyelinated axons.
| Parameter (variable name) | Default value | Test range | References |
|---|---|---|---|
| Fascicle diameter (dfasc) | 105 μm | 80–300 μm | (Kawagishi, 2008; Tweden, 2013) |
| Axon diameter (daxon) | 1 μm | 0.5–2 μm | (Giordano, 2005; Guo, 1987; Helmers, 2012; Mei et al., 1980; Robinson, 1972; Waataja, 2011) |
| Axonal area fraction (AAF)a | 0.435b | 0.20–0.737c | (Agostini, 1957; Altman and Plonsey, 1989) |
| Endoneurial resistivity (ρendo-micro) | 0.65 Ω-m | 0.5–2 Q-m | (Altman and Plonsey, 1989; Geddes and Baker, 1967b) |
| Intracellular resistivity (ρa) | 0.7 Ω-m | 0.5–4 Q-m | (McIntyre et al., 2002; McIntyre and Grill, 2000) |
| Specific membrane resistance (Rm) | 0.2 Ω-m2 | 0.0025–0.4 Ω-m2 d | (Barrett and Crill, 1974; Cole and Hodgkin, 1939) |
Table 5 (Supplement C) shows calculations for estimating the axonal area fraction for a single fascicle based on cat posterior abdominal vagus nerve morphology.
Although we estimated an axonal area fraction of 0.45 from cat posterior abdominal vagus nerve histology slides (Table 5, Supplement C), we could only achieve AAF = 0.435 in the finite element model with the default parameters.
The lower bound of this range was chosen arbitrarily simply to test model sensitivity to this parameter. The upper bound is the maximum extent to which we could pack 2 μm axons into a 105 μm diameter fascicle.
The lower bound of this range represents the minimum membrane resistance for squid giant axon if all axons in the fascicle are active simultaneously.