Computational models of compound nerve action potentials: Efficient filter-based methods to quantify effects of tissue conductivities, conduction distance, and nerve fiber parameters
Fig 2
Overview of modeling CNAP recordings from rat cervical vagus nerve.
(A) A bipolar stimulation electrode activated the nerve fibers at the proximal end of the nerve. A tripolar electrode recorded the CNAP at each contact—in a monopolar configuration—at the distal end of the nerve. The volume conductor model represented the monofascicular nerve as a cylinder with a perineurium (not illustrated) and an anisotropic endoneurium, and it represented the electrodes as electrode contacts within insulating cuffs. The recording electrode had a cuff opening of either 0° or 16°. A conductive material (“surround”) filled the space within and around the nerve and cuffs. (B) Template creation inputs included the stimulation volume conductor model, a stimulation waveform, and a set of 193 (myelinated) or 97 (unmyelinated) discrete fiber diameters that defined a population of nerve fibers at the centroid of the nerve to simulate in a biophysical model. (C) Template creation outputs included CV and transmembrane currents for each of the 193 (myelinated) or 97 (unmyelinated) simulated fibers. (D) CNAP calculation inputs included a recording volume conductor model, fiber diameter measurements, fiber locations, and the template creation outputs (i.e., transmembrane current templates and CV vs. fiber diameter relationship). Fiber diameter measurements and random fiber locations defined a population of 1,676 (myelinated; not illustrated) or 13,283 (unmyelinated; illustrated) nerve fibers to be recorded. Fiber diameter measurements were obtained from a publicly available dataset [47] and transformed by the shape-adjusted ellipse method [48]. (E) We interpolated transmembrane current templates across all fiber diameters. We calculated the recording sensitivity functions at all fiber locations via the recording volume conductor model. We calculated a filter for each fiber by inserting zeros into the recording sensitivity function such that the time between non-zero samples equaled the internodal length divided by CV. We generated SFAPs by convolving each filter with an interpolated transmembrane current template, and we superposed SFAPs to generate CNAPs.