Abstract
Objective. Fast neural electrical impedance tomography (EIT) is a method which permits imaging of neuronal activity in nerves by measuring the associated impedance changes (dZ). Due to the small magnitudes of dZ signals, EIT parameters require optimization, which can be done using in silico modelling: apart from predicting the best parameters for imaging, it can also help to validate experimental data and explain the nature of the observed dZ. This has previously been completed for unmyelinated fibres, but an extension to myelinated fibres is required for the development of a full nerve model which could aid imaging neuronal traffic at the fascicular level and optimise neuromodulation of the supplied internal organs to treat various diseases. Approach. An active finite element method (FEM) model of a myelinated fibre coupled with external space was developed. A spatial dimension was added to the experimentally validated space-clamped model of a human sensory fibre using the double cable paradigm. Electrical parameters of the model were changed so that nodal and internodal membrane potential as well as propagation velocity agreed with experimental values. Impedance changes were simulated during activity under various conditions and the optimal parameters for imaging were determined. Main results. When using AC, dZ could be recorded only at frequencies above 4 kHz, which is supported by experimental data. Optimal bandwidths for dZ measurement were found to increase with AC frequency. Significance. The novel fully bi-directionally coupled FEM model of a myelinated fibre was able to optimize EIT for myelinated fibres and explain the biophysical basis of the measured signals.
Highlights
There is currently increasing interest in exploring ways of imaging fast electrical activity inside peripheral and autonomic nerves
In spite of general agreement, the differences between the experimental and simulated results can be explained by: 1) the whole nerve was used instead of a single fibre which changes the compound action potential (AP) latency due to dispersion in nerve and 2) in the experimental results, a bandwidth of 3 kHz was used for all frequencies, which this current study has revealed is not the optimal parameter setting to use (Fig. 8)
The impedance changes simulated with the developed bidirectionally coupled finite element method (FEM) model of a myelinated fibre were maximal at DC and were different if the in-phase and anti-phase signal were subtracted during signal processing ce Author et al the way of signal processing
Summary
There is currently increasing interest in exploring ways of imaging fast electrical activity inside peripheral and autonomic nerves. Author et al of the capacitance of the lipid neuronal membrane, as this allows these applied currents to flow through it in both activated and inactivated states of the tissue. This general behaviour was previously predicted with passive [12] and active [7] models of nerve fibres. The latter active model predicted and explained the finer structure of the dZ decrease with frequency and its increase at certain frequencies as well; it described the dependence of the dZ on experimental parameters including amplitude and frequency of the injected current and size and position of the electrodes. A goal to develop an accurate full myelinated model with active ion channels was set in this study
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