Electrodes applied to most peripheral nerves can electrically
access a large number of axons within the nerve. In many
cases, advantage may be taken of the
fascicular arrangement of the axons within the nerve trunk. By placing multiple
electrode contacts around or within the nerve, the field of influence of the
applied electrical stimulus can be directed to selectively activate subsets
of axons within the nerve. In the figure, a radial arrangement
of four electrical
contacts is shown arrayed around a nerve trunk with four fascicles. By orchestrating
the electrical field spatially and temporally from specific contacts, different
subpopulations of the axons may be selectively activated.
Numerical Analysis of Electric Field Generated by a Nerve
A 3-D finite element model of an unifascicular nerve was
developed to study the potential distribution from a nerve
cuff electrode . A schematic model nerve is shown in the
figure. The z-axis runs in the direction of the axons in
the nerve trunk. The 3-D domain representing the nerve, surrounding
tissue and the electrode were assumed purely resistive. The
nerve model consisted of 28 layers with 256 volume elements
along the axis of the nerve. A finer mesh was used in the
region inside and close to the electrode.
The x-y and the x-z planes were considered to be two planes
of symmetry. Therefore, only one-fourth of the problem was
modeled using 7769 nodes and 1680 volume elements.
The electrical potential distribution was obtained by solving
the Laplace Equation within a 3-D domain. The solution
was obtained by minimizing a functional given by the equation
at the bottom of the Figure.
Geometry of the Nerve
of the represented nerve is shown in the figure. It was modeled
as a three-mm. diameter, unifascicular anisotropic structure
s-longitudinal = 1.0 S/m and s-transverse = 0.1 S/m, surrounded
by a 30 µm perineurium with sigma = 0.00063 S/m.
Two layers of encapsulation tissue, a 150 µm loose
layer with s = 1.0 S/m and a 50 µm tight layer with
s = 0.0659 S/m (so that the effective sigma = 0.22/m) and
a 50 µm
layer of 0.9% saline (sigma =2.0 S/m) were considered to
between the nerve and the electrode.
The electrode contacts were 30 µm thick with s = 10^4
S/m and with surface area of 1.57 sq. mm. They were embedded
on the inner surface of a 1 mm thick insulated nerve cuff
(sigma = 10^-6 S/m). The surrounding body fluid was represented
by a 20 mm layer of saline.