The Field from an Isolated Nerve in a Volume Conductor

Experimental recordings of electric field potentials from an active frog sciatic nerve in an "infinite" volume conductor were obtained. The dorsal and ventral roots supplying the isolated sciatic nerve were dissected and stimulated separately and in combination. The measured electric field potentials from dorsal/ventral root stimulation, as well as whole nerve stimulation, were recorded at the nerve surface and at several radii from the nerve. The field potentials, at these radius values, were predicted from a solution of Laplace\´s equation. The Fourier transform development of the solution of Laplace\´s equation, as presented by Clark and Plonsey [4], [5], allowed the determination of the field potentials to be viewed as a one-dimensional linear filtering procedure with the surface potential as an input and the field potential at a given radius as an output. Standard techniques in linear system theory were utilized to perform convolution via the discrete Fourier transform (DFT) and to perform inverse filtering via optimal filtering methods. With these techniques the surface potential could be predicted given a field potential measurement at a known radial distance from the nerve.