Effects of limb perturbation on surface recorded nerve action potentials

The field action potentials are a convolution of source strength and the propagating-point source responses (PPSRs). Surface-recorded peripheral nerve potentials are strongly influenced by the volume-conductor properties of the limb. In this paper the PPSRs are solved based on finite-element formulation, which allows solutions to forward volume-conductor problems involving curved nerves in arbitrarily shaped, inhomogeneous, anisotropic limbs. Effects of limb perturbation on surface-recorded nerve action potentials are simulated based on circular and elliptic cylinders with a uniform cross-section consisting of multiple inhomogeneous and anisotropic regions (fat, muscles, and bone layers). The simulations show that a simple homogeneous model is adequate only for a very superficial nerve that lies completely in the subcutaneous fat layer. Deeper nerves that lie on or within the muscle require a multiple-layer model that takes both the muscle and the fat into account. The cross section only needs to be modeled accurately between the recording site and the nerve location. A circular limb model is adequate for clinical recordings. However, an accurate knowledge of the nerve path is essential for accurate modeling of the action potentials, in both needle and surface recordings.