(Invited) An Electrochemical Perspective on Electrophysiological Measurements

Abstract

Electrophysiological measurement such as EEG and peripheral nerve conduction studies are commonly used in neurology research and medical diagnoses. Bioelectrochemical structures and reactions at the molecular to micrometer scale propagate charge through space. Electrophysiological measurements made across distances on the scale of centimeters map collective effects of many charge propagation events. In peripheral nerves, charge propagates along nerves in the limbs at ~50 m/s. Voltages measured from the surface of the skin are in the range of 6 to 8 µV for peripheral nerves and as high as 40-60 µV in EEG studies. Charge propagation events that generate measured electrophysiological signals can be viewed with the same electrostatic fundamentals that describe how potentials are established in electrochemical systems. Polarization of a synaptic membrane generates a flux of positive charge and then a charge balancing flux of negative charge as the membrane depolarizes. The flux separates charge that generates a dipole. The dipole propagates down a peripheral nerve as the membrane separates charge sequentially. A sketch of the dipole is based in electrostatics. From an electrostatic perspective, the separation and magnitude of the charges establishes an electrical potential Φ(x,y,z). Consider a single dipole at an instant in time. Evaluation of charge separation yields a map of potential in space as shown in the Figure. The gradient in the electrical potential establishes the electric field. Migration is driven by potential gradients such that the electric field may play a role in dipole propagation. The Figure is an instantaneous snapshot of potential associated with a dipole in space. In electrophysiological measurements, multiple dipoles events yield the measured voltages as nerve impulses that propagate in time along a peripheral nerve tract. A sketch of the correlations between electrophysiological measurements and electrochemical fundamentals is presented. .... Figure 1