Bioelectric currents

Reid, B., Nuccitelli, R and Zhao, M. (2007) Non-invasive measurement of bioelectric currents with a vibrating probe. Nat. Protoc. 2, 661-669. 

FIGURE 19.1 Bioelectric current loop, concept drawing. Current enters the membrane atz = zi, flows intracellularly and emerges at Z2. Flow in the extracellular volume may be along the membrane (solid) or throughout the surrounding volume conductor (dashed). 

Parallel with the discovery of new sources of electricity, the detection of small bioelectric currents became possible. Soon after 0rsted s discovery in 1820, the first galvanometers appeared. The problem was twofold to increase sensitivity and to make the new sources follow the rapid changes of muscle and nerve currents. 

Electric fields can arise from a static distribution of charge, whereas electric currents are charges in motion. Since current flow in a volume of fluid is not confined to a linear path as in a wire, currents can flow in many directions. When describing bioelectric currents, we often use a derivation of Ohm s law ... 

FIGURE 17.5 The volume conductivity of the body means that bioelectric current generators such as the heart produce electric fields detectable elsewhere on the body. 

However, there are still electrode-detectable bioelectric fields from myelinated nerves during action events, since volume currents flow, as shown in Fig. 17.13. These bioelectric currents are dipolar in nature, meaning that ionic currents flow spatially from the source at the leading edge of depolarization to a sink at the repolarization edge. Separation distances between these nodes in mammals are on the order of a millimeter. 

The mechanical contraction of the heart follows the time course of its longer bioelectric current flows. Bioelectric currents initiate and define the motor activity of the heart cells. If die heart cells simply twitched during an action event rather than producing a sustained force over a longer duration, there would not be time for the myocardium to move against the inertia of blood in the ventricles. 

When discussing bioelectric currents, electrophysiologists usually use die term dipole moment p. This is defined as... 

Finally, there are also bioelectric currents flowing in the retina. They are detectable using corneal electrodes, and the monitored biopotential is known as an electroretinogram (ERG). It changes in amplitude and waveshape with pulsed illumination. A strobe light, for example, will produce a transient ERG biopotential of nearly 1 mV. 

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