ABSTRACT
CONTENTS 2.1 Introduction ......................................................................................................................... 35
2.1.1 Sources of Endogenous Electric Fields................................................................ 36 2.1.2 Methods for Measuring Endogenous Electric Fields........................................ 38
2.1.2.1 Self-Referencing Probe ............................................................................ 38 2.1.2.2 Microelectrode Techniques for Measuring Endogenous
Electric Fields............................................................................................ 38 2.1.2.3 Voltage-Sensitive Fluorescent Dyes for Measuring
Endogenous Electric Fields .................................................................... 38 2.2 Measurements of Endogenous Extracellular Electric Fields ....................................... 38
2.2.1 Amputated Limbs................................................................................................... 38 2.2.2 Embryonic Electric Fields Beneath the Skin....................................................... 39 2.2.3 Fields Associated with the Neural Tube ............................................................ 43 2.2.4 Fields Associated with Epithelial Wounds ........................................................ 43
2.3 Measurements of Endogenous Intracellular Electric Fields ........................................ 46 2.3.1 Nurse Cell Complex in Insects ............................................................................. 46 2.3.2 Development of Left-Right Polarity in Chick and Frog .................................. 46
2.4 Methods for Modifying Endogenous Electric Fields .................................................... 47 2.4.1 Passing Current between Electrodes ................................................................... 47 2.4.2 Low-Resistance Shunts .......................................................................................... 48 2.4.3 Placing Tissues in an External Electric Field ..................................................... 48
2.5 Summary .............................................................................................................................. 48 References ..................................................................................................................................... 49
In a volume presenting the biological effects of electromagnetic fields it is appropriate to review the information we have regarding endogenous electric fields in the body. After all, imposed electromagnetic fields may augment the naturally occurring ones, so a complete understanding of the possible effects of imposed fields requires consideration of those electric fields already present. Here, I will provide a brief overview of the direct current (DC) endogenous fields that have been best characterized in animals and will touch on the evidence that these electric fields are required for the function of various cellular and organ systems. Other well-known variable fields that are generated by various electrically excitable organs such as the heart (electrocardiogram), brain (electroencephalogram), and
eye (electrooculogram) will not be covered here. Another very comprehensive review of endogenous fields that will be of interest has appeared quite recently in Physiological Reviews by McCaig et al. (2005), and another review of the roles of such fields in development was presented by Levin (2003). In order to put these endogenous fields in perspective for the reader, I would like to
summarize here their main characteristics. Unlike much of the material considered in this volume, these endogenous fields are small and very slowly changing. Endogenous fields typically fall into the 10-to 100-V/m range and are generally very steady, DC fields generated by the flow of ionic currents through cells and embryos. This can be compared with the much higher fields required to electroporate cells (3V/cell diameter or 3 105V/m for a 10-mm-diameter cell), which are usually only applied for a short time on the order of a millisecond.
