Electromagnetic effects – From cell biology to medicine
Introduction
Here we focus on how cells are influenced by electrical fields (EF), magnetic fields (MF) and alternating electromagnetic fields (EMF) (the term “EMF” is also used to summarize the whole field of “electric”, “magnetic” and combined “electromagnetic” effects). We begin by reviewing the response of cells to direct current electrical fields (DC EF), which are static electric fields that are generated from the cell mostly by ion transporters. Our focus first turns to mechanisms coupling DC EF to known cell biological phenomena, such as cell adhesion and migration, embryonic and tissue development, and wound healing. We consider these DC EF-induced reaction cascades in the context of known physical principles that link via molecular mechanisms to cell behavior. Later in the review we compare these reactions to those described in response to static magnetic fields (SMF), which come in most cases from outside (e.g. earth magnetic field).
Nothing in a living organism is static, however, because movements of cells, tissues and organs are ever-present phenomenon. Thus in living tissue we mostly find alternating fields as well as a combination of EF and MF. This point is addressed in the last chapters.
EMF frequencies (Table 1) in the body are normally in the range of extremely low frequencies (ELF). These EMF include the action potentials of nerves and heart tissue, skeletal muscle vibrations and frequencies elicited by rhythmic activities within other body tissues. Thus, we concentrate on these frequencies in the present review.
Confusion still persists in the field as to the mechanisms by which high and very high (till microwave) frequencies – encompassing also the mobile radio communication (main area of 900–1800 MHz) – EMF act at the cell and molecular biological level. This is largely due to the numerous “frequency windows” for the biological action (frequencies where a biological respond is found) and by the mixtures of modulated frequencies and carrier frequencies. Researchers have thus concentrated on the thermal effects of radiation at a tissue-specific absorption rate (SAR).
However, as is the case with ELF EMF, the energetic threshold to produce cell-specific information at defined frequencies can be much lower than that required for unspecific heating of the tissue. Interestingly, for therapeutic purposes (e.g. repair of bone fractures, wound healing, etc.) ELF EMF are used as directly applied frequencies or modulated as pulsed magnetic fields. For compiling “hard” data the situation is not very easy: the literature dealing with electric and magnetic field stimuli is full of a bewildering array of model systems, clinical situations, signal configurations and stimulation devices. Thus, we concentrate mainly on cell biological findings in renowned journals and look for basic principles in the mechanisms of EMF influence on biological systems.
The most complex, and currently the most speculative, component of research on EMF is relating the molecular and cell biological findings to observations in multicellular systems and organisms. It is extremely difficult, even in tissues or cells studied in vitro, to determine which responses directly result from EMF. In the body, the wide range of interactions that are likely to occur are too manifold to be defined by clear causal relationships and are thus not discussed here.
Finally, as an outlook to future research topics, we will link areas of EMF research to thermodynamics and quantum physics approaches that are sure to produce novel insight into cell biology.
Section snippets
History of electromagnetic field research
The idea of bioelectricity, which refers to the EMF produced by living matter, has long provoked scientific research. Most famously, Galvani elicited muscle contractions from preparations of frog nerves and muscle in the late 1700s using electricity from lightning storms and static electricity generators. He later demonstrated that a frog leg can be made to twitch merely by touching it with the cut end of the sciatic nerve from the opposite leg (1794). Although Galvani believed this experiment
DC EF strength in the environment
Because of the high conductivity of body tissues relative to air, and because EF are conducted around the body, exposure to DC EF via air does not produce an internal field but instead builds up surface charge on the body (Tenforde, 1991; see Bracken et al., 2004). At sufficiently high levels, the induced surface charge can lead to hair deflection and cutaneous perception of strong static electric fields. The mean threshold for field perception by human volunteers is about 22 kV/m, when standing
Static magnetic fields (SMF)
Compared to DC EF, endogenous sources of static magnetic fields are presumably negligible because in living systems everything from molecule to organelle is in motion. Even SMF can induce EMF if the change in movement is rhythmic. Thus one can apply Faraday's law that “a changing magnetic field is associated with a changing electric field”. In most cases of endogenous MF generation by muscle, nerve, piezo, streaming and other E potentials, the concomitantly generated magnetic (H) component is
Bone, muscles and nerves
As we already mentioned, everything in living systems is in motion and changing MF are associated with changing EF. Endogenous EMF and PEMF arise from the movement of muscles, tendons, etc. and the actions of the musculoskeletal system itself. Mechanical deformation of dry bone caused piezoelectricity, i.e. bending strain couples to the spatial gradients of permanent dipoles in collagen molecules (Hastings and Mahmud, 1988). However, in the moist surroundings of living bone, small piezoelectric
Aspects for future research
Metabolic oscillations and concommittant, ELF EMF signals are generated by distinct molecule arrays (see above). Possibly, this is also important for information processing of the cell. The point is that these information “routes” would add to all other hitherto known cell biological signaling and information pathways and this would not be an alternative explanation of cell physiology.
We are still far away to understand all these endogenous processes. Not until we really know more about their
Acknowledgments
The author likes to thank Dr. Th. Monsees, Dr. H.H. Epperlein, Dr. N. Özkucur, M. Valtink, Dr. C. Roehlecke, Dr. L. Knels, S. Perike, R. Ali, K. Heidel, B. Rost and K. Pehlke for their excellent experimental work, I. Beck for the graphical work, L. Rohde, Y. Knobloch, C. Nipproschke and T. Schwalm for editing, and the DFG (FU 220/9-1) for the financial support.
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