摘要:

AbstractA workshop on the dosimetry of extremely‐low‐frequency fields was held to assess current knowledge in this field and to develop a set of recommendations for new research that meets the needs of health risk assessment, in particular, the assessment of cancer risk. The workshop was sponsored by the Electric Power Research Institute and was held on March 20–22, 1991, in Carmel, California. Major topics of the workshop were microdosimetry of induced electric fields, scaling of induced fields among biological systems from cells to humans, and the problem of defining a biologically effective “dose.” A number of research recommendations were developed, the most important of which are to (1) characterize the natural background electric and magnetic fields in tissues and near cells, (2) improve experimental exposure geometries to allow accurate characterization of induced fields in samples, (3) design experiments to distinguish between electric and magnetic field mechanisms, (4) develop standard in vitro biological systems with reproducible and well‐established responses to fields, and (5) develop definition of dose with respect to fields at the primary site of interaction. 1992 Wile

ISSN:0197-8462    

DOI:10.1002/bem.2250130702

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

ISSN:0197-8462    

DOI:10.1002/bem.2250130703

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractEnvironmental and laboratory exposure to electric and magnetic fields (EMF) in the extremely‐low‐frequency range (ELF) produces electrical quantities that interact directly with the exposed biological system on a scale small compared to the size of the human body but large with respect to cellular dimensions. The purpose of this paper is to describe these macroscopic electrical quantities and their characterization through measurements on living systems and experimental models. Electric field exposure results in a total induced current, surface electric fields, internal electric fields, and internal currents. Magnetic field exposure results in internal magnetic field, internal electric fields, and internal currents. Basic properties of fields and matter determine the methods by which these quantities can be measured. Quantification or dosimetry for these parameters on a macroscopic basis can be directed to the whole body, a cross section across the body, a local surface area, or a local volume. Models of varying degrees of sophistication have been used to establish spatial distributions of external fields and internal fields and currents. 1992 Wiley‐Liss

ISSN:0197-8462    

DOI:10.1002/bem.2250130704

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractSome numerical and analytical methods used to estimate the internal electric fields and current densities produced within human and animal models by low‐frequency electric and magnetic fields are surveyed. A major goal of such modeling is the design of laboratory experiments on cellular systems or animal models to produce a dosage comparable to that experienced by humans in a particular situation. Specific comparisons are made between the results of ellipsoidal approximations and finite‐difference methods applied to irregularly‐shaped, homogeneous, human and rat models for applied 60 Hz electric (10 kV/m) and magnetic (10−4T) fields. For scaling purposes, the induced current densities in various parts of the body are compared for rat and human models for both types of field. In addition, the current density distribution induced in rectangular culture dishes by applied magnetic fields is also described. The extension of these methods to inhomogeneous models and localized sources may not be simple. 1992 Wiley‐

ISSN:0197-8462    

DOI:10.1002/bem.2250130705

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractWe have used the finite‐difference time‐domain (FDTD) method to calculate induced current densities in a 1.31‐cm (nominal 1/2 in) resolution anatomically based model of the human body for exposure to purely electric, purely magnetic, and combined electric and magnetic fields at 60 Hz. This model based on anatomic sectional diagrams consists of 45,024 cubic cells of dimension 1.31 cm for which the volume‐averaged tissue properties are prescribed. It is recognized that the conductivities of several tissues (skeletal muscle, bone, etc.) are highly anisotropic for power‐line frequencies. This has, however, been neglected in the first instance and will be included in future calculations. Because of the quasi‐static nature of coupling at the power‐line frequencies, a higher quasi‐static frequency f′ may be used for irradiation of the model, and the internal fields E′ thus calculated can be scaled back to the frequency of interest, e.g., 60 Hz. Since in the FDTD method one needs to calculate in the time domain until convergence is obtained (typically 3–4 time periods), this frequency scaling to 5‐10 MHz for f′ reduces the needed number of iterations by over 5 orders of magnitude. The data calculated for the induced current and its variation as a function of height are in excellent agreement with the data published in the literature. The average current densities calculated for the various sections of the body for the magnetic field component (H) are considerably smaller (by a factor of 20–50) than those due to the vertically polarized electric field component when the ratio E/H is 377 ohms. We have also used the previously described impedance method to calculate the induced current densities for the anatomically based model of the human body for the various orientations of the time‐varying magnetic fields, namely from side to side, front to back, or from top to bottom of the model, respe

ISSN:0197-8462    

DOI:10.1002/bem.2250130706

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

ISSN:0197-8462    

DOI:10.1002/bem.2250130707

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractThe objective of this paper is to review some of the fundamental mechanisms for the interaction of electric and magnetic fields with biological systems at variable levels of field strengths and to examine several possible ways by which weak fields may influence these systems.We begin with a review of the basic equations by which electric or magnetic fields interact with biological fluids and follow it with a look at the effects of inserting a simple cell membrane. The initial starting points are the force equations on charged particles and dipoles. We examine their effects on current flow, the orientation of long‐chain molecules, and the forces which can be exerted by particles of magnetite on membranes. This is followed by a very simple model for the effects of a cell membrane on the overall current distribution and a model for current flow through a membrane. Some sources of nonlinearities which might serve as mechanisms for converting weak electrical signals from one frequency to a more biologically significant frequency are described.Additionally, three models by which a biological system may extract weak signals from noise are presented. The first of these is the injection‐locking of oscillating processes where the signal‐to‐noise ratio may be less than unity. The second is parametric amplification which allows the external signal and the biological process to be at different frequencies and where stability requirements on the external pump frequency discriminates against the noise. The third approach is to examine a computer model for a neural network which can be trained to identify a 60 Hz field at signal‐to‐noise ratios much less than one. The key to each of these models for possible interactions of magnetic fields with biological systems is the long‐term coherence of the signal with respect to the noise. Finally, we briefly examine the possibility of using scanning force and tunneling microscopes to give a better description of the characteristics of the cell surface. 1992 Wi

ISSN:0197-8462    

DOI:10.1002/bem.2250130708

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractAdvances in the speed of signal processing enable application of a Fourier‐synthesized function as a small perturbation (1 mV) superposed on voltage clamp steps to rapidly (<1 sec) acquire cell membrane complex driving‐point functions (impedance or admittance) in several frequency bands ranging from 1 Hz to 10 kHz. Curve fits of admittance models to these data yield a complete quantitative linear description of membrane conduction systems and their kinetics. Furthermore, the rate constants between microscopic states of an ion channel can be calculated from conductance parameters derived from model curve fits of membrane admittances. Additionally, the power spectrum of membrane thermal noise is obtainable from impedance determinations by use of the Nyquist relation. Consequently, rapid driving‐point function determinations provide the most complete macroscopic assessment of membrane conduction properties presently available. Admittance determinations of the potassium conduction system in squid giant axon and the potassium conducting “inward rectifier” in snail neuron are used to illustrate the above points. 1992 Wiley

ISSN:0197-8462    

DOI:10.1002/bem.2250130709

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

摘要:

AbstractDue to the apparent lack of a biophysical mechanism, the question of whether weak, low‐frequency magnetic fields are able to influence living organisms has long been one of the most controversial subjects in any field of science. However, two developments during the past decade have changed this perception dramatically, the first being the discovery that many organisms, including humans, biochemically precipitate the ferrimagnetic mineral magnetite (Fe3O4). In the magnetotactic bacteria, the geomagnetic response is based on either biogenic magnetite or greigite (Fe3S4), and reasonably good evidence exists that this is also the case in higher animals such as the honey bee. Second, the development of simple behavioral conditioning experiments for training honey bees to discriminate magnetic fields demonstrates conclusively that at least one terrestrial animal is capable of detecting earth‐strength magnetic fields through a sensory process. In turn, the existence of this ability implies the presence of specialized receptors which interact at the cellular level with weak magnetic fields in a fashion exceeding thermal noise. A simple calculation shows that magnetosomes moving in response to earth‐strength ELF fields are capable of opening trans‐membrane ion channels, in a fashion similar to those predicted by ionic resonance models. Hence, the presence of trace levels of biogenic magnetite in virtually all human tissues examined suggests that similar biophysical processes may explain a variety of weak field ELF bioeffects. 1992 Wiley‐

ISSN:0197-8462    

DOI:10.1002/bem.2250130710

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY

ISSN:0197-8462    

DOI:10.1002/bem.2250130711

出版商:Wiley Subscription Services, Inc., A Wiley Company    

年代:1992

数据来源: WILEY