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Biomedical subjects

A T Barker

Publications and source records attributed to A T Barker.

10 recordsLinked to original sources
PubMed1991

The effect of low-frequency pulsed magnetic fields on chick embryonic growth.

There are many claims made for biological effects of low-frequency pulsed magnetic fields ranging from beneficial to harmful, but few have been independently verified. The ubiquitous nature of both natural and man-made magnetic fields makes the possibility of biological interaction a potentially important subject. We have investigated the claimed stimulatory action of low-frequency pulsed magnetic fields, of a type used clinically, on the growth of embryonic chicks. Four day old embryos were exposed to a magnetic field, peak field strength 2.1 mT pulsed in 5 ms bursts repeated at 15 Hz, for 100 h. Embryo weights and long-bone lengths were compared to sham-exposed controls. Particular care was taken to reduce temperature differences between the test and control groups because this model has a known sensitivity to small temperature changes. We found no increase in embryonic growth due to this low-frequency pulsed magnetic field and hence have been unable to confirm earlier findings by other workers using the same model. We conclude that rigorous design of experimental protocol and a full description of the physical parameters are essential in studies purporting to show effects of electromagnetic fields if the results are to be confirmed by other workers.

Animals↗
PubMed1991

An introduction to the basic principles of magnetic nerve stimulation.

Magnetic nerve stimulation is a new method for the noninvasive stimulation of neuromuscular tissue. The technique, developed at the University of Sheffield, United Kingdom, is being increasingly used for both clinical studies and basic research, with some 500 stimulators presently in use worldwide. This paper looks at the development of magnetic stimulation as a clinical tool. The basic physics principles of the technique are outlined, and the different magnetic field waveforms, coil geometrics, and orientations that can be used are discussed. The depth of penetration of magnetic stimulation is compared to that of conventional electrical stimulation using surface electrodes. It is shown that the former generates lower electric fields at the surface of the body, resulting in greater penetration and the ability to stimulate deep nerves without pain. Magnetic stimulation has several other advantages over electrical stimulation, including being able to stimulate the human brain without discomfort due to the magnetic fields passing through the skull without attenuation. These advantages, along with the limitations of the technique, are discussed. Finally, data relating to the safety of brain stimulation are summarised in terms of the electromagnetic parameters used. It is concluded that the present generation of magnetic stimulators do not cause acute hazards, provided their electrical and mechanical design meets the relevant electromedical safety standards.

Brain↗
PubMed1991

Magnetic nerve stimulation: the effect of waveform on efficiency, determination of neural membrane time constants and the measurement of stimulator output.

We describe the first investigation into the effect on stimulation efficiency of varying the output of a commercial magnetic stimulator based on our original clinical design. Over the range of magnetic field waveforms considered, it is shown that the stored energy required to achieve stimulation, both cortically and in the periphery, varies by approximately 2:1. Greater efficiency is obtained by using shorter risetime magnetic fields. This results in more effective stimuli for the same stored energy, or, for the same stimulus, a decrease in energy storage, power dissipation and peak currents, thus simplifying hardware design. A novel method of processing the data obtained from different waveforms is presented which enables neural membrane time constant to be calculated. Data from normal subjects is presented showing both peripheral and neural time constants to be of order 150 microseconds. The cortical measurements represent the first non-invasive determination of cortical membrane time constant in man. Time constant measurements using magnetic stimulation may be clinically useful because they give information concerning the electrical properties of the nervous system not available from present techniques. Finally a method of quantifying the output of magnetic stimulators and coils is described which enables laboratory comparisons to be made, and takes into account magnetic field waveforms and coil geometry. The proposed symbol for this new measurement is Et150 with units volt seconds/meter.

Adult↗