| MadSci Network: Neuroscience |
I believe that the answer to your question is related to the frequency of the magnetic field and the currents that are induced by the magnetic field. Unfortunately, I don't have a strong understanding of electromagnetism, so I can't provide you with a detailed explanation for this answer. However, I can provide you with information and links to web pages that I believe will be helpful.
For example, I found the following information on two web pages about a technique termed Transcranial Magnetic Stimulation (TMS) http://www.biomag.helsinki.f i/tms/basic.html http://www.biomag.helsinki. fi/tms/safety.html
"Cell membrane sustains a potential difference between intra- and extracellular space. In static cells the transmembrane potential is about - 70 mV (intracellural more negative). An externally applied electric field may deviate the cell's membrane potential, that is, depolarize the membrane and hence activate excitable tissue. Electric fields suitable for neural stimulation can be generated non-invasively applying electromagnetic induction. In magnetic stimulation a time-varying magnetic field induces a current flow in the tissue. The magnetic field strengths of up to several Tesla required in magnetic stimulation are achieved by driving the stimulating coil with brief current pulses of several kiloamperes." "If used properly, single-pulse TMS has no known harmful side effects. TMS has been used since 1985 and today some 3,000 stimulators are in use. If used properly, also repetitive TMS (rTMS) is thought to be safe." However later on this page, it is noted that, "Single-pulse TMS has produced seizures in patients, but not in normal subjects. rTMS has caused seizures in patients and in normal volunteers."The brain is exposed to strong magnetic fields during magnetic resonance imaging (MRI). The principles of MRI are described in a web page describing MRI at Microsoft® Encarta® Online Encyclopedia 2001 http://encarta.msn.com
"MRI is possible in the human body because the body is filled with small biological "magnets," the most abundant and responsive of which is the proton, the nucleus of the hydrogen atom. The principles of MRI take advantage of the random distribution of protons, which possess fundamental magnetic properties. Once the patient is placed in the cylindrical magnet, the diagnostic process follows three basic steps. First, MRI creates a steady state within the body by placing the body in a steady magnetic field that is 30,000 times stronger than the earth's magnetic field. Then MRI stimulates the body with radio waves to change the steady-state orientation of protons. It then stops the radio waves and "listens" to the body's electromagnetic transmissions at a selected frequency. The transmitted signal is used to construct internal images of the body......Because it does not use ionizing radiation, MRI is risk free except for patients with cardiac pacemakers, patients who might have an iron filings next to their eyes (for example, sheet metal workers), patients with inner ear transplants, and patients with aneurysm clips in their brains."Finally, the following is from a draft document entitled "A Primer on Medical Device Interactions with Magnetic Resonance Imaging Systems", which was published by the U.S. Department of Health and Human Services Food and Drug Administration Center for Devices and Radiological Health http://www. fmrib.ox.ac.uk/~peterj/safety_docs/fda_primer.html
"When this gradient magnetic field is applied, the magnetic field intensity changes rapidly, giving rise to a time-varying magnetic field. During the rise time of the magnetic field, a voltage is induced in an electrical conductor, even when it is stationary in the field. However, in most MRI systems, the currents induced by the pulsed magnetic gradient field are about 1,000 times smaller than those induced by the pulsed [radio frquency]RF component and are therefore not of great concern with regard to thermal injuries.....With regard to biological effects, one main concern with [the radio frequency] component of MR is the production of heat in tissue.MRI is also discussed on the following MadSci pages:
Moderator Note: In addition, I would just add that the magnetic fields generated by the electrical currents in the brain are in the femto Tesla range. These local fields mollify external fields to some extent. In addition, varying magnetic or electromagnetic fields are more likely to affect biological organisms than the static magnetic fields generated in MRI's. The magnetic fields generated in MRI's specifically affect the protons in atoms. In order to affect the general flow of ions themselves in biological organisms (which is necessary in order to "stop" the brain functioning), the field has to be strong enough locally to prevent undue heating from molecular collisions. If you would like to see an abstract on the intensity of static magnetic fields necessary to rotate (and possibly dissociate) ion-protein complexes in E. Coli cells, please see this PubMed article. Finally, to see the effects (or lack thereof) of 0.2 T static magnetic fields on human neurons (and any resulting inhibition of signal transduction), please see this PubMed abstract. You might also want to check this PubMed abstract to see the effects of low-frequency electromagnetic fields on nerves in the human body. -- RJS.
Try the links in the MadSci Library for more information on Neuroscience.