Why isn't the brain affected by external magnetic fields?

Question inspired by this thread, about magnetic psychiatric therapy quackery, and in particular, Post #37 there.

I’ll just repeat the question that I posted there:

Come to think of, why isn’t the brain affected by external magnetic fields anyway? We’ve all heard the people complaining that they get headaches or fuzzy vision or whatever from standing under high-voltage lines, and we’ve all read that there’s no good evidence to believe that. And we know that even high magnetic fields like in MRI machines aren’t harmful (that being a point made in this thread).

But, why? As mandala notes, our nervous system is an electrical system. One would naturally expect it to be affected by external magnetic fields. Why isn’t it?

It is affected. Note that this require oscillating magnetic fields to induce a current in the brain, a simple static magnet won’t work. http://en.wikipedia.org/wiki/Transcranial_magnetic_stimulation

Kinda interesting, then, that it doesn’t produce stronger effects than described there. Like, it mentions sporadic cases of seizures. So it’s interesting that strong magnetic fields like that don’t totally frotz out the brain, with who-knows-what gruesome effects.

It says that TMS magnetic fields are about as strong as MRI fields. I’ve seen some photos showing the strength of those – One showed a metal chair that someone had brought into the room that had gotten sucked into the MRI machine. Kinda surprising that a field that strong doesn’t seriously scramble the brain, at least transiently.

Qualified guess. Magnetic force on an electrical particle is proportional to speed. Electricity in the body moves too slowly to experience significant forces.

This only applies to static fields of course.

For the changing magnetic fields in TMS one important factor is how rapidly the field changes, since that’s proportional to the induced current.

From here Scholarpedia: Transcranial magnetic stimulation

The natural assumption then is that the fields in regular MRIs have much higher rise times.

Neurons are really small, and the synapses (connections between neurons) are even smaller. It would take a really strong change in a magnetic field to generate significant voltage over such a short distance.

Also, neurons don’t conduct electricity like wires. They operate by electrochemical activity, where a series of chemical reactions carries charge from one place to another. While a changing electric field can induce current in a wire, it can’t directly cause these chemical reactions to happen. An induced electric field could introduce enough voltage to cause a synapse to fire, but it won’t push current around in the brain as if it were a wired circuit.

That gives me an idea. How about we use oscillating magnetic fields to speed up the electrical activity in neurons, on the same principal as in an atomic particle accelerator, by which we could amp up human mental activity to Vulcan proportions! :stuck_out_tongue: Or kinda like turbocharging a modern CPU by jacking up its clock rate, requiring only that one be verrrrrry careful not to push too far and toast it. :eek:

Help fight me some grade-school-level iggorance here. How is “electrochemical” activity different from any chemical activity. (Is it?) I mean, all “ordinary” chemical reactions are driven by the activities and proclivities of atoms’ electrons, n’est ce pas? Can magnetic fields affect that in any case, one way or another? Is neuron activity significantly different than that somehow?

Each neuron has an internal static electrical charge. When the static charge gets high enough the neuron fires, sending a pulse of current down its axon to the synapses that connect it to other neurons. The synapses release neurotransmitters that open ion channels in the other neurons, slightly raising or lowering their internal static charge … which may trigger (or inhibit) those other neurons from firing.

A strong-enough magnetic field *could *induce a current in the axons of your neurons. But it’s unlikely to do so in a way that will mimic the effects of the normal pulse that occurs when the neuron fires. So it’s unlikely to trigger the release of neurotransmitters that would allow the pulse to propagate to other neurons.

There’s several reasons this won’t work, but the simplest and most obvious one is that the long axons in your brain go in different physical directions, so a magnetic field pointed in a particular direction would actually slow down axons going in the “wrong way”. Actually, this wouldn’t happen, either, but the details are very complex.

What matters is the rate of change of the magnetic field relative to the conductor, or the movement of the conductor in the filed (in either sense so that the conductor cuts the lines of force.)

A TMS probe has a very fast rise time, plus a very high intensity, and that is why it affects the brain. An MRI scanner has a static field of a similar order of magnitude, but it is as stable as engineering can make it. The gradient coils are driven to change the field (these are what make the noise when the scanner is operating) but they are vastly lower strength. They do however manage to induce currents in metal objects, which is why some people feel tingling in fillings, and there is zero chance you can have a pacemaker and be scanned.

To elaborate on what The Hamster King said:

Axons are not wires! There is no substantial net current flow along the length of the axon. Signals are propagated as action potentials, which are waves of voltage depolarization that move along the length of the axon. This happens essentially because there are a set of electrochemical gradients from one side of the membrane to the other, and the conductance of the membrane is a function of voltage and time. The major currents are radial, as ions move across the membrane.

Furthermore, neurons are not electrically connected to each other. When one neuron communicates to another at a synapse, it does so with non-charged chemical signals, which trigger a different set of changing conductances in the membrane of the receiving neuron.

So there really are no electrical circuits in between neurons. Thus a changing magnetic field does not significantly induce current in neurons. (If it did, the field of electrophysiology would be a hell of a lot easier, since you could do some kinds of experiments without physically poking a bunch of electrodes into a neuron).

However, all of the neurons are bathed in the same conductive extracellular fluid. This means that TMS can cause localized eddy currents. Here my understanding is lacking, but I imagine these eddy currents can change the extracellular potential, and therefore alter the thresholds at which an axon can send a signal.