Re: Can a nerve such as the great sciatic radiate a magnetic field (pulse) ?

I'll give you the short answer first, and then go into detail about the 
reasons that the technique works, if you are interested in that. In short, 
people are already using something similar to what you are describing as a 
part of clinical medicine. They apply some sort of stimulation to a part 
of the body, and measure the brainwaves from the skull. The idea is that 
if there was a break in transmission, you would not see a change. A 
similar idea can be used in the visual system (measure differences in 
brain activity between when a person is in a dark room vs when they are shown 
a picture on a screen), or in the auditory system, by playing clicks in a 
person's ear. 

For more information about these tests and what they can be used for check 
out : 
http://www.bgsm.edu/neurology/department/diagneuro/ep.html

Now for the more detailed answer. Basically, as i'm sure you know, the 
brain contains lots of cells called neurons. When these neurons get 
excited, they cause a tiny current to flow which changes the electric 
field around them. A single neuron by itself doesn't create enough of an 
electric field for you to measure it outside the skull, but when you have 
millions of them all firing, the electric field adds up enough that you 
can measure it. By now you're probably wondering - why is she talking 
about electric fields, when i want to know about magnetic fields. Well to 
put it simply, when an electric field changes it creates a magnetic field. 
So when neurons get excited, and change the electric field around them, 
they also change the magnetic field around them. So in answer to the first 
part of your question, I would think that the great sciatic nerve would 
radiate both an electric field and a magnetic field. Whether those 
fields would be strong enough to be effectively measured non-invasively, I 
don't know.

Anyways, continuing the story, if you have neurons in the brain (since 
these techinques are more often used in the brain) then at any given time, 
some of them may be firing, and some may not be firing. The more neurons 
that are firing at a given time, the bigger the signal you can measure. 
How is this useful ? Well you can easily imagine that when a person is 
sitting in a dark room and they are shown a nice bright picture, the 
neurons in the area of their brain that processes visual information will 
get all excited and the electric and magnetic fields in that region will 
increase. Similarly, if you step on someone's foot, (I don't recommended 
that though), the area of their brain that processes sensory information 
from the foot  will get excited and once again the electric/magnetic 
fields will increase. You can have skull caps that contain electrodes to 
pick up the electrical field signal in all the different areas of the 
skull (this is called ElectroEncephaloGraphy-EEG), or the electrodes can 
pick up the magnetic field (in which case we call it 
MagnetoEncephaloGraphy - MEG). EEGs and MEGs are better at picking out 
different parts of the signal, and so people are looking at ways of using 
both of them together to get the most information possible from them. 

A final note on your idea. It might be possible to use a simiilar 
technique to measure (noninvasively) nerve conduction along the nerve 
(even before it got to the brain). My guess is that it would be a bit 
tricky for several reasons. One is that muscles also create electric 
fields (which are the basis for ECGs (ElectroCardioGrams) used to study 
the heart). This may make it harder to get readings from the nerve you are 
looking for. 

I hope this made sense and answered your question.
Vidya

A bit technical reference on neural imaging techniques (EEG, MEG, fMRI, 
etc) is 
http://www.cogneuro.med.utah.edu/methods.html 

I'm sorry that I couldn't find a good basic reference on EEG/MEG techniques. If 
I run across one, I'll let you know.