jerrystraub

Thanks but I think you both misunderstood my question. I already know that microelectrodes can be inserted and patchclamp techniques can be used to study action potentials in individual neurons and I basically understand how that would work. What I'm trying to understand is how action potentials are generated and recorded during diagnostic nerve conduction studies where they simply apply electrodes to the skin like they use when doing an EKG. It seems like the stimulating electrode would just generate a localized current deep to wherever it's placed. Any voltage gradients generated would be across the whole cell and not just across the cell's membrane and wouldn't cause any ion channels to open and, hence, wouldn't cause any depolaization leading to action potentials. So I don't understand how a surface electrode can cause specific neurons to depolarize sufficiently to trigger action potentials. And if they can, then how would it "know" which neurons were being stimulated since one might presume a lot of different neurons deep to the electrode. Similarly, the reording electrode has to be able to pick up very small currents -- only about 0.3 microamps for a median nerve -- from a pretty small nerve bundle -- only 2 mm diameter for the median nerve. And it has to do that to the exclusion of all the other neuron "traffic" that might be passing beneath the recording electrode. I'm not an engineer and simply fail to see how the apparatus used in a clinical nerve conduction study functions. Like I said, I have read many article on the device and they allude to generating action potentials and then measuring them but I haven't found an explanation of exactly how they do that.

I'm having a bit of difficulty understanding the physiology of nerve conduction studies. As described, the device generates a voltage difference beneath a surface electrode which then causes depolarization of neurons deep to the electrode. This allegedly causes action potentials which move proximally or distally and which are detected by another surface electrode. Knowing the time and distance thus allows calculation of a conduction "velocity." First, I vaguely recall being taught that the only way to reliably generate action potentials was by either physically "clamping" a nerve or by inserting a microelectrode into the nerve. To me, it seems like simply applying a voltage difference to the skin surface would only cause a localized current that might, in turn, cause some superficial muscle fibers to twitch or might stimulate pain fibers if the current generated sufficient heat. Can surface electrodes cause depolarization of specific neurons? Has anyone actually stuck a microelectrode into a neuron cell and measured the action potential generated by a surface electrode. My literature search skills are admittedly rusty but I couldn't find any papers that discussed anything of that sort. I would be inclined to think there would be wide variability depending upon different types of neurons, their distance from the stimulus, etc -- yet most of the recordings I've seen in normal subjects so far show near identical recordings of AP amplitude and duration all along the recording path. It would make sense in an EKG that the wave is consistent since it records a huge muscle sending all of its signals in a particular direction at one time. But in something like a median nerve, which is only 2 mm in diameter and generates only 1microamp of current, given the high bioimpedance of tissue and the other factors, it just seems that any recording from a surface electrode would look rather "sloppy." Also, I assume that if an action potential were generated then it would also have to propogate distally/proximally as well and therefore cause some effect -- e.g. a muscle twitch if you were studying motor fibers. By the same token, an action potential traveling along a sensory fiber would continue past the proximal electrode to the brain and be perceived as touch, hot, cold, or whatever other sensory components were activated. But as I understand it, neither of these two logical (to me at least) consequences occurs during a NCT. Anyway, that's the current state of my ignorance. I've read a slew of articles about NCTs but they never seem to address my specific questions. Any enlightening comments would be appreciated.