Nerve stimulation concepts

[ee101sg]

Hi there,

I am trying to think through some EE concepts as they relate to stimulating nerves (such as transcutaneous nerve stimulation -- TENS). I am running into the following questions, which I find conflicting answers for online:

1. What are the considerations when deciding whether to use AC or DC waveforms for this application? I learned traditionally that DC current is less dangerous than AC current, but it seems like many circuits (like TENS) deliver an AC output for nerve stimulation.

2. Many circuits I see control the output current, rather than voltage. (For example specifying 10mA output rather than 10V output). Why does this matter?

3. My last question relates to the specific case in which a ganglia is stimulated using a DC pulsed output through an electrode on the skin (like in TENS). Is a second ground electrode on the skin (in addition to the output electrode) needed? I know there is always a second electrode, but it doesn't make much sense to me because we want as much of the current as possible hitting the ganglia rather than directing the electric field at a 90deg angle across the skin.

Thanks for your help and please let me know if I can clarify any of these questions!

 

[KlausST]

Hi,

maybe one problem is the definition for "DC":
* is it 0Hz, a straight line (either positive or negative), no waveform at all?
* or any waveform that is purely negative or purely positive

For me personally:
I think about the first. A straight horizontal line. Thus, for me "pulsed DC" does not exist.
Thus when I hear about "pulsed DC" I think about: AC signals (frequencies) combined with a 0Hz (DC) signal.

When you differentiate between AC and DC .. what is your definition?

Best is if you give clear values/specifications:
Example:
* 1V AC ... is rather ambiguous. --> 1V RMS 50Hz sine wave is rather clear.
* 1V pulsed DC is rather ambiguous. --> 0V/1V square wave, 50Hz, 30% duty cycle is rather clear.
***

Then you talk about "dangerous".
Touch a 12V battery (pure DC, straight horizontal line) wiht your fingers .. you will barely feel anything.
Touch a 12V AC 50HZ square wave ... you may feel it.
None of both is considered dangerous, because the current will be too low.

While you may feel nothing when touching 12V with your fingers ... you will clearly feel 1.5V when touching with your tongue. Touching a 9V battery (for me) with the tongue is really unpleasant ... I don´t want to try 12V at all.
This means ... it matters "where on your body" you touch the signal. There are places that are mor conductive than others (resulting in higher current) and there are places which are more sensitive than others.

****

* The "danger" of an DC signal is that H2O (water) is decomposed into hydrogen and oxygen. And on higher currents the skin may get burnt at the contact area.
* The danger of low frequency AC is that when run across the heart it can mess up your heart rhythm. This may cause immediate stop of heartbeat ... and it may even may cause late heart rhythm problems even after many hours... depending on current amplitude, time of exposure, frequency, current path...
* The danger of high frequency AC is that it travels along the skin ... and my burn wide areas of the skin .. not only at the contact area.
* The danger of microwave frequency AC .. even non conductive .. may cause your H2O molecules to resonate ... = heat up ... like the goods in a microwave stove.

******

Now you asked about voltge and current:
Because of size of contact area and because of contact skin conductivity the overall resistance varies in a wide range. Thus the current varies in a wide range. Thus intruducing current ... overcomes the personal skin conductivity problem .... and results in more predictable reaction.

*****
All this is not new. It has been studied in detail. You may find a lot of good documents in the internet. Just do a search.

Klaus

 

[Aussie Susan]

The 'danger' factor comes from the current that is inside the body. The skin is a 'reasonable' insulator but it only takes a few mA to actually stimulate a nerve (which includes muscle) cells. (The way it was put to me is, the body is wet and gooey inside but the skin stops all that from leaking out.) This is why the voltage is not as important as the current - when you get inside the body.

This is also why the current is controlled. you may need to increase the voltage to get the required current inside the body, but it is the current that can kill.

Regarding the placement of the 2nd electrode, as implied above, the current will flow along the 'path of least resistance'. Inside the body has a MUCH orders of magnitude lower resistance than the skin. Therefore the 'equivalent circuit' of two spaced electrodes is a very high resistor between them (i.e. along the skin) in parallel with two high resistors (across the skin and into the body at both electrodes) in series with a very low resistor (through the body). The greater current will flow through the body.

Whatever you do, remember that patient safety is ABSOLUTELY CRITICAL. This is why health related products need such strict regulation and testing.

Susan