Q: Does AC make neurons depolarize?

Madhusudan Natarajan nutty at brain
Fri Feb 7 19:32:12 EST 1997

tob at world wrote:

: Can you settle a bet for me?
: I say the voltage reverses faster than the time it takes a neuron to
: depolarize. I also say that if that were so, we would go epileptic every

You are right in that at a very high frequency, the time for ONE cycle is 
much shorter than the time needed for sodium channels to reach threshold 
(the fastest channels, and thus the predominant timing ion for our 
discussion). Thus at one cycle, we would definitely not be able to excite 
a neuron. By the same reasoning, I would believe that an A.C will 
not excite a neuron.. BUT gut instinct tells me to bet otherwise. 
I dont know why, but for some vague reason ideas keep popping 
into my head about tetanizing stimuli, which do not allow a nerve 
to recover ; cause summation of voltage and thus depolarization.
It could be succesfully argued that the negative phase of the a.c will 
hyperpolarize the neuron to the same extent that the positive phase 
depolarizes it...BUT I'd still bet the other way.

: time we were near a radio transmitter, since electromagnetic waves of a
: few watts passing through our skulls have an instantaneous voltage that
: meets the conditions of the bet.

By the same (your) argument, going near a very large capacitor which has a 
very high voltage applied across it, will cause you brain to be in an 
unchanging electric field. Any slow movement will cause a large induced 
emf (according to faraday) in your neurons, which will be large enough to 
depolarize them. This (obviously) does not occur .. (I wonder why) So 
your final argument cannot be offered as proof that an A.C will not cause 
a neuron to depolarize.

Sorry if I am coming up with more questions than answers.

 : Who's right and why? Thanks.

:         Tom

Madhusudan Natarajan		
					..and still partly at..	
Ward 5-223 : Ph (312) 503 0202		
Dept. of Physiology			Dept. of Biomedical Engineering
Northwestern University			University of Akron

"I have no data yet. It is a capital mistake to theorize before one has 
data. Insensibly one begins to twist facts to suit theories, instead of 
theories to suit facts."    - Sherlock Holmes

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