Current flow in neurons (was Re: Auditory Impulse Travel and Distance)

Bruce Raoul Parnas brp at dino.berkeley.edu
Mon Jun 24 12:54:06 EST 1991


In article <13584 at uhccux.uhcc.Hawaii.Edu> bjones at uhunix1.uhcc.hawaii.edu (Brad Jones) writes:
>In article <SLEHAR.91Jun18084520 at park.bu.edu> slehar at park.bu.edu (Steve
>voltage change than sites near the current injection.  The reason this
>is important is that non-spiking neurons are able to conduct electrical
>signals much faster than spiking ones (but not as far).  Conduction
>velocity in non-spiking neurons is near-instantaneous.  Conduction
>velocity in spiking neurons depends on the rise time of the voltage
>change at the sodium channels, and the sodium channel density and
>distribution.


In the strictly mathematical sense, this conduction is nearly instantaneous-
any set of equations used to model this system will immediately show a
non-zero response at all points in the cell.  However, is this practically
useful?  While the potential at some distant point will "instantaneously"
be several femtovolts, a system which works on millivolt signals will not
detect this.  In fact, it will be far smaller than intrinsic noise, which
sets a lower bound on detectability.  The voltage at these points must rise
above some value before it may be detected, and this takes time.  Conduction
velocity should refer to the conduction of signals, and not to femtovolts.
In this sense, the regenerative phase of spiking neurons makes them conduct
SIGNALS faster than their non-spiking counterparts which (as was mentioned in
Brad's message) are at the mercy of membrane capacitance.  So, it seems to
me that it all really depends on how we decide to define conduction in nerves.

>
>    Brad Jones -- bjones at uhunix.uhcc.hawaii.edu - bjones at uhunix.bitnet


bruce
(brp at bandit.berkeley.edu)



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