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

Bradley R. Jones bjones at uhunix1.uhcc.Hawaii.Edu
Sat Jun 22 03:36:36 EST 1991

In article <SLEHAR.91Jun18084520 at park.bu.edu> slehar at park.bu.edu (Steve
Lehar) writes:
>The  reason why  the brain  uses  neural spiking, and  encodes  signal
>magnitude as spiking frequency  is exactly  to avoid   the degredation
>with distance that is experienced by the  alternative method of neural
>signaling, i.e. the density of ions of a particular charge.
>The ions, injected at the site of  neural input must diffuse passively
>along the neuron, which works ok as long as they don't have to diffuse
>too far.   When you get one  of  those neurons  with an extremely long
>axon  however, there may be  little or no charge  left by the time the
>signal gets to the end, so the signal decays with distance.

While correct in a general sense, this description is not technically
accurate.  Passive electrical conduction in neurons does not occur at
the diffusion rate of the ions in the cell.  Electrical conduction in
neurons occurs just as in wires: at the speed of light.  The charged
partical doesn't have to move the entire distance of current flow, it
bumps a neighboring ion which bumps another and so on.  The reason
there is spatial decrement of electrical signals in neurons is because
current leaks out across the membrane capacitor as the current flows
along the cell.

>again   to recover.   That gulp of  ions   diffuses outward,  and what
>happens next  depends critically on the  density of electrically gated
>channels in  the local viscinity.   If the next  one is too  far away,
>then the  charge will  not be strong  enough  to   trigger it, and the
>charge diffuses slowly in space and  time.
 ^^^^^^ ^^^^^^^^ ^^^^^^ ^^ ^^^^^ ^^^  ^^^^
As I said, the charge travels at the speed of light.  The amplitude
decrement is due to the current loss at the membrane capacitance.  The
temporal slowing is also due to the distributed capacitance.  But,
_and_this_is_important_, when current is injected at a point in a
neuron the initial deflection will be measured at all points of the
cell nearly instantaneously.  The _rise_time_ is what is slowed by the
membrane capactitance, so more distant sites show a more slowly rising
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

    Brad Jones -- bjones at uhunix.uhcc.hawaii.edu - bjones at uhunix.bitnet
    Bekesy Laboratory of Neurobiology, Pacific Biomedical Research Center
    University of Hawaii, Honolulu, HI  96822

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