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

Brad Jones bjones at uhunix1.uhcc.hawaii.edu
Tue Jun 25 08:32:23 EST 1991

In article <1991Jun24.175406.24957 at agate.berkeley.edu> brp at dino.berkeley.edu (Bruce Raoul Parnas) writes:
>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
....[section deleted]...
>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.

Yes, this is a good point.  _Information_ transfer in a non-spiking
neuron is not necessarily faster than it would be in a spiking neuron.
Many variables affect how fast the input signal reaches a level
sufficient to release transmitter at a distant site.  In order to
maximize the length constant, a neuron may have a very high membrane
resistance which will increase the time constant and lead to a slower
rise time of the passive signal at a distant site.  Still, for a short
neuron having a membrane potential near or above the activation
threshold for Ca channels in the presynaptic membrane (e.g. retinal
bipolar cells), information transfer must be faster without spikes
than it would be with them.  This is because there is no inter-spike
refractory period associated with the passive conduction.

Also, the speed of passive conduction is very important in myelinated
nerve where discrete spiking nodes are separated by non-spiking regions
a millimeter or so in length.  Of course the specialization that makes
this useful is the decreased membrane capacitance and increased
membrane resistance afforded by the Schwann cell wrapping.

    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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