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From: brp@bandit.berkeley.edu (Bruce Raoul Parnas)
Newsgroups: bionet.neuroscience
Subject: Re: Current flow in neurons (was Re: Auditory Impulse Travel and Distance)
Message-ID: <1991Jun28.120554.6634@agate.berkeley.edu>
Date: 28 Jun 91 12:05:54 GMT
References: <13584@uhccux.uhcc.Hawaii.Edu> <1991Jun24.175406.24957@agate.berkeley.edu> <13628@uhccux.uhcc.Hawaii.Edu>
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In article <13628@uhccux.uhcc.Hawaii.Edu> bjones@uhunix1.uhcc.hawaii.edu (Brad Jones) writes:

>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.

The information that gets lost in this scheme is timing.  The result of a
continuous modulation of Ca channels is a smearing of the temporal information
present at the soma as the signal traverses the axon.  In some systems this is
ok, but in sensory systems this won't work.  Spikes provide a means for having
precise synchrony to temporal events (not single neurons, of course, but the
ensemble response of neural populations).  Again, it all depends on the
"information" that one wishes to transfer.  If time is relevant, spikes are
very important.  Many neural network models forego spikes in favor of analog
(continuous) signals that represent something like mean spike rate.  In this
scheme the temporal information is lost and this is, i believe, simply not an
adequate model for neurons in sensory systems.  Long live spikes!

>    Brad Jones -- bjones@uhunix.uhcc.hawaii.edu - bjones@uhunix.bitnet
bruce
(brp@bandit.berkeley.edu)