Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!uflorida!gatech!emory!boothe
From: boothe@mathcs.emory.edu (Ronald Boothe {guest})
Newsgroups: comp.ai.neural-nets
Subject: Re: wanted: neurobiology references
Message-ID: <3971@emory.mathcs.emory.edu>
Date: 23 Apr 89 14:17:12 GMT
References: <4486@psuvax1.cs.psu.edu> <17450@cup.portal.com> <17490@cup.portal.com>
Reply-To: boothe@emory.UUCP (Ronald Boothe {guest})
Organization: Emory University
Lines: 36

In article <17490@cup.portal.com> mmm@cup.portal.com (Mark Robert Thorson) writes:
>> I would expect synaptic weights to be proportional to the axon circumference
>> where it joins the cell body, but I have no evidence to support that belief.
>
>Opps, I meant "dendrite circumference", of course.  And now that I think
>about it, that's wrong too.  I was taught that there are two kinds of 
>conduction in nerves cells, "electrotonic" and "propagative".  The former
>might be described as an electrolytic and resistive form of conduction,
>while the latter involves action potentials originating in the axon hillock.
>
For most neurons in brain you can probably ignore propagative conduction by
dendrites and just consider the effects of electrotonic conduction.  This
conduction will be dissipated by the space constant of the cell membrane and
therefore the input of each synaptic input needs to be weighted by its 
distance from the axon hillock.  In addition, many dendrites have branches
and varicosities which can alter the resistance to current flow along the
dendrite, so the geometry of the dendrites also must be taken into account.
Finally, a majority of excitatory synapses onto dendrites contact specialized
anatomical structures called spines. These spines are shaped like mushrooms
with the thin stalk projecting from the dendrite, and the synaptic input 
coming onto the head of the spine.  This long thin stalk provides resistance
to current flow, so the weight of each synaptic input is also influenced by the
length and diameter of the stalk (some think a good biological mechanism for 
altering the weights of specific inputs is to change the shapes of the spines).

There is lots of recent work on this topic in the neuroscience literature.
I don't recall specific references right now, but some of the influentual
early work was done by W. Rall.  A check of the citation index to see who
is making reference to the old Rall papers should turn up current literature.


-- 
Ronald Boothe {guest}
Emory University, Atlanta, GA 
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