From newshub.ccs.yorku.ca!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!rpi!uwm.edu!linac!att!news.cs.indiana.edu!arizona.edu!NSMA.AriZonA.EdU!bill Mon Dec 16 11:02:11 EST 1991
Article 2149 of comp.ai.philosophy:
Path: newshub.ccs.yorku.ca!ists!helios.physics.utoronto.ca!news-server.csri.toronto.edu!rpi!uwm.edu!linac!att!news.cs.indiana.edu!arizona.edu!NSMA.AriZonA.EdU!bill
Newsgroups: comp.ai.philosophy
Subject: Re: How does one model anesthesia in AI?
Message-ID: <1991Dec15.200552.2266@arizona.edu>
>From: bill@NSMA.AriZonA.EdU (Bill Skaggs)
Date: 15 Dec 91 20:05:51 MST
Reply-To: bill@NSMA.AriZonA.EdU (Bill Skaggs)
References: <59809@netnews.upenn.edu> <12665@pitt.UUCP> <60022@netnews.upenn.edu> <12686@pitt.UUCP> <60318@netnews.upenn.edu>
Distribution: world,local
Organization: Center for Neural Systems, Memory, and Aging
Nntp-Posting-Host: ca3.nsma.arizona.edu
Lines: 31

  A few points to clear up some neuroscience-confusion that has
been propagated here recently:

  In general neurons communicate in three ways:  1) by releasing
chemicals (neurotransmitters) that diffuse across narrow gaps and
bind to receptors; 2) by releasing chemicals (hormones and neuro-
modulators) that diffuse over broad distances and bind to receptors;
3) by direct electrical connections (gap junctions).  All of
these mechanisms are common in humans as well as lower animals.
A fourth possible way is "ephaptic transmission":  this occurs
when cells create extracellular electrical potentials that 
influence the behavior of other cells.  It is functionally
important in certain fish cells but as far as I know there is
no good evidence that it is important in mammalian brains.

  Most neurons have axons, but some do not.  There are several
kinds of axonless ("amacrine") cells in the human brain.

  Most neurons can fire action potentials, but some do not, including
some kinds in the human brain.  However, cells whose axons travel
long distances (i.e. more than a millimeter or so) *always* fire
action potentials, because graded potentials cannot propagate
very far.

  Interestingly, although gap junctions, amacrine cells, and nonspiking
cells are all common in the human brain, they are quite rare
in the cerebral cortex.  So the part of the brain that does the
most complex computations restricts itself to the simplest
and most "digital" mechanisms.  One point for connectionism!

	-- Bill


