From newshub.ccs.yorku.ca!ists!helios.physics.utoronto.ca!news-server.ecf!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!think.com!yale.edu!jvnc.net!netnews.upenn.edu!libra.wistar.upenn.edu Thu Dec 26 23:58:36 EST 1991
Article 2408 of comp.ai.philosophy:
Path: newshub.ccs.yorku.ca!ists!helios.physics.utoronto.ca!news-server.ecf!news-server.csri.toronto.edu!bonnie.concordia.ca!uunet!think.com!yale.edu!jvnc.net!netnews.upenn.edu!libra.wistar.upenn.edu
>From: weemba@libra.wistar.upenn.edu (Matthew P Wiener)
Newsgroups: comp.ai.philosophy
Subject: Re: Ignore QM and be happy
Message-ID: <61072@netnews.upenn.edu>
Date: 25 Dec 91 23:15:19 GMT
References: <1991Dec24.054745.16805@bronze.ucs.indiana.edu> <61056@netnews.upenn.edu> <1991Dec25.061455.29709@news.media.mit.edu>
Sender: news@netnews.upenn.edu
Reply-To: weemba@libra.wistar.upenn.edu (Matthew P Wiener)
Organization: The Wistar Institute of Anatomy and Biology
Lines: 33
Supersedes: <61067@netnews.upenn.edu>
Nntp-Posting-Host: libra.wistar.upenn.edu
Nntp-Posting-Host: libra.wistar.upenn.edu
In-reply-to: minsky@media.mit.edu (Marvin Minsky)

In article <1991Dec25.061455.29709@news.media.mit.edu>, minsky@media (Marvin Minsky) writes:
>>For at the picolevel, there is never such a notion as releasing
>>neurotransmitter "at just the right times".  The work of Katz in the
>>50s on the probabilistic release of acetylcholine vesicles is
>>textbook knowledge by now; I cited Eccles on how this accords with a
>>quantum mechanical description.

>I must have missed this.  Can you review how Eccles shows that QM
>effects are important, say, as compared to thermal effects, for
>anything so large as a vesicle?

It's in his FACING REALITY, right now unavailable to me.  But the
following is an easy calculation.  (So easy that I only had to redo
this message once.)

	presynaptic membrane width:	5e-9  m
	synapse emission time:		1e-3  s
	neurotransmitter vesicle mass:	3e-17 g

So a vesicle's worth of released neurotransmitter has momentum roughly
3e-17*5e-9/1e-3=1.5e-22 gm/s.  Using this figure for order of magnitude
in momentum uncertainty, we use the uncertainty relations to identify a
positional uncertainty (6.6e-31 gmm/s)/(1.5e-22gm/s)=4e-9 m, about the
width of the presynaptic membrane.

The counterintuitiveness of quantum effects for such a large object has
an obvious explanation.  The synapse transition is unusually fast, about
a thousand times faster than, say, vesicle transport down the axon.  If
I remember correctly, that slower speed is roughly the same as cellular
component thermal diffusion.  Somebody fix the slug in me...in an ideal
gas, 300K vesicles would have thermal velocites ~~ 1m/s.
-- 
-Matthew P Wiener (weemba@libra.wistar.upenn.edu)


