X-Message-Number: 3890
Date: Wed, 22 Feb 1995 21:20:27 -0800
From: John K Clark <>
Subject: SCI.CRYONICS Uploading

In 3876   (Joseph J. Strout) Wrote:

	      >The voltage exists between the inside and outside of the cell,
	      >across the membrane.  It spreads by the movement of             
	      >electrons, as in any conductor. 
	       
In a conductor like a metal, current is caused by the movement
of electrons but neurons are not made of metal. Free electrons
can not exist in an aqueous solution, especially one that has a
lot of chlorine in it as neurons do. A chlorine atom would grab
a free electron and become an Cl- ion so fast it makes my head
spin. A chlorine ion is  65 thousand times as heavy as an
electron, a potassium ion is even heavier and that makes them slow.  
	       
	     >You accurately described the propagation of the action
	     >potential, but the spread and integration of PSPs is different  
	     >because it does not depend on voltage-gated ion channels. 
	     
That's true, but  I'm sure you know there are 2 types of
channels, voltage gated ion channels and ligand gated ion
channels. It's the ligand gated ion channels that change the
membrane potential by opening up and allowing ions to cross the
membrane when a  neurotransmitter from a presynaptic neuron
binds to it. Ligand gates work well with PSP because they are
transducers, the more of the external neurotransmitter the
stronger the ion current, but they don't work at light speed and
they deal in ions not electrons.

The only other thing I can figure your talking about is the
cabal properties of dendrites, that is the passive spread of
electrical signals with no amplification from voltage gated
channels. Trouble is dendrites make terrible cables, their
resistance is high but much worse their electrical capacitance
is very high so the signal is slow even by biological standards.
Because the membrane has such a high capacitance it takes a long
time for membrane potential to build up from ion current flow.
The high capacitance also distorts the signal, turning a sharp
spike into a gentle rise and fall after traveling only a
millimeter or two.  

			>These signals are short-range but (lightspeed) fast.
		
As you point out, in the cell body the grand postsynaptic
potential is the spatial integration of all the excitations, but
the signal ( at least the one that is passed on by the axon
hillock through the axon for long distances) is also the result
of a TEMPORAL integration of the excitations. The size of the
net PSP depends on the rate of firing of the incoming
excitations so can't be light speed fast. The bottom line is
that signals can not be processed and transmitted from the
synapses to the axon hillock at anything close to light speed
and could benefit from a speed up as much as the action
potential would.
		
More generally, I don't understand why you think timing problems
represents an insurmountable obstacle in speeding up the brain.
Computer designers come across this problem constantly and have
always been able to find clever solutions like catching. If
information arrives before another part is ready for it just
store it until needed, that way all the parts are kept busy and
time is not wasted.
		
				  John K Clark          

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