X-Message-Number: 3876
From:  (Joseph J. Strout)
Subject: SCI.CRYONICS Re: Uploading
Date: Mon, 20 Feb 1995 09:57:48 -0800 (PST)


John Clark wrote:

> In the spine the axon on a neuron can be almost 3 feet long, the
> biggest cell in the body. Are you saying a nerve signal travels
> those 3 feet at the speed of light, or are you saying nothing
> can send a signal over those 3 feet faster than 300 miles per
> hour, the top speed a neuron is capable of? It really doesn't
> matter because both statements are untrue.

My posts must be unusually opaque; I'll try again.  There are SEVERAL
kinds of signals in the nervous system.  Action potentials, which
carry signals down long axons, travel slowly.  Membrane potentials
carry signals among the dendrites and cell body, and spread at
lightspeed.

> Just because something uses voltage doesn't mean it can send
> messages at light speed. A constant voltage can generate a
> static electric field but a static field of any kind can't
> convey information, you need to vary the voltage

Yes, it varies in response to a variety of factors; the most obvious
come from synapses, and we call this response a post-synaptic
potential (PSP).

> , also voltage
> doesn't move through things it exists between things. Current
> (amps) is what moves through things.

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.  The ion currents are responsible for maintaining (and
changing) the voltage locally, but the changes spread to other areas
of the cell as in any conductor.  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.  These signals are short-range but (lightspeed) fast.

The idea that action potentials are the only information-carrying
signals in the brain is a sadly common one.  A couple basic textbooks
might prove enlightening:

Shepherd, G.  Neurobiology (2nd Ed.).  Oxford University Press, 1988
    (esp. Chapters 5-7).

Kandel, E., Schwartz, J., & Jessel, T.  Principles of Neural Science
    (3rd Ed.).  Appleton & Lange, 1991 (esp. Chapter 6).

,------------------------------------------------------------------.
|    Joseph J. Strout           Department of Neuroscience, UCSD   |
|               http://sdcc3.ucsd.edu/~jstrout/    |
`------------------------------------------------------------------'



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