From: Martin Striz (email@example.com)
Date: Tue Feb 01 2005 - 22:07:43 MST
--- Phil Goetz <firstname.lastname@example.org> wrote:
> --- Martin Striz <email@example.com> wrote:
> > --- Phil Goetz <firstname.lastname@example.org> wrote:
> > > Neurons use redundancy to tolerate noise. In
> > > exchange, they can use much, much lower voltage
> > levels
> > > (by about 5 orders of magnitude) than integrated
> > > circuits do, because they don't need to be
> > completely
> > > reliable. This may lead to lower overall power
> > > requirements for a computation. You could work
> > out
> > > the numbers for known systems, but it wouldn't be
> > > easy.
> > When you say 5 orders of magnitude, are you taking
> > into account the distance?
> > A 100 mV difference across a 5 nm membrane
> > translates into about 100,000 V per
> > centimeter (Cf. MBoC, Alberts et al.). There's
> > actually a lot of electrical
> > potential energy in that noggin.
> > Martin
> I hadn't thought of that. Would you measure the
> distance across the membrane, or the distance
> that the signal travels down the neuron?
Let's define our terms to be clear. Voltage is a measure of electric potential
energy -- I think most people confuse it with current. It has units of Joules
per Coulomb, or energy per unit charge. A solution of ions will mix evenly,
but if the charges are separated (in this case Na+ and K+ ions, and to a lesser
degree Cl- and Ca++), electric potential energy is created (which can be used
to do work). So it's the distance between Na+ and K+ ions, which is the 5 nm
cell membrane. Most people imagine the ions floating around randomly, waiting
for to be discharged (switch sides to regain equilibrium), but due to the
charges they hug each side of the membrane closely, so this is the only
distance that's significant.
A circuit, at least in the simple cases that I've studied, might have a battery
that creates a voltage potential, starting from the negative terminal, all the
way to the positive terminal (these are the two charges in this system), so the
entire distance around is counted. The equation used to quatitate the voltage
is V=kQq/r, and the only important thing to take out of that is that the
voltage drops off proportionally with distance (r). I don't know how big
circuits are in microchips, but I trust they are much bigger than 5 nm.
> The human brain runs on about 75W, which is less
> than a desktop PC. Its computing power is believed
> to be orders of magnitude greater than the power
> of that desktop PC. So the brain still comes out
> on top.
Watts a measure of power, or energy per unit time. So yeah, the brain uses
less energy per second than a computer. That's probably because the "circuits"
are so small. This is probably a better way to compare energy demand between
computers and brains.
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