You are probably in the right range for "elementary events" in
the brain within several orders of magnitude at least. That is,
counting as you do either ion channel or transistor gate switches.
And it is channel switching that is the important factor because
it ultimately is responsible for both the active action potential
and the graded events that occur in so many local cells and
circuits.
Ultimately the limit would be set by two factors. First, each
significant event must be sufficiently large in energy to exceed
the random thermal fluctuation for that physical system. Second,
the total energy dissipation must be low enough to allow thermal
dissipation without overheating. The same two factors would apply
to the biological and the electronic system.
The real question (as the alternative Richard points out) would
have to take into account the information content of all that energy
consumption. I would guess that your 1 GHz Pentium-III undergoes
the same number of gate switches whether it is calculating an
incredibly complex algorithm, downloading dirty pictures from
the internet, or just sitting in a loop executing NOPs. The brain has
the same considerations -- as was discussed in a previous post.
The metabolic rate of the brain (as a whole) doesn't really change
much no matter how "deep" your thoughts.
"Iain McClatchie" <iain at 10xinc.com> wrote in message
news:399F9568.D5F86B0A at 10xinc.com...
> Richard,
>> Thanks for the reply. Let me clarify a little more of what I was
> attempting to get at.
>> My understanding is that a "firing" of a neuron causes an action
> potential to propagate down the axon. I think I misused the term
> "action potential". Perhaps you could tell me the right term for
> the event which is a neuron triggering and propagating an AP down
> the axon. I'll call it a "firing" here.
>> Anyway, it seems I ought to be able, at least in principle, to count
> the number of neuron "firings" per second in the entire human brain.
> I'd like to know this number because I'd like to compare it to the
> bandwidths at various points inside a computer. Here, I'll give you
> the other number: a 1 GHz Pentium-III has about 10^15 transistor gate
> voltage changes per second (30 watts, 1 GHz, operating at 2 volts,
> and the average transistor is about 5 or 10 fF). My guess is that
> this number is about the same as the number of times, per second,
> that a gated ion channel turns on or off in the human brain. My
> guess is that there are maybe 10^11 neuron firings per second.
>> I thought perhaps I could get to this number by using an energy
> consumption argument. The voltages and currents and resistances in
> the membrane might be changing, but in the end, if sufficient charge
> (sodium and potassium ions) crosses a capacitor (the cell membrane)
> to change the field across that capacitor from one voltage to
> another, that dissipates an amount of energy that I can calculate --
> I do it all the time when designing CMOS.
>> I think the chemical work of ions moving down gradients is exactly
> the electrical work of charge moving across a capacitor. And to
> provide the energy for that work, the brain burns ATP->ADP to pump
> the ions back up these gradients to recharge the capacitor for the
> next firing. When you point out the other work the brain does, in
> processing and reprocessing the synaptic transmitters, you're right,
> and I didn't know how to account for that except with a fudge factor.
>> You point out that there is a lot of variation in neuron surface
> area, and in the current density on that surface, so maybe that's
> the wrong way to chase down the number I'm looking for.
>> You also point out that gated ion channels typically see currents
> of a few pA, which would lead to energy dissipation of a few fJ per
> action potential per gated ion channel. And assuming that this
> energy dissipation is 10% of the brain's power dissipation, that
> gives about 10^15 of these events per second for a 25 watt brain.
>> Right?
>> -Iain McClatchie 650-364-0520 voice
>http://www.10xinc.com 650-364-0530 FAX
>iain at 10xinc.com 650-906-8832 cell
