[extropy-chat] A couple weird questions...

[A B]

Hi Eric,
   
  Thanks for your response. At the beginning of the thread titled "Dead Time of the Brain" (a long, long time ago), I made a back-of-the-envelope calculation which seemed to show that there exists a significant delay between the firing/discharging of *any* two neurons in the brain, arbitrarily located *anywhere* in the brain, ranging from a 3 nm separation to 6 inch separation (- it doesn't matter). Here's a quick recap of the calculation:
   
  1 Second / 1000 Firings / 100 Billion Neurons / 10^ -43 Seconds = ...
   
  ... Approximately 10^29 Planck Intervals during which not a single neuron anywhere could be firing within the human brain (based on its limited parameters). The "1000 Firings" above represents the high upper limit on the frequency of cycles/firings of the fastest of neurons within the brain.
   
  My first presentation of this was very informal and did not include any treatment of the issue of Simultaneity. That is the oversight I'm trying to address now. Here is a rough draft of my new version.
   
  When I divided by 100 Billion in the above calculation, that represented an (artificial) *perfectly* staggered time-distribution of neuronal firings - such that every neuron in the brain fired after a uniform period following the firing of some different neuron located elsewhere (arbitrarily located). A person could readily object, that this treatment ignored the "reality" that neurons in the brain are *not* perfectly staggered in their firing sequence; in other words, that at any chosen instantaneous "moment" multiple neurons would be firing in what amounted to a Simultaneity; their collective firing distributions will be so apparently random that many neurons "must" be firing at the same time. This is essentially the same objection you just raised when you wrote:
   
  "I doubt that there is any time during which there are not any neurons in the process of firing within a normally functioning human brain."
   
  And this is a very reasonable objection, however, it is ultimately incorrect. The theory of Relativity has shown that true Simultaneity does *not* exist; it is an abstracted human *illusion*. ( I know that Entanglement exists, but it is irrelevant to this topic, and  it doesn't conduct information). It's based on the limitations imposed by the speed of light. Any two physical events *cannot* be Simultaneous, they are temporally separated in proportion to the distance between them (determined by the speed of light limitation).
   
  The minimal distance I could use was the the relevant distance between two immediately adjacent brain neurons (two Somas that are bumping shoulders). I did a pretty poor job of stating my question clearly, here is what I meant to ask:
   
  What is a lower-bound estimate of the average distance between a central point in the middle of one neuronal Soma and a central point in the middle of the neighboring neuronal Soma (two Somas that are bumping shoulders)?
   
  Or, a more straightforward way of asking the same question is: What is a lower-bound estimate on the diameter of the Soma of the average neuron within the brain (ignoring peripheral neurons)?
   
  I really appreciate your help Eric, is there any chance that you or someone else can also answer this question for me so that I can finish with my calculations? It would be much appreciated.
   
  Once I have this number, I will calculate the the time necessary for light to traverse this distance. This number will most likely provide a new, lower limit on the *very minimum* duration of time span during which no neurons are firing, however, it will still be a larger than zero number, and in fact probably significantly larger than zero. In other words, this particular conclusion is still valid: There is indeed a repeating, small span of time during which not a single neuron is discharging/firing anywhere in the normal human brain.
   
  Best Wishes,
   
  Jeffrey Herrlich   
  
Eric Messick <exi at syzygy.com> wrote:
  Jeffrey Herrlich:
> What would be a very lower-bound estimate for the physical distance
> between the Soma/Main-Body of one neuron and the Soma/Main-Body of an
> adjacent neuron in the Human brain? (Doesn't matter what units are
> used)

It's not clear to me what you're asking about here. Neurons typically
only interact via synapses. Without a synapse, why does the
inter-cellular distance matter?

> What would be a very lower-bound estimate for the distance between a
> Terminal Button and the Dendrite's surface? (Again, units don't
> matter)

There are two types of synapses: electrical and chemical.

In electrical synapses, the cell membranes are directly connected by
"gap junction channels" which directly connect the cytoplasm of the
two cells. There, the distance between the cell membranes is about
3.5nm. There is no significant delay for signal propagation, and it
is usually bidirectional.

In chemical synapses, the membrane distance is 20-40nm, the signaling
delay is at least 0.3ms, and usually 1-5ms or longer.

This is from Principles of Neural Science, Fourth Edition, by Kandel,
Schwartz, and Jessell, published in 2000. Chapter 10, Overview of
Synaptic Transmission, p176.

> The reason I ask, is that I'm trying to refine my estimate of the
> time delay between neuronal discharges by incorporating a discussion
> about the illusion of Simultaneity of discharges - involving distances
> between neurons and the speed of light. Any input is greatly
> appreciated.

Action potentials (the firing of the neuron) have a duration of about
1ms, and propagate along axons at rates of 1-100 m/sec. The
refractory period between firings is a few ms. Repetitive firing
properties vary widely among different types of neurons.

I doubt that there is any time during which there are not any neurons
in the process of firing within a normally functioning human brain.

-eric
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