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

[Eric Messick]

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