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A Brief Sketch of the Brain
What's the scale of things here?
The following lengths (from POSNER p. 305) give approximate sizes for structures
in the nervous system:
0.001 mm: synapses (tip of a connection between neurons)
0.1 mm: neurons (brain cell)
1 mm: local circuits (small networks of cells)
10 mm: maps (spatially organized topographic maps)
100 mm: systems (e.g., the visual system)
1000 mm: the central nervous system (including spinal cord)
How many things are we talking about?
Short answer: a LOT.
Long answer: Per cubic millimeter (mm^3), there are about 10^5 neurons
and 10^9 synapses. For you computer types, that's about 2^16 neurons and
2^30 synapses. It's estimated that there are around 10^12 (or 2^40) neurons
and 10^15 (or 2^50) synapses in the nervous system. A typical brain cell
receives inputs from thousands of other cells, and the influence of each
connection is 1-5% of threshold -- that is, only 1-5% of what the cell
needs to respond.
How fast does the brain work?
Not very fast by computer standards. An action potential (i.e., nerve impulse)
lasts about 1 ms (millisecond). Axons, the long output connection from
a cell, come in two basic types: myelinated and unmyelinated. Myelinated
axons have an extra layer of "insulation," which allows the action potential
to travel about 10 to 100 meters per second. Unmyelinated axons are slower,
transmitting at only about 1 meter per second. When the signal reaches
the end of the line, it has to cross the synapse to influence the next
cell; this process takes about 5 ms. The effect can last from a millisecond
to many minutes, depending mostly on the type of the synapse. [Posner p.
339-340]
How are memories stored in the brain?
This is one of the great questions of neuroscience, and research has nearly
converged on an answer. Short-term memories -- those which last a few minutes
or hours -- may be stored in a variety of ways, including protein activation
and inactivation within neurons, or simply cycles of neural activity. More
important to uploading is the issue of long-term memory. It appears that
long-term memories are stored by structural changes in neural processes.
These changes include the number of branches a neural process makes and
the number and efficacy of synapses. BYRNE ET AL present an excellent overview
of recent work documenting these changes in Aplysia
(a marine mollusc). There is currently no reason to believe that memories
in humans operate by a different mechanism. [It may also be speculated
that these same morphological characteristics are responsible for other
aspects of personality, though this has yet to be demonstrated.]
OK, so neurons and connections, is that it?
Well, there's more in the brain than that. There are also various forms
of "glia" cells (meaning "glue"), which serve mostly support functions.
The most important of these, from a processing standpoint, are the astrocytes;
astrocytes may affect information processing by regulating the neuronal
microenvironment, by slow signaling mechanisms (e.g., calcium waves), or
by other poorly understood processes. Also, there are all sorts of hormones
and other chemical interactions in the brain which definitely do have an
effect on the mind; these chemical interactions would need to be simulated
for the upload to be accurate.
More Brain
Facts and Figures are available from the University
of Washington.
brainfacts.html . . . . . . . . 2/20/98 . . . . . . .
Joe Strout
BREMONT@WANADOO.FR
