"Gee Wiz!" Facts About the Brain
It is tempting to think that the main principles of neural development will have been
discovered by the end of the century and that the cellular and molecular basis of the
mind will be the challenge for the next. An alternative view is that this feeling
that understanding is just a few steps away is a recurring and necessary delusion that
keeps scientists from dwelling on the extent of the complexity they face and how much
more remains to be discovered.
- Martin Raff
- The cortex has 15 million neurons per sq. cm, or 146000 per sq.
mm. This holds throughout the brain, regardless of cortical
thickness, except in vision areas, where there is 2 1/2 times
this number. [C 51]
- A typical neuron may have one to ten thousand input connections,
but may have as many as 200 thousand. [A 40]
- A typical neuron may make 1000 connections to other neurons.
- There are approximately 100 billion (i.e. 1011) synapses per sq.
cm, or 10 million per sq. mm. This estimate is based on a
nominal 7000 synapses per neuron.
- The area of the cortex is 2200 sq. cm = 0.22 sq. m.
The neocortex is 700 sq. cm. [C 45]
- Therefore the human cortex contains approximately 30 billion
(3 X 1010) neurons. [C 51]
Other estimates run from 10 billion to 100 billion
(1010 - 1011) neurons and
1014 to 1015 synapses. [A 5]
- The chimp and gorilla have about 500 sq. cm of cortex and
therefore 7-8 billion neurons. [C 51]
- A rat has 4-5 sq. cm of cortex, and therefore about 65 million
neurons. [C 51]
- There are approximately 100 million receptor cells in the retina.
They feed into approximately one million ganglion cells (also in
- After age 40, about 1000 neurons die per day.
(I seem to have forgotten the source of this...)
- Typical spikes (action potentials) are 1 - 10 msec. long.
- Maximum spike rate is several hundred per second.
- It takes at least N msec. to distinguish N values by rate coding
(due to the Gabor uncertainty principle).
Therefore, in 100 msec. a value can be transmitted with 1-2 digits
of precision. [M 166]
- The synaptic delay is about 1/2 msec. [A]
- A typical postsynaptic potential has a rise time
of 1 - 2 msec. and a decay time of 3 - 5 msec.
Much longer decay times occur, tens to hundreds of msec. [A 39-40]
- The membrane time constant is typically 1 - 2 msec. [A 30]
- The membrane length constant is typically 2 - 5 mm. [A 30]
- A typical mental rotation rate is 450 degrees per second.
- [A] Anderson, An Introduction to Neural Networks.
- [C] Changeaux, Neuronal Man.
- [G] Gazzaniga, The Cognitive Neurosciences.
- [M] MacLennan, "Information processing in the Dendritic Net,"
in Pribram, Rethinking Neural Networks, 161-197.
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Wed Jan 12 17:11:31 EST 2000