white surface of the brain

Richard Norman rsnorman at mediaone.net
Thu Sep 6 08:17:51 EST 2001

On Thu, 06 Sep 2001 12:04:30 GMT, remove_this!helbrecht at gmx.net
("Wolfram") wrote:

>Hello everybody,
>someone told me, neurons in the white surface of the brain
>operates like bridges, transmitting signals from A to B about
>three times faster, than "normal" grey neurons. Is that true?

"operates like bridges" sounds like you are still trying to find
the "job" of neurons??

The vertebrate CNS is divided into two components, grey and
white.  However, the surface of the brain (do you mean
cerebral cortex?) is grey matter, not white.  Grey matter consists
of the neuronal cell bodies, the dendrites, and the synapses (plus
all the local axons) while the white matter is bundles of axons.  The
myelination on the larger axons gives this material its whitish color
and sheen.   But white vs grey neurons?  Where did you hear that.

The speed of conduction of action potentials along an axon depends
on the axon diameter.  For myelinated cells, the speed is roughly
directly proportional to diameter. For unmyelinated cells it is
roughly proportional to the square root of diameter.  So large neurons
conduct faster than small neurons and myelinated large neurons
conduct very much faster than small unmyelinated ones.

But "operates like a bridge" sounds suspicious.  Perhaps this is a
reference to those cells that act a s "projection neurons".  See
Shepherd,, Neurobiology, for a discussion of microcircuits, local
circuits, and projection circuits.

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