[ExI] Neuroscience Question

[Jeff Miller]

> --- A B <austriaaugust at yahoo.com> wrote:
> > I have a geeky question. Do the neurons of different
> > major reasons of the brain differ significantly in
> > physical structure?


As I understand it, cell types vary significantly among the fundamental
brain regions (cortex, cerebellum, thalamus, hippocampus, brainstem, etc).

HOWEVER, the cerebral cortex (aka isocortex, aka neocortex), consists of
the same neuron types, structured in a similar way, throughout. (This is
the inspiration for one of its names, "isocortex".) That is the leading
theory, at least --- with the exception of one neuron type (see below).

The isocortex constitutes approximately 85% of the human brain (by mass),
and is responsible for a large part of its advanced activities, including
sensory processing (visual, auditory, somatosensory), motor control,
language understanding and generation, logic, math, and spatial
visualization.

I'm not a neuroscientist, but I play one in
_Yudkowski Returns: The Rise And Fall And Rise Again of Dr. Eliezer
Yudkowski_.    ;)

(Actually I'm an "AGI" researcher with a keen interest in modeling the brain.)


Are you familiar with the work of Vernon Mountcastle? He pioneered this
theory in:

"An Organizing Principle for Cerebral Function: The Unit Model and the
Distributed System", The Mindful Brain (Gerald M. Edelman and Vernon B.
rountcastle, eds.) (1978) Cambridge, MA, MIT Press.

This excellent paper of his is available online:
"The columnar organization of the neocortex", VB Mountcastle, 1997
http://brain.oxfordjournals.org/cgi/content/abstract/120/4/701

Vernon Mountcastle
http://en.wikipedia.org/wiki/Vernon_Mountcastle



The exceptional neuron type is called a "spindle" neuron. I will quote
briefly from Koch's "The Quest for Consciousness":

"... In fact, small and large excitatory pyramidal neurons and spiny
stellate cells, as well as inhibitory basket, nonspiny stellate cells,
double bouquet neurons, and other members of the diverse menagerie of
inhibitory neurons, are found in all mammals.

The sole exception, so far, are spindle neurons, a class of giant cells
restricted to two neocortical regions in the frontal lobe. Found in high
densities in humans, they are much sparser in the great apes and
altogether absent in monkeys, cats, and rodents. A few tantalizing hints
point toward their possible involvement in self monitoring and self
awareness.

...

Spindle neurons, the Korkzieher cells of von Economo and Koskinas (1925),
are characterized by elongated and large cell bodies in the lower part of
layer 5, the output layer of the cortex (Nimchinsky et al., 1999). Absent
in newborn infants, their numbers stabilize in adults at about 40,000
neurons in the anterior cingulate cortex and 100,000 or so in FI, another
frontal area. These regions are involved in self-evaluation, monitoring,
and attentional control."




Jeff Miller
"The geek shall inherit the Earth."