Leslie Kay lmk2 at garnet.berkeley.edu
Sat Apr 23 17:02:35 EST 1994

In article <2p6msn$ohk at portal.gmu.edu>,
HARRY R. ERWIN <herwin at mason1.gmu.edu> wrote:
>Has anyone considered how neocortex evolved? I've heard that it probably
>came from paleocortex, but paleocortex is characterized by afferent
>sensory input arriving at Layer 1, while neocortex receives its input
>via the thalamus in layer 4. The pyramidal cells appear to be organized
>similarly in both, but neocortex has 6 layers while paleo has 3. If neo
>came from paleo, how did it lose its layer 1 inputs? Has anyone tried to

I have in front of me "Anatomic Organization and Physiology of the Limbic
Cortex", Lopes da Silva, Witter, Boeijinga, and Lohman (1990) Physiological
Reviews 70:453-511.

On the second page they give a nice heirarchy of the laminar cortices, 
citing three major subdivisions: isocortex (or neocortex), mesocortex,
and allocortex.  Allocortex encompasses paleocortex and archicortex 
(hippocampus).  Mesocortex encompasses all the "transition" areas like
entorhinal, and is further divided into proisocortex (bordering isocortex),
and periallocortex (bordering allocortex).  Periallocortex is further
subdivided into peripaleocortex and periarchicortex.  Entorhinal cortex
is listed as periarchicortex, but one can also see that it is also peripaleo-
cortex and perhaps even proisocortex.  Entorhinal cortex (EC) receives input in both deep and superficial
layers (deep being the pyramidal cells and superficial being the setellate
cells (oops, stellate cells).  Its laminar structure is more clear the
further one gets from the pyriform cortex (paleocortex) and thus closer
to the archicortex and nearby neocortical areas.  It is a kind of vestigial
paleocortex (the superficial layers) separated from its less clearly
laminated deep layers by the lamina dissecans, which is a kind of seventh
layer consisting mostly of dendrites of deep lying pyramidal cells but
also some scattered interneurons.  In addition, the EC receives thalamic
input to superficial layers (from nucleus reuniens), and it appears from the
diagram, also to the deep layers, probably disynaptically via the subiculum.
(Witter, et al (1989) Functional Organization of the Extrinsic and
Intrinsic circuitry of the parahippocampal region, Progress in Neurobiology
33:161-253).  It should be noted also that the EC does not project to
the thalamus.

This won't explain how the neocortex "lost" its layer I inputs, but it
does shed some light on the continuity of lamination from paleo and
archicortex to neocortex, and by extension the development of thalamic
input into both superficial and deep layers in the EC.  Since the EC
is a kind of integration cortex, it may require the dual inputs (to
both superficial and deep layers) that neocortex does not.

Leslie Kay
lmk2 at garnet.berkeley.edu

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