[Neuroscience] confusion about cerecellar circuity

Peter H. via neur-sci%40net.bio.net (by pjwholm from yahoo.es)
Tue Feb 23 08:29:37 EST 2010

To begin with, I would like to mention that as a default I assume the
confusion to be on my side. But:

My favourite neuroanatomy book (*1) states about the layered structure
of the cerebellar cortex that "These layers are, passing from
superficial to deep, the cell-poor molecular layer, the monolayered
Purkinje layer and the granular layer.". And I have never seen any
other neuroanatomy book stating or drawing anything different.

But as everybody here can see at http://brainmaps.org , the Purkinje
cells of fish, birds and mammals are embedded in the external margin
of the molecular layer, i.e. they are situated in between the
molecular layer and the surface of the cerebellum. Between the
Purkinje layer and the cerebellar surface you can see two types of
cells, the bigger ones supposedly Granular and the smaller ones
supposedly Glia.

As a consequence, the molecular layer is interposed in between the
Purkinje cells and the deep nuclei of the cerebellum (dentatus,
interpositus and fastigii). In fact, in some places at the posterior
(or caudal) margin of the cerebellar cortex the molecular layer
becomes practically continuous on its inner side with the deep nuclei.
At such places, the somata of deep nucleus neurons can be seen to
approach the molecular layer on its inner side to the same distance,
as have the Purkinje cells furthest from it on its outer side, i.e.
next to none (*2). But no such continuity can be detected in such
areas between the Granular cell layer and the deep nuclei.

And all this naturally leads me to the following two questions, i.e.:
How do the axons of the Purkinje cells reach the deep nuclei of the
cerebellum? Do they perhaps penetrate the molecular layer?
And how do the Mossy Fibers reach the Granule cells?

Besides that, the space in between the deep nuclei and the molecular
layer is filled with myelinated fibers, which obviously connect these
two structures. So given the above circumstances, could these fibers
be Purkinje axons and/or Mossy Fibers?

I would greatly appreciate if anybody here could clear up these
questions for me.

Thank you very much in advance

Peter Holm

*1: Nieuwenhuys R., Voogd J., van Huijzen Chr.: The Human Central
Nervous System 3rd Ed. 1988 Berlin: Springer-Verlag  p.222 top left

*2: See Macaca Mulatta, Nissl, horizontal, histo, whole-brain,
section 0205 hemisphere at bottom in http://brainmaps.org

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