282 Neuroanatomy: Draw It to Know It
Cerebral Cytoarchitecture ( Advanced )
Here, we will draw the cytoarchitecture of the cerebral
cortex. First, indicate that three diff erent histologic pat-
terns of cerebral cortex exist: neocortex, allocortex, and
mesocortex. Neocortex (aka isocortex) is the phyloge-
netically newest cortical cytoarchitecture; it constitutes
roughly 90 % of the cerebral cortex and comprises six his-
tologically distinct layers. Allocortex is phylogenetically
older and comprises from three to fi ve layers (the actual
number is inconsistently defi ned). Allocortex subdivides
into paleocortex and archicortex. Th e most prominent
example of paleocortex is the olfactory cortex and the
most prominent example of archicortex is the hippocam-
pus. Mesocortex represents transitional cortex between
neocortex and allocortex and it subdivides into perial-
locortex and proisocortex. Periallocortex further subdi-
vides into peripaleocortex and periarchicortex.
Now, in order to learn the six histologic layers of the
neocortex, let’s discuss the neocortical cell populations,
themselves. Th e cerebral cortex contains roughly 50 bil-
lion neurons, which are commonly divided into pyrami-
dal and non-pyramidal cells. Indicate that pyramidal
cells are 10 to 80 micrometers ( μ m) in diameter and are
shaped like a fl ask with a single thick, cortically oriented
apical dendrite and multiple basal dendrites. Indicate
that one important pyramidal cell type is the Betz cell;
by at least one commonly held defi nition, Betz cells lie
primarily within the primary motor cortex but also
extend, to a lesser extent, into the premotor cortex—
note that select authors instead defi ne Betz cells as
being solely confi ned to the primary motor cortex. Th en,
indicate that the non-pyramidal cells are much smaller:
they are 5 to 15 μ m in diameter. Th e most notable
non-pyramidal cell type is the stellate (or granule) cell.
Other non-pyramidal cell types include the basket,
fusiform, horizontal, Martinotti, and neurogliaform
cells.^15 – 17
Next, let’s label the six cytoarchitectural layers of the
neocortex from outside to inside. Label the outermost
cytoarchitectural layer as layer I, the molecular (or plexi-
form) layer; indicate that it is primarily a nerve fi ber
layer, meaning that it is cell sparse and predominantly
comprises axons and dendritic processes. Th en, show
that the next innermost layer is layer II, the external
granular layer; indicate that it predominantly contains
non-pyramidal cells and much fewer small pyramidal
cells. Next, label layer III, the external pyramidal layer;
indicate that it predominantly contains pyramidal cells
of varying sizes. Also, layer III is sparsely populated with
non-pyramidal cells. Now, label layer IV, the internal
granular layer; indicate that it is densely packed with
non-pyramidal cells. Layer IV is the narrowest cellular
layer and it contains the horizontally oriented external
band of Baillarger, which is a prominent thalamocortical
nerve fi ber layer. In the primary visual cortex, this nerve
fi ber band is called the line of Gennari. Next, label layer V,
the internal pyramidal (or ganglionic) layer; indicate that
it contains the largest pyramidal cells, most notably the
Betz cells which are the major cortical motor neurons.
Th e horizontally oriented internal band of Baillarger lies
deep within this layer. Now, label layer VI, the multiform
(or fusiform) layer; indicate that it contains a wide variety
of pyramidal and non-pyramidal cells. Layer VI blends
into the underlying white matter.^10 , 18
Although the specifi c cellular constitution and nerve
fi ber density of each histologic layer varies across the
cerebral cortex, certain areas of similarity exist. In
1909, Korbinian Brodmann published the most widely
recognized cytoarchitectural maps of the human brain,
which distinguish 52 diff erent cytoarchitectural areas.
Each area or group of areas subserves a unique function.
In the next diagram, we will draw select, fundamental
Brodmann areas and learn their signifi cance.