CHAPTER 7
Nervous Tissue
127
Figure 7-10A.
Cerebral cortex.
Nissl stain,
3
24
The
cerebral cortex
is a layer of densely packed neurons
about 2 mm thick that forms the surface of the hemi-
spheres of the brain. The cortex is organized into
layers
(indicated by
red lines
and
Roman numerals
) on the basis
of the
size
,
shape
, and
packing density
of the neurons
in different regions of the cortex. For example,
layers II
and
IV
in this photomicrograph consist of small, tightly
packed neurons (mainly granule cells), whereas
layers III
and
V
consist of larger neurons (mostly pyramidal cells).
This example is taken from the association cortex of the
parietal lobe (
red box in inset
).
Layer I
consists predomi-
nantly of horizontally running dendrites and axons; the
nuclei in this region belong to glial cells. The white matter
consists of axons entering and leaving this small region of
cortex and connecting it with other cortical regions, with
subcortical structures such as the thalamus and with the
spinal cord.
I
II
Pia
White matter
(myelinated axons)
III
IV
V
VI
A
CLINICAL CORRELATION
Figure 7-10C.
Alzheimer Disease.
Bielschowsky
stain,
3
272; inset
3
550
Alzheimer disease
is the most common form of
dementia
in the elderly (60%–80% of cases). Patients
show
progressive memory loss
,
personality changes
,
and
cognitive impairments
. Pathologically, Alzheimer
disease is characterized by extracellular deposition
of
A
b
protein (plaques)
,
intracellular neurof brillary
tangles
(
inset:
N, normal neuron; T, neuron cell body
± lled with dark-staining tangles), and
loss o±
neurons
and synapses in the cerebral cortex and some subcor-
tical regions. Gross anatomy shows
atrophy
of the
affected regions. The most prominent theory is that
mutation of genes such as those located on chromo-
some 21 lead to overproduction of A
β
precursor.
There is no cure for Alzheimer
disease. Treatments
include pharmaceutical and
psychosocial therapies
and supportive caregiving.
Amyloid plaques
Amyloid plaques
Amyloid plaques
N
T
C
S
AD
BD
S
S
A
B
Figure 7-10B.
Cerebral cortex, pyramidal cells.
Golgi
preparation,
3
136
The most numerous neurons in the
cerebral cortex
are
pyramidal cells
, named for their triangular cell bodies
(singular,
soma
; plural
somata
;
S
in the
inset
and
photo-
micrograph). Pyramidal cells are also characterized by a
long
apical dendrite
(
AD
) that extends into layer I and
has many lateral branches; several
basal dendrites
(
BD
)
that extend laterally from the base of the soma; and a long
axon
(
A
) that leaves the cortex and extends, in the white
matter, either to some other region of the cortex or to sub-
cortical structures, such as the basal nuclei, brainstem, or
spinal cord. Pyramidal cells are found in layers II through
VI of the cortex but are most obvious in layers III and V.
Somata range in size from 10 to over 50
μ
m, with the larg-
est located in layer V of primary motor cortex. The sam-
ple shown here is taken from layer III. It includes several
medium pyramidal cells and many small pyramidal cells.
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