Biomedical Engineering Reference
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ing distributed passive sources to preserve current conservation. IPSPs produce local
membrane current sources with more distant distributed passive sinks. Much of our
conscious experience must involve, in some largely unknown manner, the interac-
tion of cortical neurons. The cortex is also believed to be the structure that generates
most of the electric potentials measured on the scalp.
The cortex (or neocortex in mammals) is composed of
gray matter
, so called
because it contains a predominance of cell bodies that turn gray when stained, but
living cortical tissue is actually pink. Just below the gray matter cortex is a second
major region, the so-called
white matter
, composed of myelinated nerve fibers
(
axons
). White matter interconnections between cortical regions (
association fibers
or
corticocortical fibers
) are quite dense. Each square centimeter of human neocor-
tex may contain 10
7
input and output fibers, mostly corticocortical axons intercon-
necting cortical regions separated by 1 to about 15 cm, as shown in Figure 1.2. A
much smaller fraction of axons that enter or leave the underside of human cortical
surface radiates from (and to) the thalamus (
thalamocortical fibers
). This fraction is
only a few percent in humans, but substantially larger in lower mammals [3, 4].
(a)
(b)
Figure 1.2
(a) Some of the superficial corticocortical fibers of the lateral aspect of the cerebrum
obtained by dissection of a fresh human brain. (b) A few of the deeper corticocortical fibers of the lat-
eral aspect of the cerebrum. The total number of corticocortical fibers is roughly 10
10
; for every fiber
shown here about 100 million are not shown. (
After:
[5, 6].)
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