Biomedical Engineering Reference
In-Depth Information
3mm
A
Δ
A
1
D
2
C
3
4
6
7
B
z
1
ΔΦ
2.5 mm
8
0.6 mm
9
J
10
11
12
E
12
F
14
15
16
sr w
(',
,
t
G
Figure 1.4
The macrocolumn is defined by the spatial extent of axon branches E that remain within
the cortex (recurrent collaterals). The large pyramidal cell C is one of 10
5
to 10
6
neurons in the
macrocolumn. Nearly all pyramidal cells send an axon G into the white matter; most reenter the cor-
tex at some distant location (corticocortical fibers). Each large pyramidal cell has 10
4
to 10
5
synaptic
inputs F causing microcurrent sources and sinks
s
(
r
,
w
,
t
). Field measurements can be expected to
fluctuate greatly when small electrode contacts A are moved over distances of the order of cell body
diameters. Small-scale recordings measure space-averaged potential over some volume B depending
on the size of the electrode contact and can be expected to reveal scale-dependent dynamics, includ-
ing dominant frequency bands. An instantaneous imbalance in sources or sinks in regions D and E will
produce a “mesosource,” that is, a dipole moment per unit volume
P
(
r
,
t
) in the macrocolumn.
(
From:
[4]. © 1995 Oxford University Press. Reprinted with permission.)
2. The magnitude of the mesosource depends on the magnitudes of the
microsource function
s
(
r
,
w
,
t
) and source separations
w
within the mass
W
.
Thus, cortical columns with large source-sink separations (perhaps
produced by excitatory and inhibitory synapses) may be expected to
generate relatively large mesosources. By contrast, random mixtures of
sources and sinks within
W
produce small mesosources, the so-called
closed
fields
of electrophysiology.
3. Mesosource magnitude also depends on microsource phase synchronization;
large mesosources occur when multiple synapses tend to activate at the same
time.
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