Biomedical Engineering Reference
In-Depth Information
A
B
500
0.5s
1s
2s
0
40
50
400
400
300
300
200
200
100
100
0
0
10
20
30
40
50
60
70
0
0.5
1
1.5
2
2.5
Spacing (
µ
m)
Time (s)
Figure 4.8
Area over threshold as a function of the total cross-sectional area of source for different num-
bers of evenly spaced fibres of diameter 2µ
m
. A. Affected area against spacing for 100 fibres
for NO synthesis of length 0.5, 1 and 2 seconds. B. Affected area over time due to 100 fibres
arranged as a single source (spacing = 2) or separated by 40 or 50µ
m
for 2 seconds of NO
synthesis. Note the delay till effective co-operation of the separated sources.
period which would be tunable by changing the spacing), but once past this point,
large regions are rapidly 'turned-on' by the cloud of NO.
‡
Another factor which affects the length of the delay is the thickness of the fibres
used. Examining the delay from plexuses composed entirely of fibres of various fixed
diameters, we see that the delay is longest for the thinnest fibres (
Figure 4.9)
.
This is
because the fibres are now too small to achieve an above threshold signal singly and
so must cooperate, although the subsequent rise in the affected area is not as steep
as for ordered arrays. This is expected since the random nature of the plexus means
that the distribution of concentrations is less uniform and the interaction effect is
less pronounced. Examining the delay for plexuses of other diameters we see that it
rises steeply to that seen for the 1
m
plexuses showing that the interaction needed
is much greater for the smaller fibres (Figure 4.9A). As a property of a signal, the
obvious role for such a delay is as a low pass filter since there has to be nearly
200
ms
of synthesis before the thin plexus will respond. In the case of the signal
mediating increased blood flow, this means that there would need to be significant
sustained activity before blood flow increased, whereas for a thick plexus blood flow
would react to every short burst of neuronal activity. Other features seen to vary
with fibre thickness are maximum concentration, how centred a cloud is on a target
region, and the variability of concentrations over a region; all of which have sensible
interpretations in terms of neuronal signalling to blood vessels.
µ
‡
A form of signalling which might also be useful in an artificial neural network (see Section 4.4).
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