Structure-Based Models of NO Diffusion in the Nervous System - Computational Neuroscience: A Comprehensive Approach - page 115

A B

t = 25ms
t = 50ms
t = 100ms

t = 175ms
t = 300ms
t = 1.5s

12 12

8 8

4 4

0 0

200 100 0 100 200 200 100 0 100 200

Distance ( m) Distance ( m)

µ µ

200 µ m 200 m

µ

Figure 4.3
Concentration of NO plotted against distance from the centre of a hollow spherical source
of inner radius 50µ m and outer radius 100µ m for a 100 ms burst of synthesis starting at time
t = 0. The graphics underneath each plot depict the structure. A. Concentration of NO at times
t = 25 , 50 and 100 ms , two time points during and one at the end of synthesis. B. Concentration
of NO after synthesis at times t = 175 , 300 and 1 . 5 s . The reservoir effect following the end of
synthesis is clearly seen as the centrally accumulated NO is trapped by the higher surrounding
concentrations.

rise in the synthesising area (the cytoplasm). After the end of synthesis, this reser-
voir continues to fill up for about 200 ms as the NO in the cytoplasm diffuses away
from its point of origin to points of lower concentration in the nucleus. However, the
concentration outside the cell still rises slowly as the NO is dissipated over a larger
volume. Later, the situation changes somewhat, as we are now in the position where
the concentration in the nucleus is roughly equal to the concentration in the cyto-
plasm, giving a wide flat peak to the concentration profile. Until this point, the NO
which had diffused into the centre had been 'trapped' and could not be dissipated due
to the higher concentration present in the surrounding cytoplasm. Now though, we
see this reservoir spreading away from the cell in a wave of high concentration which
starts to raise the distal concentrations to significant levels. However, the concentra-
tion at the centre remains high and does not spread outwards very quickly since the
concentration gradient is virtually flat, meaning there is very little diffusive pressure
on the NO in this area. It is this effect that produces the unexpected time delay at
distant points.
Examination of the concentration at 225

m from the centre of the cell ( Figure
4.2) , shows that it remains low until about 400 ms after synthesis has stopped. It
peaks shortly afterwards and stays relatively high for a relatively long period. This
has implications for the temporal dynamics of NO-signalling in a neurobiological

µ