Biomedical Engineering Reference
In-Depth Information
Figure 7.4
Comparison between model inter-spike interval distributions (lines) at different lev-
els of synaptic input, and distributions observed in an oxytocin cell at different times
during infusion of hypertonic saline (points). Oxytocin cell inter-spike interval dis-
tributions were constructed over 1000s, model cell distributions over a simulated
25000s, normalised for comparison. Oxytocin cell distributions correspond to mean
firing rates of 12.8Hz, 9.3Hz 4.8Hz and 2.5Hz. Model cell distributions were con-
structed for equal average numbers of EPSPs and IPSPs, over a range of PSP frequen-
cies that matched the range in firing rates observed during the period of recording
analysed, producing output rates close to the average firing rates of oxytocin cells. A
single value of l (0.08) produces good fits for this cell at all levels of activity.
about 0.5Hz (lower range of spontaneous rates) to about 10Hz (peak sustained rates).
This range was spanned in the model cells by a narrow range of
R
E
typically by a
change in
R
E
from 110/s to 180/s. Osmotic stimulation is accompanied by a direct
depolarisation of 3-5mV [17], and an equivalent change in
T
0
leads to a compression
of the range of
R
E
needed. This suggests that tonically active cells subject to EPSP
input alone will respond strongly to osmotic stimuli as a result of the direct osmotic
depolarisation, even with no change in synaptic input. Furthermore, similar changes
in
R
E
from a different initial rate, but accompanied by the same osmotic depolari-
sation, result in very different amplitudes of responses. This suggests that oxytocin
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