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In-Depth Information
Fig. A.3
Circuit model of
a segment of dendrite
Table A.3
Electrical
parameters
R
1
¼
200 M
C
1
¼
31.4 pF
R
LOSS1
¼
106 M
I
Na1
2.11 nA
¼
I
K1
0.955 nA
¼
xcur
refers to a subcircuit
cur1
that provides the appropriate charging and
discharging currents, as explained below.
Controlled Current Source Subcircuit
The current sources in the given code are represented by a subcircuit termed
cur1
.
Currents are arranged to switch at given voltage values. To accomplish this, node
voltage v
n
must be sampled. This requires care since node resistances in the model
are high, hundreds of megohms (10
6
Ω). Sampling will not disturb the node if it is
done through tens of gigaohms (10
9
). In order to control the current sources, three
control voltages v
1
, v
2
, v
3
are going to be created. The currents will be delivered by
voltage-controlled current sources. These control voltages will depend on node
voltage v
n
.
Starting Sodium Current To begin, sodium current switches on as node voltage
increases through about
Ω
55 mV. Figure
A.4
illustrates the output of a basic
nonlinear amplifier or comparator whose purpose is to generate a control voltage v
1
.
A schematic for the amplifier appears in Fig.
A.5
. Note that the reference voltage
is
55 mV
and there is no feedback resistor.
The amplifier is modeled with a voltage-controlled voltage source E that
amplifies by a factor of 100 the voltage across the two (+,
) inputs to the voltage
source. Ideal diodes are used to clip the output to be between
1, +1 V. This is
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