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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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