BIOMIMETIC CORTICAL NANOCIRCUITS - Bio-Inspired and Nanoscale Integrated Computing - page 465

1

AP
PSP

0.8

0.6

0.4

0.2

0

0 1 2 3

Time in seconds

4 5 6

10 − 11

×

Figure 17.3. The action potential and PSP at the dendritic trunk under normal
operation.

efficiency of the reuptake mechanism to be tuned. The action potential is inverted
and used to block pulldown of the synaptic cleft potential to delay reuptake of the
neurotransmitter at the presynaptic neuron while there is a positive action
potential. Some antidepressants inhibit neurotransmitter reuptake, and this is
modeled by lowering the reuptake control voltage in the synapse circuit.
The increase in potential in the synthetic synaptic cleft will temporarily turn
on the transistor connected to the ion electromotive force control, E-channel,
causing the potential at Synaptic Interior to rise. This models the change in
biological membrane potential due to the increased conductance of neurotrans-
mitter gated ion channels and the subsequent influx of charge carrying ions. A
pull-down network 3 models the action of the biological ion pump. As with the
reuptake mechanism the efficiency of the ion pump may be tuned via the pump
control voltage.
The synaptic interior potential is transferred through a resistive connection to
the dendritic trunk, which carries it to the cell body of the neuron. The synaptic
weight control allows the importance of this synapse to be tuned, as is often done
in learning algorithms. The potential on the dendrite trunk represents the
postsynaptic potential (PSP). The dendrite trunk terminates on the gate of a
single NFET that acts as a load for the synaptic circuit for testing purposes. This
load would be replaced with a block of circuitry representing the dendritic tree
structure, cell body and axon of the postsynaptic neuron when the synapse is used
in a complete neuron model.
In the archetypical biological neuron we are modeling, potentials range from
around 75mV to +40mV. with action potentials peaking around +40mV.
Since the carbon nanotubes are designed to operate with V dd around 0.9 V and
with 0 V. (Ground) as the lowest potential, the potentials were scaled in the
synaptic circuit accordingly, with 0v. circuit potential corresponding to 75mV
biological potential and 0.9 V circuit potential corresponding to 40mV biological

3 Three transistors in series provide the R pulldown resistance.