Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 43

Fig. 3.11 Selecting the first
pulse and blocking all others
Having a weak synapse is one way to achieve a single pulse. An equivalent way
is to employ a neural circuit to provide a single pulse is given in Fig. 3.11 .This
circuit prevents the transmission of pulses beyond the first pulse.
The figure shows XOR logic (XOR logic is discussed below) to prevent any
pulses after the first pulse. In the XOR, pulses applied to one input pass through, but
simultaneous pulses at both inputs result in no output. The first pulse triggers a STM
neuron STM1 whose several output pulses are timed to balance each additional
pulse in the burst to prevent the transmission of pulses beyond the first. Other less
tricky methods of producing a single pulse are possible using enabled AND and
NOT gates, described below, which makes pulse timing less critical (the design of a
circuit is left as a exercise). The label Pulse(1) means exactly one pulse.
Long-Term Potentiation
This refers to a phenomenon long thought to be important to learning, under which a
previously triggered synaptic system becomes easier to trigger in the future [ 3 ]. One
intuitive explanation is that a presynaptic vesicle delivers a limited count of excit-
atory neurotransmitters that trigger a receptor with LTP, but that the configuration of
the postsynaptic receptor is such that it cannot be triggered without LTP. That is, the
neuron responds to the limited count of excitatory neurotransmitters only if LTP has
been created beforehand. LTP may be created by a signal such as an overvoltage that
changes the chemistry of the receptor's membrane, causing it to be more sensitive
than otherwise it would be without LTP. Without LTP, triggering would require a
full bundle of excitatory neurotransmitters.
LTP Circuit Model
LTP can be modeled in circuit terms as trapped static charge that has been
permanently placed into the membrane capacitance of a receptor. The receptor
with LTP must be able to be queried, so the connecting synapse is modeled as a
regular synapse as above, that is, a transconductance amplifier that transmits one
packet of charge for each voltage pulse applied to its input.