Circuit Elements Required for Neural Systems - Brain Theory from a Circuits and Systems Perspective - page 50

Fig. 3.19 NOT gate concept for a neuron

Table 3.1 Neurotransmitter
codes

(+) Presence of excitatory neurotransmitters

( ) Presence of inhibitory neurotransmitters

(0) Absence of neurotransmitters

But there is no chance of signals “a” and “b” producing an output, since signal “b”
does not pass through to reach junction y. Thus the output of axon “z” is false
(no output pulses) when the inputs going to branch “a” are true.
If branch “a” is not excited, as in the right-most figure, the inhibitory
neurotransmitters from the primed input are likewise not applied. Branch “b”is
now free to propagate an action potential. This signal is not stopped by branch “a”
where an action potential merely propagates harmlessly back down branch “a” and,
most importantly, also toward the soma. Thus, when the inputs to branch “a” are
false, the soma output is true. What is accomplished is a NOT gate using a neuron,
assuming a linked combination of pulses to “a” and inhibitory neurotransmitters to
“b” and “y.”
The external neuron driving branch “a” must also provide inhibitory inputs to
branches “b” and “y.” The inputs to branches “e” as well as “b” may be obtained
from an external neuron. Branch “e” serves as an enable, as does the input to branch
“b.” To summarize using Boolean notation:

a 0 Þ

z ¼ e ð b ;

z, e, a, b are Boolean signals and a 0 means NOT(a). To enable it, let e

¼ b ¼ 1so

a 0 . Note that no dendritic gates are involved. It also may be noted that the
timing of the precharge due to branch “e” is not critical; but it must be present to
have a valid output. The input to branch “b” is similarly not critical. These auxiliary
signals may be stopped only after operations are completed. For enabled neural
logic, signals need be present only during a certain relevant time frame.
Focusing on basic neural components as above is convenient when explaining
the logical properties of a neuron. Nevertheless there are several proposed ways to
achieve a complete set of Boolean logic [ 7 - 12 ].

that z ¼