In recent years physiologists have become quite adept at probing
into neurons with intracellular microelectrodes. They are now able,
in fact, to measure (a) the voltage change across the postsynaptic
membrane elicited by a single presynaptic impulse (see, for examples,
references 1 and 2) and (b) the voltage-current characteristics
across a localized region of the nerve cell membrane (3), (4), (5),
(6). With microelectrodes, physiologists have been able to examine
not only the all-or-none spike generating and propagating properties
of axons but also the electrical properties of somatic and dendritic
structures in individual neurons. The resulting observations have
led many physiologists to believe that the individual nerve cell
is a potentially complex information-processing system far removed
from the simple two-state device envisioned by many early modelers.
This new concept of the neuron is well summarized by Bullock in his
1959 _Science_ article (10). In the light of recent physiological
literature, one cannot justifiably omit the diverse forms of somatic
and dendritic behavior when assessing the information-processing
capabilities of single neurons. This is true regardless of the means of
assessment—whether one uses mathematical idealizations, electrochemical
models, or electronic analogs. We have been interested specifically in
electronic analogs of the neuron; and in view of the widely diversified
behavior which we must simulate, our first goal has been to find a
unifying concept about which to design our analogs. We believe we have
found such a concept in the Modern Ionic Hypothesis, and in this paper
we will discuss an electronic analog of the neuron which was based on
this hypothesis and which simulated not only the properties of the
axon but also the various subthreshold properties of the somata and
dendrites of neurons.
We begin with a brief summary of the various types of subthreshold
activity which have been observed in the somatic and dendritic
structures of neurons. This is followed by a brief discussion of the
Hodgkin-Huxley data and of the Modern Ionic Hypothesis. An electronic
analog based on the Hodgkin-Huxley data is then introduced, and we show
how this analog can be used to provide all of the various types of
somatic and dendritic activity.