E. DONG, G. C. M. WIEDERHOLD 
1103 
D. B. Jackson, Abbott Laboratories, North 
Chicago: You mentioned in your abstract that 
you're now using a small computer. Is this 
system you describe small? I mean this wasn't 
a small system. . . . 
Dr. Dong: No. 
Dr. Jackson : I wondered what it was ? 
Dr. Dong: We carried out all these experi- 
ments that I described here on ACME which 
j is a 360-50 with an 1800. Since that time, we 
j have purchased a PDP-12, and we started to 
do the same type of experimentation. As I said, 
1 1 it took us four days to carry out the first ex- 
periments with the ACME system. It took us six 
weeks to program the machine line in which to 
carry out the same thing on a PDP-12. For rea- 
sons of economy, it was cheaper to do it on 
the PDP-12 when you're going into a production 
j mode. 
I Chairman Warner: I'd like to just make a 
couple of comments about your observations 
: of phase sensitivity of the vagal stimulus on 
1 heart rate and its relation particularly to the 
model you presented initially. Bob Tuckett in 
our laboratory has been studying this phenom- 
j enon for a couple of years and has done many 
! of the same kinds of things you describe here. 
But, he has gone a step further I suppose in the 
sense that he's tried to take this observation 
and test it against the model. He found that by 
altering the model so that heart rate now, in- 
stead of being a function of frequency of stim- 
I ulation, changes the equation to be a discrete 
function, where each stimulus represents a unit 
1 input to the model. With that addition, there is 
I only one other parameter, which is a parameter 
that describes the delay in transmission of the 
signal from the point of stimulation on the 
vagus to the sinoauricular node. This model 
does describe the phase sensitivity. 
! I differ with you in one comment you made, 
i and that was that the important part of the 
model is the proportionality between acetylcho- 
line concentration and heart rate. Of course, 
that is an element in the model. But the interest- 
ing features of the model are the kinetics, the 
dynamic features of the relationship of the 
rate of destruction for acetylcholine and the 
release of acetylcholine at the time the stimulus 
arises. These two phenomena will account for 
the features that you present in your experi- 
mental data. It might be interesting to test your 
data against that same phenomena. Would you 
like to make any closing remarks? 
Dr. Dong: Time didn't permit the demon- 
stration of a different model which we modified. 
There is a well-known axone model of Hodg- 
kins and Huxley which was modified by Noble 
to simulate the membrane potential. We modi- 
fied that model too by adding a simple circuit 
to it, and simulated the whole thing on the ACME 
system. It also will describe phase sensitivity. 
Although the decay of heart rate from stimula- 
tion to control state was previously described, I 
think, as a single exponential (and experimental 
data does not confirm that), it appears as if the 
decay of heart rate from stimulated state to 
control state is actually at least two phases: 
one having a very slow phase and one having 
a very, very fast phase. This modification was 
added to Noble's model so that it can simulate 
that, as well as phase sensitivity. What I want 
to say here today was simply that the use of this 
large system has given us great insight and con- 
trol over acquiring data as well as analyzing 
data and I think that it's extremely useful for 
physiological research. It is well to have both 
of these types of systems as well as other more 
dedicated systems. 
Chairman : Thank you Dr. Dong. I will just 
conclude by saying there is no such thing as a 
unique model to any system and the final cri- 
teria for selection of a model for describing a 
particular phenomenon must be based on 
aesthetics. 
