A Physiological And Biophysical Study On The Blood Pressure Signal Encoding By Aortic Arch Baroreceptors

The encoding mechanism of sensory nerve is of fundamental importance in neuroscience. The idea of frequency coding has been established in classical neurophysiology. The intensity of external signal is believed encoded in the mean frequency of firing trains, and the frequency of impulse is positively correlated to external stimulus density. However, in a number of recent studies, the mean firing frequency was found not increased but decreased when the temperature was increased. In addition, mean firing frequency could not. reflect the temporal variations of the complex and dynamic external signal. With combination of neuroscience, nonlinear dynamics, information technology and computer technology, a scientific approach that combined theory and experiment, neurodynamics, has formed. Using conceptions of nonlinear dynamics, the firing pattern and pattern transition (bifurcation) regularities were studied. The results provided a basis for a deeper understanding of complex and variable firing rhythm, identification of the relationship between dynamic firing rhythm and dynamic external signal, and discussion of how dynamic firing encode dynamic external signal. It also gives a new viewpoint and method to theoretically study the neural firing rhythm and neural coding mechanism.In the present study, using methods of nonlinear dynamics as well as physiological experiments, the biophysical mechanism and physiological significance of the encoding of blood pressure by aortic arch baroreceptor was studied. The aortic arch baroreceptor of rabbit was chosen as an experimental model. The neural firing and aortic blood pressure were recorded simultaneously. The biological relevant theoretical model was formed by adding a blood pressure signal into the classic HH model (simulating the baroreceptor). The theoretical model was analyzed theoretically, and the experimental results were compared with the numeric simulation. The theoretical analysis could reproduce the present experimental results and used to guide future experiment. On the other hand, the experimental results verified the theoretical analysis.The main results are as follows. 1 There were two kinds of behaviors in autonomous HH model. With respect to increase of the depolarization current I, corresponding to BP in physiological significance, the behavior of the autonomous HH model was the polarized resting when I was lower than I₀, and then was changed into period 1 firing (a firing pattern whose interspike intervals (ISis) were equal) when I was increased, at last changed into the depolarization resting when I was higher than I₁. I₀ and I₁ were two threshold values of the firing generation and the HH model could generate firing when depolarization current was between I₀ and I₁. The firing frequency of the period 1 firing increased with respect the increase of I, implying that the frequency rate coding mechanism was adequate to some extent in autonomous HH model. The values of I₀ and 1₁ were different when the configuration of the multiple physiological parameters of the autonomous HH model was modified. The bifurcation structure and changing regularity of frequency in autonomous HH model were similar to those in observed in the baroreceptor stimulated by static blood pressure, showing that HH model was a suitable model describing the behaviour of baroreceptors.2 The generation of the dynamic firing patterns, as well as the neural coding mechanisms were elucidated by theoretical analysis and numeric simulation with HH model stimulated by dynamic blood pressure BP(t). The analytical conclusions were also verified in the experiment.For HH model stimulated by dynamic blood pressure BP(t), the behavior was sustained firing as I₀＜BP(t)＜I₁, while remained at rest when BP(t)＜I₀ or BP(t)＞I₁.Within the firing region and under the action of BP(t), the behavior of the dynamic HH model was driven to 'evolve' on the static bifurcation structures of autonomous HH model, resultingin adynamic changein ISI series. The successive ISIs and spontaneous frequency within one period of blood pressure signal could be changed with respect to dynamic variation of BP(t). The spontaneous frequency of the dynamic firing reflected the temporal variation of the blood pressure within the heartbeat period. The temporal pattern of the firing train then encoded the temporal variation and waveform of the blood pressure signal.With in this firing region, when the mean blood pressure increased, the mean firing frequency increased correspondingly. Therefore, the mean firing frequency could represent the mean blood pressure.2.1 With respect to the positional relations to I₀ and I₁, the changing range of the blood pressure could fell into five different regions of parameter I and form three types of firing patterns. The patterns could appear in turn, during the increase of mean blood pressure. Pattern type 1 was composed of bursts corresponding to the peak of the blood pressure pulse and quiescence corresponding to the trough. Pattern type 2 was continuous firing with variation in the values of ISis in each period. Pattern type 3 was a paradoxical one composed of firing in the trough and quiescence in the peak of blood pressure in each heart period.The above numeric simulation results could be observed in the experiments recording the unit firing from baroreceptors and blood pressure simultaneously. Some of the single unit could experience the three pattern types during the increase of mean blood pressure.2.2 Pattern type 3 was a novel firing rhythm found in baroreceptors, since the temporal relations between firing and blood pressure variation were paradoxical. The receptor fired during the trough and ceased fire during the peak in each period.2.3 Within the continuous firing segment of each of the three types, the instantaneous firing frequency could represent the blood pressure. The instantaneous firing frequency changed just according to the variation of blood pressure.2.4 The results of theoretical analysis and numerical simulation both indicated thai the frequency with the firing segment (with the resting segment omitted) could represent the mean blood pressure changes very faithfully.In the case of the pattern type 3, if the firing frequency was calculated as averaged by all the time course including both firing segment and resting segment, the average frequency might exhibit a drop when mean blood pressure increased to certain degree. So, we suggested that in the calculation of frequency, one should consider only the firing segment to make the frequency coding idea valid.3. The fluctuation seen in blood pressure from different heart period could also be studied using the present model. The results from experiment and theoretical analysis were consistent.In the present study, by employing rabbit depressor baroreceptors under the pulsatile forcing of blood pressure as an example, we studied the dynamic firing behaviors of baroreceptors under the driving of dynamic blood pressure signals. We provided dynamic machineries for the physiological firing patterns from a perspective of nonlinear dynamical analysis. Introducing parameter I, the bifurcation structure of the baroreceptor was obtained. Using dynamic I(t), the experimental observations were simulated and analyzed. The baroreceptor system was driven by dynamic blood pressure signals to evolve on the bifurcation structure with respect to static parameter I, and thus forms the dynamic firing patterns. For a given depressor fiber and under a given blood pressure level, such an evolution could generate specific firing patterns, which represented not only the range but also the time-dependent features of the blood pressure.Baroreceptors are believed to respond uniformly, acting like one transducer. Observation of the paradoxical bursting during blood pressure elevation indicated that the physiological range of blood pressure might span the bifurcation scenario of some baroreceptors. This new finding implies the existence of working range for single baroreceptors as well as the possibility of population encoding of blood pressure information by all baroreceptors as a whole. In the cases of other sensations like audition, the working range of single receptors is limited and the complete sensory range is formed by the collaboration in receptor populations. Our results imply that baroreceptors may also have to 'collaborate' under certain blood pressure to encode the information of blood pressure. The validity of this implication should be further investigate...