Excited After Inhibition Of The Dynamics Simulation And Experimental Study Of The Phenomenon

Neural coding is an important topic in the field of neuroscience. The basic framework for understanding of neural coding has been established:frequency coding, time coding and so on. As the carieer for neural coding process, nerve fibers have been considered for a long time as the conduction channel. Its main function is that covered the potential changing siginal with a certain time sequence information to strings of action potentials and faithfully transmitted to the nerve center, or the nerve center information passed to the middle or peripheral neurons. However, the postexcitatory depression (PED) of axon after high frequency repeated excitation suggests that the sequence of action potentials was not conducted as "faithfully" as people thinking in accordance with "original" string sequence of action potention when they were produced under certain conditions. In fact, the sequence may be calculated, processed and integrated at spike initiation zone or during the process of conduction. But as an important physiological phenomenon, the mechanism of PED is unknown. This study was to explore the dynamic process and the mechanism of PED, in order to provide a reasonable explanation for the understanding of the PED phenomenon, and provide the scientific basis for the neural coding.Based on Hodgkin-Huxley model, a model describing electrical activity of axon was built in this study, which included Na+-K+ pump current, ion distribution along the axon radial and the reversal potential of Na+ and K+ expressed by the Nernst equation in this study. The model behaviors were observed with different pulse stimulation frequency, pulse stimulus intensity and Na+-K+ pump currents. Then the dynamic process and ion mechanisms of PED phenomenon were analysed. In addition, the PED phenomenon was observed in the study of single aortic arch baroreceptor fibers by using neurobiology experiments, in order to provide experimental support for the theory study of the PED phenomenon.Main results in this study were as follows:1. Under external pulse stimulation with lower frequency, action potential was generated corresponding to each pulse. Under stimulation with moderate frequency, the PED phenomenon was simulated.2. Based on simulation results, dynamic process and the different stages of ionic mechanisms of PED were analysed. The PED phenomenon can be roughly divided into four stages:(1) Continuous discharge stage, in which the ions were accumulated and equilibrium potential was slowly changed.(2) Rapid reduction oscillation stage, in which the level of membrane potential was increased, and the sodium channel inactivated, which leads to depolarization block. Then PED phenomenon was observed. This was mainly results from gradual accumulation of K+ concentration in the restricted diffusion space of myelin sheath and intracellular Na+ concentration.(3) PED phenomenon sustained stage, which was the process that the membrane potential slowly repolarized. This process was closely related to the Na+-K+ pump activity which was activated by accumulation of K+ in the rapid oscillation reduction process.(4) PED phenomenon recovery stage, which was process that the membrane potential rapidly hyperpolarized and repolarizated.3. Adjusting pulse stimulation frequency, pulse stimulus intensity and the sizes ofINaK, the impact of the three factors on PED phenomena were observed in the model. With increasing frequency pulse, the frequency of the PED generated was increased; the ranges of pulse frequency of PED produced were different acooding to pulse stimulation at different intensities; while the frequency of the PED was increased while the INaK was increasing.4. In vivo experiments, the PED phenomenon after high-frequency firing of the aortic arch baroreceptor single fiber was observed by studying the relationship between the aortic arch baroreceptors single-fiber discharge and in vivo blood pressure. The PED phenomenon was that the firing frequency of baroreceptor single fiber was increased with the vivo blood pressure increasing until it was saturation. Then the firing frequency of baroreceptor single fiber became lower to zero during the vivo blood pressure recovery process at higher blood pressure level.5. In vitro experiments, the PED phenomenon after high frequency firing of the aortic arch baroreceptor single fiber was observed by studying the relationship between the aortic arch baroreceptors single fiber discharge and in vitro pressure, which was the firing frequency was droped or it was resting at higher pressure.6. Based on never coding after high frequency firing, the dynamic simulation and biology experiments were carried on in this study. These results show that:the PED phenomenon was identified to be a depolarized block phenomenon according to the study results from dynamic simulation and experiment, which was closely related to accumulation of K+ concentration of the restricted diffusion space of myelin sheath and intracellular Na+ concentration,and activation of the Na+-K+ pump.