Study On Depolarization Block Of Arterial Baroreceptors And Nucleus Raphe Magnus Nucleus

Depolarization block happens whenever the applied current exceeds a certain level. Depolarization block draws less attention compared to excitability though it is easily observed in experiment. For searching its existence and modulation in nervous system, based on former study in our lab, we chose baroreceptor in periphery and nucleus accumbens shell (NAcSh) medium-spiny projection neurons (MSNs) in centre as objects.Excitability corresponds to firing starting and depolarization block represents to firing stopping. Baroreceptors are the sensory terminals of the baroreflex. It is well known that individual baroreceptors initiate firing at different blood pressure levels. Can they stop firing along with further increase of blood pressure? For each individual baroreceptor, is there a limited receptive range determined by two pressure thresholds at one of which firing is initiated and at the other of which firing is stopped? Present study revealed that individual baroreceptors could enter depolarization block, stop their afferent signals when the blood pressure exceeds a certain level, indicating that individual baroreceptors are sensitive to a specific range of blood pressure. The receptive ranges of individual baroreceptors differ in terms of the total range, the lower threshold, and the upper threshold. Of85baroreceptors examined in this study, the upper thresholds for about half were within the physiological blood pressure range. These results indicate that supraphysiological blood pressure is unlikely to be encoded by the recruitment of more baroreceptors. Instead, supraphysiological blood pressure levels might be signaled by an increase in the frequency of action potentials or by other mechanisms.Plasticity of excitability indicates that excitability is modulatable, is depolarization block also modulatable? Because baroreceptors cannot be recorded intracelluarly, modulation of depolarization block was mainly studied in NAc MSNs. Our studies found that, in experiments, changes in excitability and depolarization block in NAcSh MSNs are explored by increasing the intensity of injected current and blocking the SK channels by apamin. Then, a Hodgkin-Huxley type model including the main electrophysiological processes of such neurons is developed to reproduce the experimental results. Reduction of SK channel conductance also shifts the bifurcations, which is in consistence with experiment. A global bifurcation paradigm of this shift is obtained by adjusting two parameters, intensity of injected current and SK channel conductance. The results indicate that small ionic conductance other than that responsible for spike generation can modify bifurcation points and shift the bifurcation diagram and thus change neuronal excitability and adaptation.Existence of depolarization block ensures receptive ranges of baroreceptors and differences of receptive ranges between baroreceptors, directly evidences the deficiency of nowadays theory about baroreceptors encoding blood pressure signals, indicates that it is possible that segregation of duties were used by baroreceptor to encoding blood pressure signals. And, as excitability, depolarization block is modulatable, suggesting that, in physiology, same as synaptic transmission efficacy and intrinsic excitability, depolarization block is plastic, three of these cooperate to finish requests of life and meanwhile keep network safety.