Alternative view on the mechanism of cardiac rhythmogenesis

Role of indole derivative SS-68 in increasing the frequency range of cardiac rhythm control (reflex stimulation of the sinoatrial node)

Introduction: Cardiac pacing is indicated for sick sinus syndrome. It is performed with a pacemaker via electrodes implanted in the heart. This technique has several disadvantages. The search for alternative methods of cardiac pacing is underway. One of them is control of heart rhythm through stimulation of the tragus.

Objective: To perform the reflex stimulation of the sinoatrial node and to study the influence of the SS-68 substance on it.

Materials and methods: Two electrodes were fixed in the reflexogenic zone of rabbits’ auricles, volleys of electrical impulses from an electrical stimulator were applied to the electrodes, and the synchronization range of volley frequency and cardiac contractions was recorded. This range was re-recorded again after injecting the SS-68 substance (2-phenyl-1-(3-pyrrolidine-1-cyclopropyl)-1H-indole hydrochloride) intravenously at a dose of 50 µg/kg. In other experiments on frogs in a high-frequency electromagnetic field, the process of excitation of the area of the medulla oblongata associated with the heart rhythm was visualized. After the application of SS-68 (50 μM) to the surface of this zone, the process of its excitation was recorded.

Results and discussion: Stimulation of the auricular reflexogenic zone of rabbits produced a synchronization of volley frequency and heart rate in the range from 173.5 ± 2.0 to 214.0 ± 1.8 per minute. SS-68 extended this range from 168.2 ± 1.9 to 219.4 ± 1.5 per minute. In the frog’s medulla oblongata, an area synchronous to the heart rhythm glowed in the high-frequency electromagnetic field. SS-68 increased the area of glow by 131.0%.

Conclusion: The substance SS-68 increases the frequency range of heart rhythm control by activating reflex stimulation of the sinoatrial node. The main point of application of SS-68 is the medulla oblongata. Glow in the high-frequency electromagnetic field reflects the process of neuron excitation. The increase in the glow zone under the influence of SS-68 indicates synchronously excited neurons, which leads to the assimilation of the central heart rhythm generation by the sinoatrial node.

Cardiorespiratory synchronism in estimation of regulatory and adaptive organism status

The proposed method of quantitative estimation of regulatory and adaptive status (RAS) of human organism is based on complex responses of two major vegetative functions - breath and heart rates under organism exposure to a number of factors and diseases. It has been evidenced that during the follicular menstruation stage and during optimum readiness of female organism for childbirth RAS increases, however, stress impact can also cause RAS set off to decrease. Likewise, the possibility of quantitative organism stress resistance estimation is also presented. Under some pathological conditions (myocardial infarction, hypo-and hyperthyroidism, diabetes type 2), RAS goes down, and the degree of its restoration depends on the attained therapy effect. It is shown that RAS dynamics provides an innovative methodological approach to medication efficiency estimation based on its influence not only on the body organ or target function, but also on adaptive abilities of the organism.

Central control of cardiorespiratory interactions in fish

Fish control the relative flow rates of water and blood over the gills in order to optimise respiratory gas exchange. As both flows are markedly pulsatile, close beat-to-beat relationships can be predicted. Cardiorespiratory interactions in fish are controlled primarily by activity in the parasympathetic nervous system that has its origin in cardiac vagal preganglionic neurons. Recordings of efferent activity in the cardiac vagus include units firing in respiration-related bursts. Bursts of electrical stimuli delivered peripherally to the cardiac vagus or centrally to respiratory branches of cranial nerves can recruit the heart over a range of frequencies. So, phasic, efferent activity in cardiac vagi, that in the intact fish are respiration-related, can cause heart rate to be modulated by the respiratory rhythm. In elasmobranch fishes this phasic activity seems to arise primarily from central feed-forward interactions with respiratory motor neurones that have overlapping distributions with cardiac neurons in the brainstem. In teleost fish, they arise from increased levels of efferent vagal activity arising from reflex stimulation of chemoreceptors and mechanoreceptors in the orobranchial cavity. However, these differences are largely a matter of emphasis as both groups show elements of feed-forward and feed-back control of cardiorespiratory interactions.

The basis of vagal efferent control of heart rate in a neotropical fish,the pacu,Piaractus mesopotamicus

The role of the parasympathetic nervous system, operating via the vagus nerve, in determining heart rate (fH) and cardiorespiratory interactions was investigated in the neotropical fish Piaractus mesopotamicus. Motor nuclei of branches of cranial nerves VII, IX and X, supplying respiratory muscles and the heart, have an overlapping distribution in the brainstem, while the Vth motor nucleus is more rostrally located. Respiration-related efferent activity in the cardiac vagus appeared to entrain the heart to ventilation. Peripheral stimulation of the cardiac vagus with short bursts of electrical stimuli entrained the heart at a ratio of 1:1 over a range of frequencies, both below and sometimes above the intrinsic heart rate. Alternatively, at higher bursting frequencies the induced fH was slower than the applied stimulus, being recruited by a whole number fraction (1:2 to 1:6) of the stimulus frequency. These effects indicate that respiration-related changes in fH in pacu are under direct, beat-to-beat vagal control. Central burst stimulation of respiratory branches of cranial nerves VII, IX and X also entrained the heart, which implies that cardiorespiratory interactions can be generated reflexly. Central stimulation of the Vth cranial nerve was without effect on heart rate, possibly because its central projections do not overlap with cardiac vagal preganglionic neurons in the brainstem. However, bursts of activity recorded from the cardiac vagus were concurrent with bursts in this nerve, suggesting that cardiorespiratory interactions can arise within the CNS, possibly by irradiation from a central respiratory pattern generator, when respiratory drive is high.

Interaction of brain and intracardiac levels of rhythmogenesis hierarchical system at heart rhythm formation

A single-stage bilateral conduction blockade of the vagus nerves (functional denervation) by constant anodal current was carried out in 13 dogs which are under anesthesia and 3-5 days after operation in chronic experiments. In anesthetized animals, "functional denervation" led to acceleration of the heart rhythm from 102.4+/-3.2 bmp to 123.8+/-4.4 bmp. In chronic dogs "functional denervation" led to transient stoppage of the heart--a preautomatic pause with duration of 2.7+/-0.2 sec. The heartbeats recommenced with the frequency of 89.0+/-3.4 bmp versus an initial rhythm of 118+/-1.5 bpm, i.e., a rhythm deceleration took place. We conclude that in a whole organism the heart rhythm pacemaker is determined by a brain level of the hierarchical system of rhythmogenesis, while the sinoatrial node plays the role of a latent pacemaker.

Hierarchy of the heart rhythmogenesis levels is a factor in increasing the reliability of cardiac activity

Along with the existence of an intracardiac pacemaker a generator of cardiac rhythm exists in the central nervous system - in the efferent structures of the cardiovascular center of the medullar oblongata. Neural signals originating there in the form of bursts of impulses conduct to the heart along the vagus nerves and after interaction with cardiac pacemaker structures cause generation of the cardiac pulse in exact accordance with the frequency of the neural bursts. That the intrinsic cardiac rhythm generator is a life-sustaining factor that maintains the heart pumping function when the central nervous system is in a stage of deep inhibition. The brain generator is the factor that provides the heart with adaptive reactions to changes in the environment. The hierarchy of the two duplicating levels of rhythmogenesis provides reliability and functional perfection of the cardiac rhythm generation system in the whole organism.

INTEGRATION OF THE HEART RHYTHMOGENESIS LEVELS: HEART RHYTHM GENERATOR IN THE BRAIN

We propose that along with the intracardiac pacemaker, a generator of cardiac rhythm exists in the central nervous system--in the efferent structures of the cardiovascular center of the medullar oblongata. Signals in the medulla oblongata arise as a result of the hierarchic interaction of the brain structures. Neural signals originating there in the form of bursts of impulses conduct to the heart along the vagus nerves and after interaction with cardiac pacemaker structures, cause generation of the cardiac pulse in exact accordance with the frequency of "neural bursts". The intrinsic cardiac rhythm generator (the sinus node) is a life-sustaining factor that maintains the heart pumping function when the central nervous system is in a stage of deep inhibition, (e.g., under anesthesia or during unconsciousness). The brain generator is the factor that provides heart adaptive reactions in behaving organism. The integration of the two levels of rhythmogenesis in the brain and heart provides reliability and functional perfection of the cardiac rhythm generation system in the whole organism.