Study of cardiac sympathetic and vagal efferent activity during reflex responses produced by stretch of the atria

Threshold and saturation pressures of baroreflex-mediated myocardial interstitial acetylcholine release in rats

Cardiac microdialysis allows the assessment of cardiac efferent vagal nerve activity from myocardial interstitial acetylcholine (ACh) levels with minimal influence on the neural control of the heart; however, a total picture of the baroreflex-mediated myocardial interstitial ACh release including the threshold and saturation pressures has yet to be quantified. In eight anesthetized Wistar-Kyoto rats, we implanted microdialysis probes in the left ventricular free wall and measured the myocardial interstitial ACh release simultaneously with efferent sympathetic nerve activity (SNA) during a carotid sinus baroreceptor pressure input between 60 and 180 mm Hg. The baroreflex-mediated ACh release approximated a positive sigmoid curve, and its threshold and saturation pressures were not significantly different from those of an inverse sigmoid curve associated with the baroreflex-mediated SNA response (threshold: 94.3 ± 8.6 vs. 99.3 ± 6.0 mm Hg; saturation: 150.0 ± 10.3 vs. 158.8 ± 5.8 mm Hg). The sympathetic and vagal systems have certain levels of activities across most of the normal pressure range.

Sympathetic tales: subdivisons of the autonomic nervous system and the impact of developmental studies

Remarkable progress in a range of biomedical disciplines has promoted the understanding of the cellular components of the autonomic nervous system and their differentiation during development to a critical level. Characterization of the gene expression fingerprints of individual neurons and identification of the key regulators of autonomic neuron differentiation enables us to comprehend the development of different sets of autonomic neurons. Their individual functional properties emerge as a consequence of differential gene expression initiated by the action of specific developmental regulators. In this review, we delineate the anatomical and physiological observations that led to the subdivision into sympathetic and parasympathetic domains and analyze how the recent molecular insights melt into and challenge the classical description of the autonomic nervous system.

The impact of volume loading-induced low pressure baroreflex activation on arterial baroreflex-controlled sympathetic arterial pressure regulation in normal rats

Although low pressure baroreflex (LPB) has been shown to elicit various cardiovascular responses, its impact on sympathetic nerve activity (SNA) and arterial baroreflex (ABR) function has not been fully elucidated. The aim of this study was to clarify how volume loading-induced acute LPB activation impacts on SNA and ABR function in normal rats. In 20 anesthetized Sprague-Dawley rats, we isolated bilateral carotid sinuses, controlled carotid sinus pressure (CSP), and measured central venous pressure (CVP), splanchnic SNA, and arterial pressure (AP). We infused blood stepwise (3 mL/kg/step) to activate volume loading-induced LPB. Under the ABR open-loop condition, stepwise volume loading markedly increased SNA by 76.8 ± 21.6% at CVP of 3.6 ± 0.2 mmHg. In contrast, further volume loading suppressed SNA toward the baseline condition. Bilateral vagotomy totally abolished the changes in SNA by volume loading. To assess the impact of LPB on ABR function, we changed CSP stepwise. Low volume loading (CVP = 3.6 ± 0.4 mmHg) significantly shifted the sigmoidal CSP-SNA relationship (central arc) upward from baseline, whereas high volume loading (CVP = 5.4 ± 0.4 mmHg) returned it to the baseline level. Volume loading shifted the linear SNA-AP relationship (peripheral arc) upward without significant changes in slope. In conclusions, volume loading-induced acute LPB activation evoked two-phase changes, an initial increase followed by decline from baseline value, in SNA via resetting of the ABR central arc. LPB may contribute greatly to stabilize AP in response to volume status.

Differences in autonomic balance in patients with cardioinhibitory and vasodepressor type of reflex syncope during head-up tilt test and active standing

OBJECTIVE

To improve the pathophysiological understanding of cardioinhibitory and vasodepressor reflex syncope, by evaluating orthostatic effects on electrical and hemodynamic variables. To unravel the pathogeneses further, we studied these effects during both the passive head-up tilt test and the active standing test.

BACKGROUND

The current knowledge of the compromised autonomic balance in patients with reflex syncope is limited. The orthostatic responses to head-up tilt test in cardioinhibitory and vasodepressor patients differ, suggesting different pathogeneses; however, the more physiological active standing test represents daily life situations better.

METHODS

We included 74 patients; 36 cardioinhibitory and 38 vasodepressor. Patients were compared with respect to vascular hemodynamics and heart rate variability during the change from supine to upright position. Resting electrolytes, brain natriuretic peptide (BNP), pro-atrial natriuretic peptide (pro-ANP), and the C-terminal of pro-vasopressin, copeptin were measured.

RESULTS

Resting systolic blood pressure was higher in cardioinhibitory (117.8 ± 15.7 mmHg) than in vasodepressor patients (109.1 ± 15.3 mmHg, p < 0.001). Changes in heart rate tended to be smaller in cardioinhibitory patients (6.7 ± 9.8 vs. 10.7 ± 8.9 bpm, p = 0.056). Heart rate variability was lower and changed less in cardioinhibitory patients (p < 0.05). Cardioinhibitory patients had higher pro-ANP levels (63.5 ± 18.8 pM) compared to vasodepressor patients (54.2 ± 31.4 pM, p = 0.018). Responses during active standing were attenuated compared to head-up tilt in both groups.

CONCLUSIONS

This study demonstrated that cardioinhibitory patients had higher blood pressure, attenuated hemodynamic responses, and reduced autonomic regulation compared to vasodepressor patients. Furthermore, cardioinhibitory patients showed a sympathetic predominance in their modulation of autonomic responses. Orthostatic responses induced by active standing were modest and did not sufficiently explain potential pathophysiological differences between cardioinhibitory and vasodepressor patients.

Sympathetic overactivity in patients with pulmonary stenosis and improvement after percutaneous balloon valvuloplasty

OBJECTIVE

Percutaneous balloon valvulotomy (PBV) is the procedure of choice for the treatment of valvular pulmonary stenosis (PS) with similar results comparable to surgical valvotomy but less invasive.

METHODS AND RESULTS

Twenty-seven consecutive patients with PS being evaluated for PBV were enrolled in the study. Peak instantaneous transvalvular gradient, right ventricle (RV) diameter, mean atrial pressures, RV systolic pressure (RVSP), pro-brain natriuretic peptide (proBNP) levels significantly decreased immediately after PBV. Regarding heart rate variability (HRV) parameters, mean HR (heart rate), LF (low frequency) day and night, LF/HF day and night significantly decreased and standard deviation of all NN intervals (SDNN), root mean square of successive differences (RMSSD), P number of NN intervals that differed by more than 50 ms from adjacent interval divided by the total number of all NN intervals (PNN50), HF (High frequency) day and night significantly increased 1 day after PBV and these changes were shown to be preserved at the first month. The increase in SDNN was correlated with the decrease in right atrial pressure (RAP) (r =-0.5, P = 0.04); the increase in standard deviation of the 5-minute mean RR intervals (SDANN) was correlated with the decrease in proBNP (r =-0.4, P = 0.03).

CONCLUSIONS

Sympathetic overactivity and increased proBNP levels were associated with the symptomatic status of patients with PS. Associated with a decrease in atrial pressures and proBNP levels, PBV yielded a decrease in adrenergic overactivity in the patients with PS.

Responses of cardiac sympathetic nerve activity to changes in circulating volume differ in normal and heart failure sheep

Factors controlling cardiac sympathetic nerve activity (CSNA) in the normal state and those causing the large increase in activity in heart failure (HF) remain unclear. We hypothesized from previous clinical findings that activation of cardiac mechanoreceptors by the increased blood volume in HF may stimulate sympathetic nerve activity (SNA), particularly to the heart via cardiocardiac reflexes. To investigate the effect of volume expansion and depletion on CSNA we have made multiunit recordings of CSNA in conscious normal sheep and sheep paced into HF. In HF sheep (n = 9) compared with normal sheep (n = 9), resting levels of CSNA were significantly higher (34 +/- 5 vs. 93 +/- 2 bursts/100 heart beats, P < 0.05), mean arterial pressure was lower (76 +/- 3 vs. 87 +/- 2 mmHg; P < 0.05), and central venous pressure (CVP) was greater (3.0 +/- 1.0 vs. 0.0 +/- 1.0 mmHg; P < 0.05). In normal sheep (n = 6), hemorrhage (400 ml over 30 min) was associated with a significant increase in CSNA (179 +/- 16%) with a decrease in CVP (2.7 +/- 0.7 mmHg). Volume expansion (400 ml Gelofusine over 30 min) significantly decreased CSNA (35 +/- 12%) and increased CVP (4.7 +/- 1.0 mmHg). In HF sheep (n = 6) the responses of CSNA to both volume expansion and hemorrhage were severely blunted with no significant changes in CSNA or heart rate with either stimulus. In summary, these studies in a large conscious mammal demonstrate that in the normal state directly recorded CSNA increased with volume depletion and decreased with volume loading. In contrast, both of these responses were severely blunted in HF with no significant changes in CSNA during either hemorrhage or volume expansion.

Sympathetic overactivity in patients with rheumatic mitral stenosis

BACKGROUND

Mitral stenosis may increase sympathetic nervous activity by increasing left atrial pressure and reducing cardiac output. And elevated sympathetic nerve activity may be a risk factor for the development of clinical manifestations of mitral stenosis. In this study, we assessed the autonomic nervous system activity in patients with mitral stenosis by heart rate variability analysis and defined factors affecting autonomic functions.

METHODS AND RESULTS

Fifty-four patients with rheumatic mitral stenosis were compared with an age- and gender-matched control group composed of 42 healthy individuals. SDNN, RMSSD, PNN50, and HF were lower; mean heart rate (HR), LF and LF/HF ratio were higher in the patients with mitral stenosis compared to the control group. SDNN was correlated positively with left ventricle ejection fraction (LVEF), negatively with mitral valve area, left atrial (LA) diameter, and duration of symptoms. RMSSD was correlated positively with mean transmitral gradient, negatively correlated with age; PNN50 was correlated negatively with mitral valve area and positively correlated with transmitral gradient. LF was positively and HF was negatively correlated with LA diameter; LF was correlated positively, and HF was negatively correlated with duration of symptoms. LF/HF ratio was positively correlated with LA diameter and duration of symptoms, negatively with LVEF and mean valve area.

CONCLUSION

As a result, sympathetic nervous system activity is increased in patients with mitral stenosis and sympathetic overactivity worsens their symptoms. Most significant factors that affect autonomic functions in these patients are left atrial dilatation and mitral valve area.

Differential sympathetic nerve and heart rate spectral effects of nonhypotensive lower body negative pressure

Lower body negative pressure (LBNP; -5 and -15 mmHg) was applied to 14 men (mean age 44 yr) to test the hypothesis that reductions in preload without effect on stroke volume or blood pressure increase selectively muscle sympathetic nerve activity (MSNA), but not the ratio of low- to high-frequency harmonic component of spectral power (P(L)/P(H)), a coarse-graining power spectral estimate of sympathetic heart rate (HR) modulation. LBNP at -5 mmHg lowered central venous pressure and had no effect on stroke volume (Doppler) or systolic blood pressure but reduced vagal HR modulation. This latter finding, a manifestation of arterial baroreceptor unloading, refutes the concept that low levels of LBNP interrogate, selectively, cardiopulmonary reflexes. MSNA increased, whereas P(L)/P(H) and HR were unchanged. This discordance is consistent with selectivity of efferent sympathetic responses to nonhypotensive LBNP and with unloading of tonically active sympathoexcitatory atrial reflexes in some subjects. Hypotensive LBNP (-15 mmHg) increased MSNA and P(L)/P(H), but there was no correlation between these changes within subjects. Therefore, HR variability has limited utility as an estimate of the magnitude of orthostatic changes in sympathetic discharge to muscle.

Reducing cardiac filling pressure lowers norepinephrine spillover in patients with chronic heart failure

BACKGROUND

We studied the cardiac sympathetic response to selective unloading of cardiopulmonary baroreceptors in subjects with normal left ventricular (LV) function and congestive heart failure (CHF).

METHODS AND RESULTS

Eight patients with normal LV function (age 57+/-5 years, ejection fraction 58+/-2%) and 8 patients with CHF (age 60+/-2 years; ejection fraction 19+/-2%) were studied. Instrumentation consisted of an arterial line, a pulmonary artery catheter, and a coronary sinus thermodilution catheter. The radiotracer technique was used for measurement of cardiac norepinephrine spillover (CANESP) and total-body norepinephrine spillover. Lower-body negative pressure (LBNP) was applied at 2 levels: nonhypotensive and hypotensive LBNP. Nonhypotensive LBNP reduced filling pressures significantly in both groups. Arterial pressure did not change. This reduction in filling pressures caused a significant reduction in CANESP in the CHF group (from 167+/-53 to 125+/-37 pmol/min, P<0.05) but no change in the normal LV function group. Hypotensive LBNP caused a significant increase in CANESP in the normal group (73+/-13 vs 122+/-27 pmol/min, P<0.05) but no significant change in those with CHF.

CONCLUSIONS

We conclude that selective reduction in filling pressures lowers cardiac norepinephrine spillover in patients with CHF. These findings suggest that a goal of CHF management should be to reduce cardiac filling pressures while avoiding systemic hypotension.

Differential effect of acute baroreceptor unloading on cardiac and systemic sympathetic tone in congestive heart failure

OBJECTIVES

The present study was designed to identify the hemodynamic factor or factors that reflexly contribute to activation of the cardiac sympathetic nerves in patients with severe congestive heart failure (CHF).

BACKGROUND

We and others have previously shown that activation of the sympathetic nervous system is a key feature of CHF in humans. Furthermore, the degree of sympathetic activation shows marked regional heterogeneity and is most pronounced in the heart. Recent studies have shown a significant positive relation between pulmonary artery pressure and the magnitude of cardiac sympathetic activation. Of particular importance, the degree of cardiac sympathoexcitation has also been shown to be strongly associated with mortality in CHF.

METHODS

We assessed total systemic and cardiac sympathetic activity (norepinephrine [NE] spillover method) in nine patients with severe CHF and significantly elevated pulmonary artery pressure (mean [+/-SEM] pulmonary artery pressure 46 +/- 3 mm Hg) at rest and during a titrated infusion of sodium nitroprusside (SNP).

RESULTS

SNP infusion significantly reduced mean arterial blood pressure, pulmonary artery pressure and pulmonary capillary wedge pressure. During SNP infusion, the total body NE spillover rate (NESR) increased (from 7.9 +/- 1.7 to 11.2 +/- 3.1 nmol/min, p < 0.01), whereas the cardiac NESR decreased (from 522 +/- 86 to 409 +/- 71 pmol/min, p < 0.05). The ratio of cardiac/total NE spillover was also substantially reduced (from 7.8 +/- 1.3 to 4.9 +/- 0.9%, p < 0.001).

CONCLUSIONS

There is a directionally opposite change in whole-body (increase) and cardiac (reduction) sympathetic nervous activity during SNP infusion, most likely due to unloading of arterial baroreceptors and specific cardiopulmonary baroreceptors, respectively, in severe CHF. These observations support the concept of a positive feedback relation between pulmonary artery pressure/filling pressure and cardiac sympathetic tone in CHF and serve to reinforce the importance of vasodilator therapy in this condition.

Influences of the sympathetic and parasympathetic nerve transection on cardiovascular reflexes induced by volleys in the IXth nerve fibers of rat

The effect of vagotomy and sympathectomy on cardio-acceleratory and arterial hypertensive reflexes evoked by electrical stimulation of the rat glossopharyngeal (IXth) nerve was studied in relation to changes in baseline heart rate and arterial blood pressure. Uni- and bi-lateral transection of the cervical vagal trunk brought about augmentation of baseline heart rate, accompanied by a depression in reflex tachycardia; the amount of depression being inversely related to that of an increase in baseline heart rate. The latter increased more after a right vagotomy than after a left vagotomy. No appreciable change in reflex hypertension as well as in baseline blood pressure was observed by different types of vagotomy. For unilateral sympathectomy, semilunar cordotomy caudal to the obex was performed. It was found that right semilunar cordotomy significantly depressed the magnitude of cardio-acceleratory and arterial hypertensive reflexes in association with a significant decrease in baseline heart rate, though there was a less pronounced decrease in baseline blood pressure. The result obtained by a left semilunar cordotomy was similar to that of a right semilunar cordotomy, except that the decrease in reflex tachycardia was very small and statistically insignificant. Thus, the efferent activities in vagus and sympathetic nerves were more effective on the right than on the left side, in modifying reflex tachycardia and baseline heart rate, whereas, right and left sympathetic efferent outflows were equally effective in depressing the reflex increase in blood pressure.

Respiratory sinus arrhythmia: autonomic origins, physiological mechanisms, and psychophysiological implications

Respiratory sinus arrhythmia (RSA) is being used increasingly in psychophysiological studies as an index of vagal control of the heart and may be among the most selective noninvasive indices of parasympathetic control of cardiac functions. A comprehensive understanding of RSA, however, requires an appreciation of its multiple autonomic and physiological origins. We review the physiological bases of RSA and show that RSA arises from multiple tonic and phasic processes of both central and peripheral origin. These underlying mechanisms are at least partially differentiated, have distinct dynamics and consequences, and may be differentially sensitive to behavioral and cognitive events. These multiple mechanisms are relevant for psychophysiological studies of RSA, and a thorough understanding of RSA can only be achieved through an appreciation of the dynamics of its underlying origins. There is a distinction between the psychophysiological and neurophysiological domains, and conceptual and empirical bridges between these domains are needed.

Multiple modes of operation of cardiac autonomic control: development of the ideas from Cannon and Brooks to the present

Since Cannon's time much emphasis has been placed on reciprocal control of the organ function by the autonomic nerves while other modes of control has been neglected. In our laboratory, prompted by Dr. Brooks, we initiated the study of the autonomic control of the heart by recording simultaneously the activity of cardiac sympathetic and parasympathetic nerves under a variety of conditions in chloralose anesthetized dogs. We have demonstrated that the central nervous system exercises reciprocal as well as non-reciprocal pattern of control over the two arms of the autonomic outflows under different physiological and behavioral conditions, and that each mode of control has an important functional significance.

Simultaneous measurements of cardiac noradrenaline spillover and sympathetic outflow to skeletal muscle in humans

1. Muscle sympathetic nerve activity (MSA) was recorded in the peroneal nerve at the knee by microneurography in ten healthy subjects and determinations were made simultaneously of intra-arterial blood pressure, and whole-body and cardiac noradrenaline spillover to plasma. Measurements were made at rest, during isometric handgrip at 30% of maximum power and during stress induced by forced mental arithmetic. 2. At rest there were significant positive correlations between spontaneous MSA (expressed as number of sympathetic bursts min-1) and both spillover of noradrenaline from the heart and concentration of noradrenaline in coronary sinus venous plasma. 3. Both isometric handgrip and mental arithmetic led to sustained increases of blood pressure, heart rate and MSA. Plasma concentrations of noradrenaline and spillover of noradrenaline (total body and cardiac) increased. In general the effects were more pronounced during handgrip than during stress. 4. When comparing effects during handgrip and stress the ratio between the fractional increases of MSA and cardiac noradrenaline spillover were significantly greater during handgrip. 5. The data suggest (a) that there are proportional interindividual differences in the strength of resting sympathetic activity to heart and skeletal muscle which are determined by a common mechanism and (b) that handgrip and mental stress are associated with differences in balance between sympathetic outflows to heart and skeletal muscle.

The role of cardiovascular and muscle afferent systems in control of body water balance

Influences of afferent inputs from cardiovascular and muscle receptors on the activities of neurosecretory neurons in the hypothalamus, which secrete vasopressin (ADH) were studied. Recordings were made from identified neurosecretory neurons in the supraoptic (SON) and paraventricular nuclei (PVN) of cats and rats. Activation of baroreceptors in the carotid sinus and aortic arch and atrial receptors inhibited SON and PVN neuron activities, while activation of chemoreceptors in the carotid sinus excited them. Repetitive electrical stimulation of the carotid sinus and aortic nerves showed that weak stimulation produced excitation and stronger stimulation produced inhibition of SON and PVN neurons. Electrical stimulation of these nerves and the nucleus tractus solitarius (NTS) by a single or short train of pulses showed that 'fast' and 'slow' pathways between the NTS and the SON existed, while these two types of pathways were not observed between the NTS and the PVN. Evidence of direct connections from the NTS to the PVN was found by means of antidromic stimulation of the PVN. Electrical stimulations of group I afferent fibers from the gastrocnemius muscle did not change SON neuron discharges, while activation of group III and IV afferent fibers excited them. Injection of chemicals (NaCl, KCl, bradykinin) into arteries supplying the muscle excited SON neurons. The excitation disappeared after section of the muscle nerves. The results indicated that activation of small afferents from the muscle excites the SON neurons, leading to an increase in vasopressin secretion. All these studies show that afferent inputs from receptors in the cardiovascular system and in the muscle have modulatory effects on neurosecretory neurons, and participate in control of body water balance by regulating vasopressin secretion from the neurohypophysis.

Influence of atrial mechanoreceptors upon the sympathetic efferent activity elicited during a cardiogenic chemoreflex in the dog

Two different cardiogenic reflexes were studied for their effects upon the activity recorded from the thoracic cardiac nerves of 7 chloralose-anesthetized dogs. Sympathetic efferent activity in 5 right and 7 left anterior ansa multifiber preparations was quantitatively analyzed. A cardiogenic hypertensive chemoreflex was induced by administration of serotonin (100 micrograms/ml, 2 ml) into the left atrium before, during and after inflation of balloons in the right and/or left atrial appendages. Balloon inflations alone resulted in heart rate increases of 5-20 bpm but arterial blood pressure, pulse pressure and end diastolic pressure were not significantly affected. Control serotonin-induced efferent discharges were 174 +/- 10% (mean +/- S.E.M.) of resting activity with a duration of 5.2 +/- 0.4 s. During inflation of both atrial balloons, the serotonin-induced efferent discharges were 165 +/- 14% (N.S.) of resting activity with a duration of 7.8 +/- 0.7 s (P less than 0.01). Following release of the balloons, the serotonin-induced discharges were 152 +/- 17% (N.S.) of resting activity with a duration of 4.9 +/- 0.4 s (N.S.). These results suggest that afferent signals from atrial mechanoreceptors can act to increase the duration but not the total intensity of the efferent sympathetic discharges elicited during a cardiogenic chemoreflex.

Neural control of the heart: significance of double innervation re-examined

Our recent studies on the activity of cardiac sympathetic and vagal nerves in anesthetized dogs have revealed that control of these two autonomic nerves having 'antagonistic' action on the heart is not always reciprocal; both reciprocal and non-reciprocal controls exist in reflex as well as in adaptive responses. We have evidence which also suggests the functional significance of non-reciprocal control. The co-activation of both autonomic efferents to the heart is particularly useful in increasing cardiac output as, for example, in the chemoreceptor reflex. On the other hand, under 'normal' conditions the respiration related rhythmic activity of both sympathetic and vagal nerves is almost completely reciprocal. The loss of the reciprocity occurs under subnormal conditions in which sympathetic discharges shift so that both nerves fire simultaneously. This tends to reduce respiration-induced fluctuations in heart rate. During cardiac rhythms seen in the efferent nerve activity, the peak of vagal firing does not strictly coincide with the inhibitory period of the sympathetic activity; vagal impulses reach the heart slightly earlier. However, when the heart rate is irregular and slow, the vagal activity shifts in time so that it coincides more definitely with the period of sympathetic nerve inhibition. This tends to further augment rhythmic fluctuations in heart rate. These studies show that the centers involved employ both reciprocal and non-reciprocal control of cardial nerve activity in reactions regulating heart function. The relationship between discharge patterns of both autonomic nerves is involved in production of fluctuation of heart rate in normal as well as in subnormal conditions.

Patterns of reflex action, their autonomic components, and their behavioral significance

This is a discussion of the evidence that autonomic system reflexes are invariably triggered by stimuli that evoke both simple and complex somatic reflexes. Stimuli not only initiate reverberatory activity but sequences of reinforcements from higher centers. Reflexes come in chains, one reaction triggering another. Reflex actions are patterned, and these patterns are appropriate to the behavioral requirement indicated by stimuli. Reciprocal action produces a powerful braking or stimulatory action. Coactivation or synergic action of autonomic "antagonists" also occurs and can effect a greater cardiac output in certain vascular reflexes that can sympathetic action alone. Coactivation of vagus and sympathetic fibers occurring in reflex action or resulting from stimulation of specific areas in the hypothalamus has a physiologic importance to attainment of maximal cardiac function. Finally, two additional suggestions are made: (1) that the rhythmic reflex feedback discharge of the vagus from the baroreceptors aids in maintaining the regularity of cardiac rhythm as by interaction of oscillators and (2) that the autonomic system, due to its early and often conditioned response, can be determinative as well as modulatory and supportive of behavior.

Functional significance of coactivation of vagal and sympathetic cardiac nerves

Simultaneous recording of activity in the vagal and sympathetic supplies to the heart has revealed that in reflexly and centrally evoked activity these two "antagonists" do not necessarily change action reciprocally. Coactivation occurs in chemoreceptor reflexes and related reactions, upon stretching of the sinoatrial nodal region of the right atrium and when certain hypothalamic regions are stimulated. The objective of the present work was to assay the physiological importance of coactivation of the two potentially antagonistic cardiac nerves in anesthetized dogs. Output from the heart was monitored by recording volume flow in the thoracic aorta just below the aortic arch; cardiac contractility was measured as left ventricular dp/dt. Tape recordings of vagus and sympathetic nerve activity during chemoreceptor and baroreceptor reflexes, during reciprocal and nonreciprocal changes produced by hypothalamic stimulation, and during hypoxia and hypercapnia were used to trigger stimulators feeding a stimulus per action potential to cardiac vagus and sympathetic nerves after central connections were cut. The vagus stimulation alone produced a decrease in aortic blood flow; stimulation of the sympathetic nerve alone resulted in increased aortic blood flow. Simultaneous stimulation of vagus and sympathetic, however, produced an even greater cardiac output (measured by aortic blood flow). Intermediate degrees of heart rate and strength of myocardial contraction were maintained in coactivation. Obviously, an association of increased vagus and sympathetic actions, which can be effected reflexly or by action of higher centers, is of physiological benefit. In control reactions that relate cardiac function to body need, both reciprocal and synergistic actions (coactivation) of cardiac nerves are used.

Cardiovascular reflexes and interrelationships between sympathetic and parasympathetic activity

Simultaneous recordings from vagal and sympathetic nerve fibers innervating the heart have enabled us to study relationships between activity in these two autonomic nerves. Our recent studies, as well as those of others, are reviewed with respect to tonic activity and reflex actions in these two autonomic efferents. Discharges of the two nerves in relation to blood pressure pulses and to respiratory cycles are reciprocal. During slower fluctuations of hemodynamic changes which occur spontaneously, such as during Mayer waves, a reciprocal relationship between activity in the two autonomic nerves is also observed. On the other hand, in reflex responses both reciprocal and non-reciprocal patterns of reactions produced by stimulations of the hypothalamus showed varied relationships between responses in the two autonomic outflows. The functional significance of the interrelationships of the activity pattern observed in vagal and sympathetic nerves is discussed with respect to control of cardiac functions.

Introduction: control of the autonomic nervous system and the multiple integrative roles it plays in regulating cardiovascular functions

Interest in the control and action of the autonomic nervous system on cardiovascular system function has waxed and waned over the years [3]. At present it has reached a new intensity. Old concepts are being reinvestigated and new fields are being explored. As is usually the case in physiology, many are following a reductionist approach and studying the minutia of structure and function of autonomic neurons and neuron complexes. Others are approaching the totality and dealing with the autonomically controlled cardiovascular components of reactions and behavior. The topics chosen for discussion in this symposium are representative of those under intensive consideration at the present time.

Control of reciprocal and non-reciprocal action of vagal and sympathetic efferents: study of centrally induced reactions

The mechanism of control of reciprocal and non-reciprocal action between parasympathetic and sympathetic nerves was investigated. Simultaneous recordings were made from both vagal and sympathetic nerves innervating the heart of responses evoked by hypothalamic stimulation in chloralose-anesthetized dogs. Four patterns of responses could be elicited by repetitive stimulations of various sites within the hypothalamus: (1) A reciprocal pattern of cardiac vagal and sympathetic discharges accompanied by a rise in blood pressure, heart rate and a 2 to 8-fold increase in muscle blood flow. The vagal activity completely ceased, while sympathetic discharges were greatly augmented. These changes occurred quickly and often lasted throughout the stimulation, preventing baroreceptor reflex from breaking through. This pattern is similar to the cardiovascular component of the "defense reaction". (2) A reciprocal pattern of discharges accompanied by a depressor response and bradycardia; the vagal discharges increased while those of the sympathetic efferents decreased. (3) A non-reciprocal pattern of response in which activity of the two efferents increased. The blood pressure was elevated and heart rate decreased. (4) A non-reciprocal pattern of action of the two efferents in which discharges of both nerves were depressed; the blood pressure decreased and heart rate increased. Factors affecting these responses patterns were found to be: (a) secondarily occurring reflex responses due to baroreceptor activation or deactivation. These baroreceptor-related reactions always showed reciprocal changes between vagal and sympathetic nerve activity; (b) the level of tonic activity of the autonomic nerves, e.g. during a high level of tonic activity inhibitory action became less effective while the excitatory effect was greater. Thus central states maintaining influenced the pattern of reaction and the relationship between activity in these two efferent nerves. Single pulse stimulations of the hypothalamic regions from which different patterns were evoked, when stimulated repetitively, yielded patterns which were basically reciprocal and biphasic; in vagus efferents an inhibitory phase was followed by an excitation of tonic activity, while in sympathetic an excitation was followed by an inhibition. The degree and duration of these excitatory and inhibitory phases differed, depending on the site of stimulation, but only to a minor degree, and the basic pattern remained unchanged. The implication of these findings is that the hypothalamus can organized both reciprocal and non-reciprocal patterns of activity in the two autonomic limbs in response to varied afferent stimuli. Probably only subtle changes in the central states or influences on neurons of the autonomic system suffice to result in different response pattern. No pattern of response is fixed within the central control complex.

Distension of pulmonary vein--left atrial junction: heart rate responses in conscious and anesthetized dogs

The cardiac chronotropic effects of distension of pulmonary vein-left atrial junction were investigated in conscious dogs and in dogs anesthetized with intravenous alpha-chloralose (100 mg/kg) or pentobarbital (30 mg/kg). All the experiments were made on trained, chronically instrumented, closed chest animals held in horizontal position. Inflation of a single small balloon in the junction elicited a tachycardic response both in conscious and chloralose-anesthetized dogs, while in pentobarbital anesthesia no change in heart rate was found. Contrary to reports of other investigators, no transient bradycardia was found, nor any correlation between predistension heart rate and the increase in heart rate due to the distension. It is suggested that the choice of anesthetic can be done only after its modifying effect on the parameter studied is known. Only the use of trained, chronically instrumented and conscious dogs will reveal this effect.

Reciprocal and non-reciprocal action of the vagal and sympathetic nerves innervating the heart

Simultaneous recordings were made from vagal and sympathetic fibers innervating the heart in dogs anesthetized with chloralose. Reciprocal relationship between the two autonomic nerves was clearly seen in the baroreceptor reflex. Stimulation of chemoreceptors, however, evoked non-reciprocal responses of the two nerves; at the onset of the chemoreceptor reflex cardiac vagal and sympathetic discharges both increased, then, as baroreceptors became excited due to a pressor response, sympathetic nerve activity suddenly decreased while vagal discharges remained high, indicating the appearance of the reciprocal action typifying the baroreceptor reflex. Decrease in ventilatory volume and a slight increase in end-expired CO2 level augmented greatly both vagal and sympathetic discharges. As the phrenic-locked activity of the two nerves (i.e. the activity in vagus nerve occurs only in the absence of phrenic bursts while sympathetic discharges increase with phrenic bursts) increased, the alternate discharges between the two nerves became more conspicuous and the heart rate fluctuated with the respiratory (phrenic) rhythm. Thus, strong reciprocity between vagus and sympathetic can result in an oscillatory heart rate. When ventilatory volume was increased, both nerve activities decreased below control level. Mild hypoxia had similar effects to hypercapnia though changes in nerve activity were greater. When coactivation of vagal and sympathetic nerve was produced in reflex action, changes in vagal discharges occurred earlier and faster than in the sympathetic fibers. The magnitude of change in vagus activity was also far greater. The elimination of afferents in the vagi, the aortic and sinus nerves reduced cardiac vagal activity greatly. However, discharges were still present and occurred between phrenic bursts, indicating that the vagal "tone" is maintained centrally as well as peripherally by input from receptors in the cardiovascular system. The physiological significance of reciprocal and non-reciprocal control of vagal and sympathetic nerves innervating the heart was discussed.

Influence of atrial stretch receptors on hypothalamic neurosecretory neurones

1. The effects of stimulation of atrial receptors on hypothalamic neurosecretory cells were investigated in anaesthetized dogs and cats. Atrial receptors were activated by stretching the left and the right atria while action potentials of individual neurosecretory neurones in the supraoptic (s.o.n.) and paraventricular (p.v.m.) nuclei of the hypothalamus were recorded. 2. Stretching the left atrium markedly decreased firing frequencies of 'antidromically identified' s.o.n. and p.v.n. neurones in dogs and cats; 98% of neurones in dogs and 70% in cats were thus inhibited. Heart rate accelerated following a transient reflexly induced deceleration. The blood pressure was not affected. The magnitude and duration of inhibitory effects produced by atrial stretch on s.o.n. and p.v.n. neurones paralleled changes observed in heart rate. Approximately one third of 'unidentified' cells in s.o.n. and p.v.n. (those which could not be activated antidromically) were inhibited by left atrium stretch. 3. Bilateral vagotomy abolished the effects on neurosecretory neurones as well as on heart rate produced by left atrial stretch, indicating that the vagus nerves carry afferent impulses responsible for the observed changes. 4. Stretching the right atrium did not produce changes in firing frequencies of s.o.n. and p.v.n. neurones in dogs and cats, although cardiac rate was increased considerably by such stretch. Stimulation of the left and right atria was tested in the same animal in succession to permit comparison of the diverse results. 5. Interaction of responses originating from atrial receptors, baro- and chemoreceptors were studied. During carotid occlusion or stimulation of chemoreceptors activity of the neurosecretory cells was greatly augmented. Left atrial stretch when then applied evoked slight or no reduction in activity in s.o.n. and p.v.n. neurones. These results were interpreted to mean that the influence of baro- or chemoreceptors on neurosecretory neurones probably is stronger than that originating from atrial receptors. 6. Unlike the observations made in rats, no rhythmically bursting neurones (phasic cells) were found in dogs and cats. However, atrial stretch sometimes evoked a rhythmic bursting pattern which appeared only during stretch. 7. It is concluded that activation of left atrial receptors, through afferent impulses carried mainly by vagus nerves, inhibits neurosecretory neurones in s.o.n. and p.v.n. The implications of this finding in relation to the role of ADH in the maintenance of blood volume control were discussed.