Defenselike patterns of spinal sympathetic outflow involving the 10-Hz and cardiac-related rhythms

Central cardiovascular hemodynamic response to unilateral handgrip exercise with blood flow restriction

AIM

Exercise training with blood flow restriction (BFR) increases muscle size and strength. However, there is limited investigation into the effects of BFR on cardiovascular health, particularly central hemodynamic load.

PURPOSE

To determine the effects of BFR exercise on central hemodynamic load (heart rate-HR, central pressures, arterial wave reflection, and aortic stiffness).

METHODS

Fifteen males (age = 25 ± 2 years; BMI = 27 ± 2 kg/m², handgrip max voluntary contraction-MVC = 50 ± 2 kg) underwent 5-min bouts (counter-balanced, 10 min rest between) of rhythmic unilateral handgrip (1 s squeeze, 2 s relax) performed with a moderate-load (60% MVC) with and without BFR (i.e., 71 ± 5% arterial inflow flow reduction, assessed via Doppler ultrasound), and also with a low-load (40% MVC) with BFR. Outcomes included HR, central mean arterial pressure (cMAP), arterial wave reflection (augmentation index, AIx; wave reflection magnitude, RM%), aortic arterial stiffness (pulse wave velocity, aPWV), and peripheral (vastus lateralis) microcirculatory response (tissue saturation index, TSI%).

RESULTS

HR increased above baseline and time control for all handgrip bouts, but was similar between the moderate load with and without BFR conditions (moderate-load with BFR =  + 9 ± 2; moderate-load without BFR =  + 8 ± 2 bpm, p < 0.001). A similar finding was noted for central pressure (e.g., moderate load with BFR, cMAP =  + 14 ± 1 mmHg, p < 0.001). No change occurred for RM% or AIx (p > 0.05) for any testing stage. TSI% increased during the moderate-load conditions (p = 0.01), and aPWV increased above baseline following moderate-load handgrip with BFR only (p = 0.012).

CONCLUSIONS

Combined with BFR, moderate load handgrip training with BFR does not significantly augment central hemodynamic load during handgrip exercise in young healthy men.

Sympathetic preganglionic neurons: properties and inputs

The sympathetic nervous system comprises one half of the autonomic nervous system and participates in maintaining homeostasis and enabling organisms to respond in an appropriate manner to perturbations in their environment, either internal or external. The sympathetic preganglionic neurons (SPNs) lie within the spinal cord and their axons traverse the ventral horn to exit in ventral roots where they form synapses onto postganglionic neurons. Thus, these neurons are the last point at which the central nervous system can exert an effect to enable changes in sympathetic outflow. This review considers the degree of complexity of sympathetic control occurring at the level of the spinal cord. The morphology and targets of SPNs illustrate the diversity within this group, as do their diverse intrinsic properties which reveal some functional significance of these properties. SPNs show high degrees of coupled activity, mediated through gap junctions, that enables rapid and coordinated responses; these gap junctions contribute to the rhythmic activity so critical to sympathetic outflow. The main inputs onto SPNs are considered; these comprise afferent, descending, and interneuronal influences that themselves enable functionally appropriate changes in SPN activity. The complexity of inputs is further demonstrated by the plethora of receptors that mediate the different responses in SPNs; their origins and effects are plentiful and diverse. Together these different inputs and the intrinsic and coupled activity of SPNs result in the rhythmic nature of sympathetic outflow from the spinal cord, which has a variety of frequencies that can be altered in different conditions.

The posterior vermis of the cerebellum selectively inhibits 10-Hz sympathetic nerve discharge in anesthetized cats

We studied the changes in inferior cardiac sympathetic nerve discharge (SND) and mean arterial pressure (MAP) produced by aspiration or chemical inactivation (muscimol microinjection) of lobule IX (uvula) of the posterior vermis of the cerebellum in baroreceptor-denervated and baroreceptor-innervated cats anesthetized with urethane. Autospectral analysis was used to decompose SND into its frequency components. Special attention was paid to the question of whether the experimental procedures affected the rhythmic (10-Hz and cardiac-related) components of SND. Aspiration or chemical inactivation of lobule IX produced an approximately three-fold increase in the 10-Hz rhythmic component of SND (P < or = 0.05) in baroreceptor-denervated cats. Total power (0- to 20-Hz band) was unchanged. Despite the absence of a change in total power in SND, there was a statistically significant increase in MAP. In baroreceptor-innervated cats, neither aspiration nor chemical inactivation of the uvula caused a significant change in cardiac-related or total power in SND or MAP. These results are the first to demonstrate a role of cerebellar cortical neurons of the posterior vermis in regulating the frequency composition of naturally occurring SND. Specifically, these neurons selectively inhibit the 10-Hz rhythm-generating network in baroreceptor-denervated, urethane-anesthetized cats. The functional implications of these findings are discussed.

Role of serotonergic input to the ventrolateral medulla in expression of the 10-Hz sympathetic nerve rhythm

We studied the changes in inferior cardiac sympathetic nerve discharge (SND) produced by unilateral microinjections of 5-hydroxytryptamine (5-HT) receptor agonists and antagonists into the ventrolateral medulla (VLM) of urethane-anesthetized, baroreceptor-denervated cats. Microinjection of the 5-HT2 receptor antagonist LY-53857 (10 mM) into either the rostral or caudal VLM significantly reduced (P < or = 0.05) the 10-Hz rhythmic component of basal SND without affecting its lower-frequency, aperiodic component. The selective depression of 10-Hz power was accompanied by a statistically significant decrease in mean arterial pressure (MAP). Microinjection of LY-53857 into the VLM also attenuated the increase in 10-Hz power that followed tetanic stimulation of depressor sites in the caudal medullary raphé nuclei. Microinjection of the 5-HT2 receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)2-amino-propane (DOI; 10 microM) into the VLM selectively enhanced 10-Hz SND, and intravenous DOI (1 mg/kg) partially reversed the reduction in 10-Hz SND produced by 5-HT2 receptor blockade in the VLM. Microinjection of the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OHDPAT; 10 mM), into either the rostral or caudal VLM also selectively attenuated 10-Hz SND and significantly reduced MAP. The reduction in 10-Hz SND produced by 8-OHDPAT was partially reversed by intravenous WAY-100635 (1 mg/kg), which selectively blocks 5-HT1A receptors. These results support the view that serotonergic inputs to the VLM play an important role in expression of the 10-Hz rhythm in SND.

Methods of analysis and physiological relevance of rhythms in sympathetic nerve discharge

1. Like virtually all other physiological control systems, the sympathetic nervous system controlling cardiovascular function is characterized by the presence of rhythmic activity. Despite the prevalence of rhythms, their function is often not obvious, which leads to the question, what can one learn about the neural control of autonomic function by studying sympathetic nervous system rhythms? 2. Sympathetic nerve discharge (SND) is characterized by a mixture of periodicities ranging between approximately 0.04 and 10 Hz, depending on the physiological conditions, type of nerve being analysed and the species. The present article illustrates why frequency domain (power density spectral) analysis is more suitable than time domain (autocorrelation) analysis to quantify a complex signal (i.e. one with multiple frequency components) such as SND. 3. The present article entertains the possibilities that rhythmic activity may lead to more effective activation of sympathetic neurons than randomly occurring activity, that rhythmicity is important for coordinating activity in different sympathetic nerves and in formulating complex cardiovascular response patterns and that sympathetic rhythmicity may help maintain homeostasis.

Role of ventrolateral medulla in generating the 10-Hz rhythm in sympathetic nerve discharge

We recorded changes in right inferior cardiac and either left inferior cardiac or left vertebral sympathetic nerve discharge (SND) produced by unilateral microinjections of GABA-A and excitatory amino acid (EAA) receptor antagonists into the ventrolateral medulla (VLM) of urethane-anesthetized, baroreceptor-denervated cats. Unilateral microinjections of GABA-A receptor antagonists, SR-95531 or bicuculline, into single tracks in VLM anywhere between 1 and 5 mm rostral to the obex eliminated or markedly reduced 10-Hz power in SND on both sides of the body. Low-frequency components (<6 Hz) of SND were unaffected. Complete blockade of the 10-Hz rhythm occurred with a dose of SR-95531 as low as 6.25 pmol in a 50-nl volume. Unilateral microinjections of the nonselective EAA receptor antagonist, kynurenate (KYN; 7.5 nmol), into the caudal or rostral VLM significantly reduced, but did not eliminate, 10-Hz SND ipsilateral to the injection sites, while 10-Hz SND contralateral to the injection sites was not significantly changed. These observations suggest that 1) GABAergic transmission in VLM is critical for generation of the 10-Hz rhythm, 2) the caudal and rostral portions of VLM act together to generate the 10-Hz rhythm, and 3) 10-Hz rhythm generation depends, at least in part, on tonic or phasic excitatory drive to GABAergic interneurons in caudal VLM and presympathetic neurons in rostral VLM. The data also suggest that pathways interconnecting the two halves of the brain stem play an important role in promoting 10-Hz rhythm generation.

Cardiovascular effects of noradrenaline microinjected into the dorsal periaqueductal gray area of unanaesthetized rats

The periaqueductal grey area (PAG) is a mesencephalic region that is involved in the modulation of cardiovascular changes associated with behavioural responses. Among the neurotransmitters present in the PAG, noradrenaline (NA) is also known to be involved in central nervous system cardiovascular regulation. In the present study we report the cardiovascular effects of the microinjection of NA into the dorsal portion of the PAG (dPAG) of unanaesthetized rats and the peripheral mechanism involved in their mediation. Injection of NA in the dPAG of unanaesthetized rats evoked a dose-dependent pressor response accompanied by bradycardia. The magnitude of the pressor responses was higher at more rostral sites in the dPAG and decreased when NA was injected into the caudal portion of the dPAG. The responses to NA were markedly reduced in urethane-anaesthetized rats. The pressor response was potentiated by i.v. pretreatment with the ganglion blocker pentolinium and blocked by i.v. pretreatment with the vasopressin antagonist dTyr(CH2)5(Me)AVP. The results suggest that activation of noradrenergic receptors within the dPAG can evoke pressor responses, which are mediated by acute vasopressin release.

An unusual form of phase walk in a system of coupled oscillators

The authors describe an unusual form of phase walk (i.e., a progressive change in phase angle between coupled oscillators) using the 10-Hz rhythmic discharges of the inferior cardiac and vertebral postganglionic sympathetic nerves (CN and VN, respectively) in hypercapnic, baroreceptor-denervated, and vagotomized cats anesthetized with urethane. Unlike phase walk ascribable to weakened coupling (desynchronization of oscillators), the phase walk of VN 10- Hz activity relative to CN10-Hz activity 1) recurred on the time scale of the respiratory cycle, 2) was bidirectional with CN-VN phase angle increasing during expiration and decreasing during inspiration, and 3) occurred over a range equivalent to one-half the period of the 10-Hz rhythm rather than a full cycle. Moreover, this form of phase walk occurred during strong coupling of the 10-Hz oscillators, as reflected by CN-VN coherence values approaching 1.0. The authors propose that the bidirectional phase walk reflects a state of strong coupling of the 10-Hz oscillators controlling the CN and VN, the angle of which is reset from cycle to cycle by the continuously changing level of activity in their respiratory inputs. In addition, the data demonstrate that frequency and amplitude modulation of sympathetic nerve discharge can be independently regulated by respiratory inputs.

Frequency response of renal sympathetic nervous activity to aortic depressor nerve stimulation in the anaesthetized rat

1. The contribution of central baroreceptor reflex pathways to the dynamic regulation of sympathetic nervous activity (SNA) has not been properly examined thus far. The aim of this study was to characterize the transfer function of the central arc of the baroreceptor reflex (from baroreceptor afferent activity to SNA) over a wide range of frequencies. 2. In nine baroreceptor-intact and six sino-aortic baroreceptor-denervated rats anaesthetized with urethane, the renal SNA was recorded while applying sinusoidal stimulation to the aortic depressor nerve at 26 discrete frequencies ranging from 0.03 to 20 Hz. At each modulation frequency, cross-power spectrum analysis using a fast Fourier transform algorithm was performed between the stimulation and renal SNA, which provided the transfer function of the central arc. 3. In both baroreceptor intact and denervated rats, the transfer gain increased by a factor of about three between 0.03 and 1 Hz. At higher frequencies, the gain decreased but remained above the static gain of the system up to 12 Hz. There was a slight phase lead up to 0.4 Hz, then a continuously increasing phase lag. A three-element linear model satisfactorily described the experimental transfer function. The model combined a derivative gain (corner frequency approximately 0.15 Hz), an overdamped second-order low-pass filter (natural frequency approximately 1 Hz) and a fixed time delay (approximately 100 ms). 4. These results indicate that the central arc of the baroreceptor reflex shows derivative properties that are essential for compensating the filtering of fast oscillations of baroreceptor afferent activity and thus for the generation of fast oscillations of renal SNA (e.g. those related to the cardiac cycle).

Differential control of sympathetic outflow

With advances in experimental techniques, the early views of the sympathetic nervous system as a monolithic effector activated globally in situations requiring a rapid and aggressive response to life-threatening danger have been eclipsed by an organizational model featuring an extensive array of functionally specific output channels that can be simultaneously activated or inhibited in combinations that result in the patterns of autonomic activity supporting behavior and mediating homeostatic reflexes. With this perspective, the defense response is but one of the many activational states of the central autonomic network. This review summarizes evidence for the existence of tissue-specific sympathetic output pathways, which are likely to include distinct populations of premotor neurons whose target specificity could be assessed using the functional fingerprints developed from characterizations of postganglionic efferents to known targets. The differential responses in sympathetic outflows to stimulation of reflex inputs suggest that the circuits regulating the activity of sympathetic premotor neurons must have parallel access to groups of premotor neurons controlling different functions but that these connections vary in their ability to influence different sympathetic outputs. Understanding the structural and physiological substrates antecedent to premotor neurons that mediate the differential control of sympathetic outflows, including those to noncardiovascular targets, represents a challenge to our current technical and analytic approaches.

Sympathetic nerve and cardiovascular responses to chemical activation of the midbrain defense region

The changes in mean arterial pressure (MAP), renal (RBF) and femoral (FBF) blood flows, and inferior cardiac (CN) and vertebral nerve (VN) sympathetic nerve discharges (SND) produced by chemical activation (d,l-homocysteic acid) of the midbrain periaqueductal gray (PAG) were compared in baroreceptor-denervated and -innervated cats anesthetized with urethan. Defenselike cardiovascular responses in both states were similar in magnitude and consisted of increased MAP and FBF and decreased RBF; however, the nerve responses differed. In baroreceptor-denervated cats, PAG activation increased CN 10-Hz activity, decreased VN 10-Hz activity, and lengthened the CN-VN phase angle. In baroreceptor-innervated cats in which the rhythm in SND was cardiac related, PAG activation increased CN activity, but VN activity and the CN-VN phase angle were unchanged. These results demonstrate that chemical activation of PAG neurons induces differential patterns of sympathetic outflow generally consistent with accompanying defenselike cardiovascular responses. However, the mechanisms responsible for the changes in 10-Hz and cardiac-related SND appear to be different.

A defence-like reaction: an emergent property of a system of coupled non-linear oscillators

1. The present paper deals with the central mechanisms responsible for the defence-like differential pattern of spinal sympathetic outflow elicited by electrical activation of the midbrain periaqueductal grey (PAG) in urethane-anaesthetized, baroreceptor-denervated cats. The pattern is characterized by increases in the 10 Hz discharges of the inferior cardiac (CN) and renal (RN) sympathetic nerves and a decrease in 10 Hz activity of the vertebral sympathetic nerve (VN) that supplies vasoconstrictor outflow to the forelimb. 2. The model used to explain this pattern is based on the self-organizing properties of a system of coupled brainstem oscillators rather than the activation of point-to-point hard-wired connections leading to increases in sympathetic outflow to some targets and decreases to others. 3. The fact that VN 10 Hz activity was inhibited by PAG stimulus frequencies equal to or just above, but not just below, that of the free-running (control) rhythm argues against a 'hard-wired' model. 4. The evidence supporting the hypothesis that the defence-like pattern is an emergent property of a system of coupled oscillators includes changes in the phase lag of VN 10 Hz activity relative to that in the CN, temporal correlation of the changes in phase angle and 10 Hz powers and the direct relationship between the magnitude of the change in phase angle and the degree to which PAG stimulation reciprocally affected the 10 Hz discharges of the CN and VN. 5. It is proposed that changes in phase angle reflect the reorganization of the coupling of 10 Hz oscillators and that such changes in state lead to differential patterns of spinal sympathetic outflow.

"Rapid" rhythmic discharges of sympathetic nerves: sources, mechanisms of generation, and physiological relevance

Like virtually all other physiological control systems, the sympathetic nervous system controlling cardiovascular function is characterized by the presence of rhythmic activity. These include slow rhythms with frequencies at or below that of the respiration and rapid rhythms with frequencies at or above that of the heart beat. The rapid rhythms are the subject of this review. The specific questions entertained are as follows: (1) Are the rapid cardiac-related and 10-Hz rhythms inherent to central sympathetic networks, or are they imposed on sympathetic nerve discharge (SND) by extrinsic periodic inputs? (2) Does basal SND arise from an anatomically circumscribed "vasomotor center" composed of pacemaker neurons in the rostral ventrolateral medulla or from an anatomically distributed network oscillator composed of different types of brainstem neurons, none of which necessarily have intrinsic pacemaker properties? (3) Are the rapid rhythms generated by single circuits or by systems of coupled oscillators, each with a separate target? (4) Are the rapid rhythms in SND simply by-products of the sympathetic generating mechanisms, or do they subserve selective and special functions, such as the formulation of differential patterns of spinal sympathetic outflow that support particular behaviors? The controversial aspects of these issues and the state-of-the-art analytical methods used to study them are stressed in this review.

Differential pattern of spinal sympathetic outflow in response to stimulation of the caudal medullary raphe

In urethan-anesthetized cats, frequency domain analysis was used to explore the mechanisms of differential responses of inferior cardiac (CN), vertebral (VN), and renal (RN) sympathetic nerves to electrical stimulation of a discrete region of the medullary raphe (0-2 mm caudal to the obex). Raphe stimulation in baroreceptor-denervated cats at frequencies (7-12 Hz) that entrained the 10-Hz rhythm in nerve activity decreased CN and RN activities but increased VN activity. The reductions in CN and RN discharges were associated with decreased low-frequency (</=6 Hz) power and either increased (low stimulus intensity) or decreased (high stimulus intensity) 10-Hz band power. In contrast, VN 10-Hz band power was increased at all stimulus intensities, without changes in low-frequency power. High-frequency (25 Hz) stimulation decreased low-frequency activity of CN and RN discharges in both baroreceptor-denervated and baroreceptor-intact cats, without decreasing VN low-frequency activity. We propose that the differential pattern produced by raphe stimulation involves resonance at the level of the 10-Hz oscillators and differential inhibition of follower circuits that transmit both 10-Hz and low-frequency activity to sympathetic nerves.