Multiple firing of single muscle vasoconstrictor neurons during cardiac dysrhythmias in human heart failure

Effect of varying chemoreflex stress on sympathetic neural recruitment strategies during apnea

We sought to examine the effect of varying chemoreflex stress on sympathetic neural recruitment strategies during end-expiratory apnea. We hypothesized that increases in the firing frequency and probability of low-threshold axons at the asphyxic "break point" would be exaggerated during hypoxia and attenuated during hyperoxia. Multiunit muscle sympathetic nervous system activity (MSNA) (peroneal nerve microneurography) was measured in 10 healthy male subjects (31 ± 2 yr, 25 ± 1 kg/m²). Individuals completed maximal voluntary end-expiratory apnea under normoxic, hypoxic (inspired O₂ fraction: 0.17 ± 0.01), and hyperoxic (inspired O₂ fraction: 0.92 ± 0.03) conditions. Action potential (AP) patterns were examined from the filtered raw signal with wavelet-based methodology. Multiunit MSNA was increased (P ≤ 0.05) during normoxic apnea, because of an increase in the frequency and incidence of AP spikes (243 ± 75 to 519 ± 134 APs/min, P = 0.048; 412 ± 133 to 733 ± 185 APs/100 heartbeats, P = 0.02). Multiunit MSNA increased from baseline (P < 0.01) during hypoxic apnea, which was due to an increase in the frequency and incidence of APs (192 ± 59 to 952 ± 266 APs/min, P < 0.01; 326 ± 89 to 1,212 ± 327 APs/100 heartbeats, P < 0.01). Hypoxic apnea also resulted in an increase in the probability of a particular AP cluster firing more than once per burst (P < 0.01). Hyperoxia attenuated any increase in MSNA with apnea, such that no changes in multiunit MSNA or frequency or incidence of AP spikes were observed (P > 0.05). We conclude that increases in frequency and incidence of APs during apnea are potentiated during hypoxia and suppressed when individuals are hyperoxic, highlighting the important impact of chemoreflex stress in AP discharge patterns. The results may have implications for neural control of the circulation in recreational activities and/or clinical conditions prone to apnea.NEW & NOTEWORTHY Our results demonstrate that, compared with normoxic end-expiratory apnea, hypoxic apnea increases the frequency and incidence of action potential spikes as well as the probability of multiple firing. We further show that this response is suppressed when individuals are hyperoxic. These data highlight the potentially important role of chemoreflex stress in neural firing and recruitment and may have implications for neural control of the circulation in recreational and/or clinical conditions prone to apnea.

Fifty years of microneurography: learning the language of the peripheral sympathetic nervous system in humans

As a primary component of homeostasis, the sympathetic nervous system enables rapid adjustments to stress through its ability to communicate messages among organs and cause targeted and graded end organ responses. Key in this communication model is the pattern of neural signals emanating from the central to peripheral components of the sympathetic nervous system. But what is the communication strategy employed in peripheral sympathetic nerve activity (SNA)? Can we develop and interpret the system of coding in SNA that improves our understanding of the neural control of the circulation? In 1968, Hagbarth and Vallbo (Hagbarth KE, Vallbo AB. Acta Physiol Scand 74: 96-108, 1968) reported the first use of microneurographic methods to record sympathetic discharges in peripheral nerves of conscious humans, allowing quantification of SNA at rest and sympathetic responsiveness to physiological stressors in health and disease. This technique also has enabled a growing investigation into the coding patterns within, and cardiovascular outcomes associated with, postganglionic SNA. This review outlines how results obtained by microneurographic means have improved our understanding of SNA outflow patterns at the action potential level, focusing on SNA directed toward skeletal muscle in conscious humans.

Physiological and pathophysiological firing properties of single postganglionic sympathetic neurons in humans

It has long been known from microneurographic recordings in human subjects that the activity of postganglionic sympathetic axons occurs as spontaneous bursts, with muscle sympathetic nerve activity (MSNA) exhibiting strong cardiac rhythmicity via the baroreflex and skin sympathetic nerve activity showing much weaker cardiac modulation. Here we review the firing properties of single sympathetic neurons, obtained using highly selective microelectrodes. Individual vasoconstrictor neurons supplying muscle or skin, or sudomotor neurons supplying sweat glands, always discharge with a low firing probability (~30%) and at very low frequencies (~0.5 Hz). Moreover, they usually fire only once per cardiac interval but can fire greater than four times within a burst. Modeling has shown that this pattern can best be explained by individual neurons being driven by, on average, two preganglionic inputs. Unitary recordings of muscle vasoconstrictor neurons have been made in several pathophysiological states, including heart failure, hypertension, obstructive sleep apnea, bronchiectasis, chronic obstructive pulmonary disease, depression, and panic disorder. The augmented MSNA in each of these diseases features an increase in firing probability and discharge frequency of individual muscle vasoconstrictor neurons above that seen in healthy subjects, yet firing rates rarely exceed 1 Hz. However, unlike patients with heart failure, all patients with respiratory disease or panic disorder, and patients with hyperhidrosis, exhibited an increase in multiple within-burst firing, which emphasizes the different modes by which the sympathetic nervous system grades its output in pathophysiological states of high sympathetic nerve activity.

Effects of aging and coronary artery disease on sympathetic neural recruitment strategies during end-inspiratory and end-expiratory apnea

In response to acute physiological stress, the sympathetic nervous system modifies neural outflow through increased firing frequency of lower-threshold axons, recruitment of latent subpopulations of higher-threshold axons, and/or acute modifications of synaptic delays. Aging and coronary artery disease (CAD) often modify efferent muscle sympathetic nerve activity (MSNA). Therefore, we investigated whether CAD (n = 14; 61 ± 10 yr) and/or healthy aging without CAD (OH; n = 14; 59 ± 9 yr) modified these recruitment strategies that normally are observed in young healthy (YH; n = 14; 25 ± 3 yr) individuals. MSNA (microneurography) was measured at baseline and during maximal voluntary end-inspiratory (EI) and end-expiratory (EE) apneas. Action potential (AP) patterns were studied using a novel AP analysis technique. AP frequency increased in all groups during both EI- and EE-apnea (all P < 0.05). The mean AP content per integrated burst increased during EI- and EE-apnea in YH (EI: Δ6 ± 4 APs/burst; EE: Δ10 ± 6 APs/burst; both P < 0.01) and OH (EI: Δ3 ± 3 APs/burst; EE: Δ4 ± 5 APs/burst; both P < 0.01), but not in CAD (EI: Δ1 ± 3 APs/burst; EE: Δ2 ± 3 APs/burst; both P = NS). When APs were binned into "clusters" according to peak-to-peak amplitude, total clusters increased during EI- and EE-apnea in YH (EI: Δ5 ± 2; EE: Δ6 ± 4; both P < 0.01), during EI-apnea only in OH (EI: Δ1 ± 2; P < 0.01; EE: Δ1 ± 2; P = NS), and neither apnea in CAD (EI: Δ -2 ± 2; EE: Δ -1 ± 2; both P = NS). In all groups, the AP cluster size-latency profile was shifted downwards for every corresponding cluster during EI- and EE-apnea (all P < 0.01). As such, inherent dysregulation exists within the central features of apnea-related sympathetic outflow in aging and CAD.

Current Approaches to Quantifying Tonic and Reflex Autonomic Outflows Controlling Cardiovascular Function in Humans and Experimental Animals

The role of the autonomic nervous system in the pathophysiology of human and experimental models of cardiovascular disease is well established. In the recent years, there have been some rapid developments in the diagnostic approaches used to assess and monitor autonomic functions. Although most of these methods are devoted for research purposes in laboratory animals, many have still found their way to routine clinical practice. To name a few, direct long-term telemetry recording of sympathetic nerve activity (SNA) in rodents, single-unit SNA recording using microneurography in human subjects and spectral analysis of blood pressure and heart rate in both humans and animals have recently received an overwhelming attention. In this article, we therefore provide an overview of the methods and techniques used to assess tonic and reflex autonomic functions in humans and experimental animals, highlighting current advances available and procedure description, limitations and usefulness for diagnostic purposes.

Sympathetic single axonal discharge after spinal cord injury in humans: activity at rest and after bladder stimulation

STUDY DESIGN

Clinical experimental mechanistic study.

OBJECTIVES

(1) To determine in three spinal cord-injured patients whether individual muscle sympathetic nerve fibres below the level of the spinal lesion display spontaneous activity. (2) To determine in these patients if individual sympathetic vasoconstrictor fibres show a prolonged discharge following a bladder stimulus.

SETTING

University hospital in Gothenburg, Sweden.

METHODS

Microneurographic recordings of action potentials from individual muscle nerve sympathetic fibres in a peroneal nerve. Recordings of skin blood flow and electrodermal responses in a foot.

RESULTS

In all patients, there was sparse ongoing spontaneous impulse traffic in individual sympathetic fibres. Brisk mechanical pressure over the urinary bladder evoked a varying number of action potentials in individual fibres, but the activity was brief and did not continue after the end of the evoked multiunit burst.

CONCLUSION

Prolonged discharges in individual sympathetic fibres are unlikely to contribute to a long duration of blood pressure increases induced by brief bladder stimuli.

Recruitment pattern of sympathetic muscle neurons during premature ventricular contractions in heart failure patients and controls

Premature ventricular contractions (PVC) elicit larger bursts of multiunit muscle sympathetic nerve activity (MSNA), reflecting the ability to increase postganglionic axonal recruitment. We tested the hypothesis that chronic heart failure (CHF) limits the ability to recruit postganglionic sympathetic neurons as a response to PVC due to the excessive sympathetic activation in these patients. Sympathetic neurograms of sufficient signal-to-noise ratio were obtained from six CHF patients and from six similarly aged control individuals. Action potentials (APs) were extracted from the multiunit sympathetic neurograms during sinus rhythm bursts and during the post-PVC bursts. These APs were classified on the basis of the frequency per second, the content per burst, and the peak-to-peak amplitude, which formed the basis of binning the APs into active clusters. Compared with controls, CHF had higher APs per burst and higher number of active clusters per sinus rhythm burst (P < 0.05). Compared with sinus rhythm bursts, both groups increased AP frequency and the number of active clusters in the post-PVC burst (P < 0.05). However, compared with controls, the increase in burst integral, AP frequency, and APs per burst during the post-PVC burst was less in CHF patients. Nonetheless, the PVC-induced increase in active clusters per burst was similar between the groups. Thus, these CHF patients retained the ability to recruit larger APs but had a diminished ability to increase overall AP content.

Firing patterns of muscle vasoconstrictor neurons in respiratory disease

Because the cardiovascular system and respiration are so intimately coupled, disturbances in respiratory control often lead to disturbances in cardiovascular control. Obstructive Sleep Apnea (OSA), Chronic Obstructive Pulmonary Disease (COPD), and Bronchiectasis (BE) are all associated with a greatly elevated muscle vasoconstrictor drive (muscle sympathetic nerve activity, MSNA). Indeed, the increase in MSNA is comparable to that seen in congestive heart failure (CHF), in which the increase in MSNA compensates for the reduced cardiac output and thereby assists in maintaining blood pressure. However, in OSA – but not COPD or BE – the increase in MSNA can lead to hypertension. Here, the features of the sympathoexcitation in OSA, COPD, and BE are reviewed in terms of the firing properties of post-ganglionic muscle vasoconstrictor neurons. Compared to healthy subjects with low levels of resting MSNA, single-unit recordings revealed that the augmented MSNA seen in OSA, BE, COPD, and CHF were each associated with an increase in firing probability and mean firing rates of individual neurons. However, unlike patients with heart failure, all patients with respiratory disease exhibited an increase in multiple within-burst firing which, it is argued, reflects an increase in central sympathetic drive. Similar patterns to those seen in OSA, COPD, and BE were seen in healthy subjects during an acute increase in muscle vasoconstrictor drive. These observations emphasize the differences by which the sympathetic nervous system grades its output in health and disease, with an increase in firing probability of active neurons and recruitment of additional neurons being the dominant mechanisms.

Advantage of recording single-unit muscle sympathetic nerve activity in heart failure

Elevated sympathetic activation is a characteristic feature of heart failure (HF). Excessive sympathetic activation under resting conditions has been shown to increase from the early stages of the disease, and is related to prognosis. Direct recording of multiunit efferent muscle sympathetic nerve activity (MSNA) by microneurography is the best method for quantifying sympathetic nerve activity in humans. To date, this technique has been used to evaluate the actual central sympathetic outflow to the periphery in HF patients at rest and during exercise; however, because the firing occurrence of sympathetic activation is mainly synchronized by pulse pressure, multiunit MSNA, expressed as burst frequency (bursts/min) and burst incidence (bursts/100 heartbeats), may have limitations for the quantification of sympathetic nerve activity. In HF, multiunit MSNA is near the maximum level, and cannot increase further than the heartbeat. Single-unit MSNA analysis in humans is technically demanding, but provides more detailed information regarding central sympathetic firing. Although a great deal is known about the response of multiunit MSNA to stress, little information is available regarding the responses of single-unit MSNA to physiological stress and disease. The purposes of this review are to describe the differences between multiunit and single-unit MSNA during stress and to discuss the advantages of single-unit MSNA recording in improving our understanding the pathology of increased sympathetic activity in HF.

Advances in sympathetic nerve recording in humans

In humans, sympathetic activity is commonly assessed by measuring the efferent traffic in the peroneal nerve. The firing activity is the sum of several active neurons, which have the tendency to fire together in a bursting manner. While the estimation of overall sympathetic nervous activity using this multiunit recording approach has advanced our understanding of sympathetic regulation in health and disease no information is gained regarding the underling mechanisms generating the bursts of sympathetic activity. The introduction of single-unit recording has been a major step forward, enabling the examination of specific sympathetic firing patterns in diverse clinical conditions. Disturbances in sympathetic nerve firing, including high firing probabilities, high firing rates or high incidence of multiple firing, or a combination of both may impact on noradrenaline release and effector response, and therefore have clinical implications with regards to the development and progression of target organ damage. Understanding the mechanisms and consequences of specific firing patterns would permit the development of therapeutic strategies targeting these nuances of sympathetic overdrive.

Augmented single-unit muscle sympathetic nerve activity in heart failure with chronic atrial fibrillation

Atrial fibrillation (AF) is a common complication in heart failure (HF) patients. However, it remains unclear whether irregular ventricular response patterns induced by AF increase sympathetic nerve activity. We measured resting multi- and single-unit muscle sympathetic nerve activity (MSNA) in 21 age-matched HF patients with chronic AF (n = 11) rhythm or sinus rhythm (SR, n = 10). The multi-unit MSNA, which was expressed as total activity, was similar between HF + AF patients and HF + SR patients. However, the single-unit MSNA in HF + AF patients was significantly greater than that in HF + SR patients (62 ± 9 spikes min(-1) vs. 42 ± 4 spikes min(-1), P < 0.05). Moreover, the incidence of multiple firing of single-unit MSNA within a given burst was augmented in HF + AF patients as compared with HF + SR patients (48 ± 8% vs. 26 ± 3%, P < 0.01). A significant negative relationship was observed between the reduced diastolic pressure induced by a prolonged cardiac interval in AF subjects and single-unit MSNA frequency within one cardiac interval in each HF + AF subject. The firing characteristics of single-unit MSNA were different between HF patients with AF and HF patients with SR; particularly, those with a prolonged long RR interval showed multiple firings of single-unit MSNA. These findings suggest that AF per se leads to the instantaneous augmentation of single-unit MSNA induced by decreased diastolic pressure, which might partially contribute to disease progression in HF patients.

Relationship between size and latency of action potentials in human muscle sympathetic nerve activity

We employed a novel action potential detection and classification technique to study the relationship between the recruitment of sympathetic action potentials (i.e., neurons) and the size of integrated sympathetic bursts in human muscle sympathetic nerve activity (MSNA). Multifiber postganglionic sympathetic nerve activity from the common fibular nerve was collected using microneurography in 10 healthy subjects at rest and during activation of sympathetic outflow using lower body negative pressure (LBNP). Burst occurrence increased with LBNP. Integrated burst strength (size) varied from 0.22 ± 0.07 V at rest to 0.28 ± 0.09 V during LBNP. Sympathetic burst size (i.e., peak height) was directly related to the number of action potentials within a sympathetic burst both at baseline ( r = 0.75 ± 0.13; P < 0.001) and LBNP ( r = 0.75 ± 0.12; P < 0.001). Also, the amplitude of detected action potentials within sympathetic bursts was directly related to the increased burst size at both baseline ( r = 0.59 ± 0.16; P < 0.001) and LBNP ( r = 0.61 ± 0.12; P < 0.001). In addition, the number of detected action potentials and the number of distinct action potential clusters within a given sympathetic burst were correlated at baseline ( r = 0.7 ± 0.1; P < 0.001) and during LBNP ( r = 0.74 ± 0.03; P < 0.001). Furthermore, action potential latency (i.e., an inverse index of neural conduction velocity) was decreased as a function of action potential size at baseline and LBNP. LBNP did not change the number of action potentials and unique clusters per sympathetic burst. It was concluded that there exists a hierarchical pattern of recruitment of additional faster conducting neurons of larger amplitude as the sympathetic bursts become stronger (i.e., larger amplitude bursts). This fundamental pattern was evident at rest and was not altered by the level of baroreceptor unloading applied in this study.

Single-unit muscle sympathetic nervous activity and its relation to cardiac noradrenaline spillover

Recent work using single-unit sympathetic nerve recording techniques has demonstrated aberrations in the firing pattern of sympathetic nerves in a variety of patient groups. We sought to examine whether nerve firing pattern is associated with increased noradrenaline release. Using single-unit muscle sympathetic nerve recording techniques coupled with direct cardiac catheterisation and noradrenaline isotope dilution methodology we examined the relationship between single-unit firing patterns and cardiac and whole body noradrenaline spillover to plasma. Participants comprised patients with hypertension (n=6), depression (n=7) and panic disorder (n =9) who were drawn from our ongoing studies. The patient groups examined did not differ in their single-unit muscle sympathetic nerve firing characteristics nor in the rate of spillover of noradrenaline to plasma from the heart. The median incidence of multiple spikes per beat was 9%. Patients were stratified according to the firing pattern: low level of incidence (less than 9% incidence of multiple spikes per beat) and high level of incidence (greater than 9% incidence of multiple spikes per beat). High incidence of multiple spikes within a cardiac cycle was associated with higher firing rates (P <0.0001) and increased probability of firing (P <0.0001). Whole body noradrenaline spillover to plasma and (multi-unit) muscle sympathetic nerve activity in subjects with low incidence of multiple spikes was not different to that of those with high incidence of multiple spikes. In those with high incidence of multiple spikes there occurred a parallel activation of the sympathetic outflow to the heart, with cardiac noradrenaline spillover to plasma being two times that of subjects with low nerve firing rates (11.0 ± 1.5 vs. 22.0 ± 4.5 ng min⁻¹, P <0.05). This study indicates that multiple within-burst firing and increased single-unit firing rates of the sympathetic outflow to the skeletal muscle vasculature is associated with high cardiac noradrenaline spillover.

Sinusoidal galvanic vestibular stimulation (sGVS) induces a vasovagal response in the rat

Blood pressure (BP) and heart rate (HR) were studied in isoflurane-anesthetized Long-Evans rats during sinusoidal galvanic vestibular stimulation (sGVS) and sinusoidal oscillation in pitch to characterize vestibular influences on autonomic control of BP and HR. sGVS was delivered binaurally via Ag/AgCl needle electrodes inserted over the mastoids at stimulus frequencies 0.008-0.4 Hz. Two processes affecting BP and HR were induced by sGVS: 1) a transient drop in BP (≈15-20 mmHg) and HR (≈3 beat*s(-1)), followed by a slow recovery over 1-6 min; and 2) inhibitory modulations in BP (≈4.5 mmHg/g) and HR (≈0.15 beats*s(-1)/g) twice in each stimulus cycle. The BP and HR modulations were approximately in-phase with each other and were best evoked by low stimulus frequencies. A wavelet analysis indicated significant energies in BP and HR at scales related to twice and four times the stimulus frequency bands. BP and HR were also modulated by oscillation in pitch at frequencies 0.025-0.5 Hz. Sensitivities at 0.025 Hz were ≈4.5 mmHg/g (BP) and ≈0.17 beat*s(-1)/g (HR) for pitches of 20-90°. The tilt-induced BP and HR modulations were out-of-phase, but the frequencies at which responses were elicited by tilt and sGVS were the same. The results show that the sGVS-induced responses, which likely originate in the otolith organs, can exert a powerful inhibitory effect on both BP and HR at low frequencies. These responses have a striking resemblance to human vasovagal responses. Thus, sGVS-activated rats can potentially serve as a useful experimental model of the vasovagal response in humans.

Spike detection in human muscle sympathetic nerve activity using a matched wavelet approach

Sympathetic nerve recordings associated with blood pressure regulation can be recorded directly using microneurography. A general characteristic of this signal is spontaneous burst activity of spikes (action potentials) separated by silent periods against a background of considerable Gaussian noise. During measurement with electrodes, the raw muscle sympathetic nerve activity (MSNA) signal is amplified, band-pass filtered, rectified and integrated. This integration process removes information regarding action potential content and their discharge properties. This paper proposes a new method for detecting action potentials from the raw MSNA signal to enable investigation of post-ganglionic neural discharge properties. The new method is based on the design of a mother wavelet that is matched to an actual mean action potential template extracted from a real raw MSNA signal. To detect action potentials, the new matched wavelet is applied to the MSNA signal using a continuous wavelet transform following a thresholding procedure and finding of a local maxima that indicates the location of action potentials. The performance of the proposed method versus two previous wavelet-based approaches was evaluated using (1) real MSNA recorded from seven healthy participants and, (2) simulated MSNA. The results show that the new matched wavelet performs better than the previous wavelet-based methods that use a non-matched wavelet in detecting action potentials in the MSNA signal.

Firing probability and mean firing rates of human muscle vasoconstrictor neurones are elevated during chronic asphyxia

Elevated muscle sympathetic nerve activity (MSNA) features in many cardiovascular diseases, but how this sympathoexcitation is brought about differs across pathologies. Unitary recordings from post-ganglionic muscle vasoconstrictor neurones in human subjects have shown that the augmented MSNA in the obstructive sleep apnoea syndrome (OSAS) is associated with an increase in firing probability and mean firing rate, and an increase in multiple within-burst firing. Here we characterize the firing properties of muscle vasoconstrictor neurones in patients with chronic obstructive pulmonary disease (COPD), who are chronically asphyxic. We tested the hypothesis that this elevated chemical drive would shift the firing pattern from that seen in healthy subjects to that seen in OSAS. The mean firing probability (52%) and mean firing rate (0.92 Hz) of 17 muscle vasoconstrictor neurones recorded in COPD were comparable to those previously recorded in OSAS (51% and 0.96 Hz), but significantly higher than those recorded in a group of healthy subjects with high levels of resting MSNA (35% and 0.33 Hz). In COPD single neurones fired once in 63% of cardiac intervals, comparable to OSAS (59%), but significantly lower than in the healthy group (78%). Conversely, single neurones fired twice in 25% of cardiac intervals, similar to OSAS (27%), but significantly higher than in the healthy group (18%). We conclude that the chronic asphyxia associated with COPD results in an increase in the firing probability and mean firing frequency of muscle vasoconstrictor neurones and causes a shift towards multiple firing, reflecting an increase in central muscle vasoconstrictor drive.

Altered firing pattern of single-unit muscle sympathetic nerve activity during handgrip exercise in chronic heart failure

Sympathetic activation in chronic heart failure (CHF) is greatly augmented at rest but the response to exercise remains controversial. We previously demonstrated that single-unit muscle sympathetic nerve activity (MSNA) provides a more detailed description of the sympathetic response to physiological stress than multi-unit nerve recordings. The purpose of this study was to determine whether the reflex response and discharge properties of single-unit MSNA are altered during handgrip exercise (HG, 30% of maximum voluntary contraction for 3 min) in CHF patients (New York Heart Association functional class II or III, n = 16) compared with age-matched healthy control subjects (n = 13). At rest, both single-unit and multi-unit indices of sympathetic outflow were augmented in CHF compared with controls (P < 0.05). However, the percentage of cardiac intervals that contained one, two, three or four single-unit spikes were not different between the groups. Compared to the control group, HG elicited a larger increase in multi-unit total MSNA (Delta1002 +/- 50 compared with Delta636 +/- 76 units min(-1), P < 0.05) and single-unit MSNA spike incidence (Delta27 +/- 5 compared with Delta8 +/- 2 spikes (100 heart beats)(-1)), P < 0.01) in the CHF patients. More importantly, the percentage of cardiac intervals that contained two or three single-unit spikes was increased (P < 0.05) during exercise in the CHF group only (Delta8 +/- 2% and Delta5 +/- 1% for two and three spikes, respectively). These results suggest that the larger multi-unit total MSNA response observed during HG in CHF is brought about in part by an increase in the probability of multiple firing of single-unit sympathetic neurones.

Firing properties of sudomotor neurones in hyperhidrosis and thermal sweating

OBJECTIVES

Idiopathic palmar-plantar hyperhidrosis is characterized by excessive sweating of the palms and feet, and is commonly treated by transthoracic regional sympathicotomy. As the condition is believed to be due to a high sudomotor drive, we wanted to assess the firing properties of individual sudomotor neurones in this state of sympathoexcitation, extending our recent work on other pathologies associated with high sympathetic nerve activity.

METHODS

Single-unit recordings were made from eight sudomotor neurones supplying the fingers via tungsten microelectrodes inserted percutaneously into the median nerve at the wrist or upper arm.

RESULTS

Typical of sudomotor, muscle vasoconstrictor and cutaneous vasoconstrictor neurones recorded in healthy individuals in states of high sympathetic drive, all units had low firing probabilities (active in only 30.0 +/- 6.7 (SE) % of cardiac intervals) and primarily fired only once per heart beat. The percentage of cardiac intervals in which the neurones generated 1, 2, 3 or 4 spikes was 60.4 +/- 6.3, 22.9 +/- 3.9, 9.7 +/- 2.1 and 3.4 +/- 1.3%, respectively. For comparison, these values were 77.6 +/- 7.7, 15.0 +/- 4.1, 4.6 +/- 2.3 and 1.8 +/- 1.3% for eight sudomotor neurones innervating the hairy skin of the foot during thermally-induced sweating in normal subjects.

INTERPRETATION

We conclude that the firing properties of spontaneously active sudomotor neurones in subjects with hyperhidrosis are similar to those of sudomotor neurones active during thermal sweating, reflecting an increase in central sympathetic drive to the sweat glands in hyperhidrosis.

Effect of pulseless ventricular tachycardia on sympathetic activity

A szimpatikus aktivitás kamrai ritmuszavarokat serkentő hatása jól ismert. Kevésbé tudott, hogy hemodinamikailag destabilizáló kamrai ritmuszavarok idején a szimpatikus aktivitásfokozódás vérnyomás-stabilizáló (protektív) szerepet játszik. Súlyos balkamra-elégtelenségben szenvedő 62 esztendős betegünk szív-elektrofiziológiai vizsgálata során a szimpatikus izom idegaktivitásának direkt mérésével demonstráltuk az arrhythmiára adott szimpatikus választ. Az eset bemutatásával a ritmuszavarok és az autonóm idegrendszer komplex kapcsolatára hívjuk fel a figyelmet.

The activity of a single muscle sympathetic vasoconstrictor nerve unit is affected by physiological stress in humans

Recording of neural firing from single-unit muscle sympathetic nerve activity (MSNA) is a new strategy offering information about the frequency of pure sympathetic firing. However, it is uncertain whether and when single-unit MSNA would be more useful than multiunit MSNA for analysis of various physiological stresses in humans. In 15 healthy subjects, we measured single-unit and multiunit MSNA before and during handgrip exercise at 30% of maximum voluntary contraction for 3 min and during the Valsalva maneuver at 40 mmHg expiratory pressure for 15 s. Shapes of individual single-unit MSNA were proved to be consistent and suitable for further evaluation. Single-unit and multiunit MSNA exhibited similar responses during handgrip exercise. However, acceleration of neural firing determined from single-unit MSNA became steeper than multiunit MSNA during the Valsalva maneuver. During the Valsalva maneuver, unlike handgrip exercise, the distribution of multiunit burst between 0, 1, 2, 3, and 4 spikes was significantly shifted toward multiple spikes within a given burst (P < 0.05). These results indicated that evaluation of single-unit MSNA could provide more detailed and accurate information concerning the role and responses of neuronal discharges induced by various physiological stresses in humans, especially amid intense sympathetic activity.

Comparison of the firing patterns of human postganglionic sympathetic neurones and spinal alpha motoneurones during brief bursts

Focal recordings from individual postganglionic sympathetic neurones in awake human subjects have revealed common firing properties. One of the most striking features is that they tend to fire only once per sympathetic burst. Why this should be so is not known, but we propose that the short duration of the burst may limit the number of times a sympathetic neurone can fire. Indeed, while the normal variation in cardiac interval and burst duration is too narrow to reveal a correlation between burst duration and the number of spikes generated, we know that spike generation is doubled when burst duration is doubled following ectopic heart beats. To test the hypothesis that the burst duration constrains the firing of individual sympathetic neurones to one per burst, we used the human skeletomotor system as a model for the sympathetic nervous system, which allowed us to vary burst duration and amplitude experimentally. Intramuscular recordings were made from 27 single motor units (alpha motoneurones) in the tibialis anterior or soleus muscles of seven subjects; multiunit EMG activity was recorded via surface electrodes and blood pressure was recorded continuously. Subjects were instructed to generate EMG bursts of varying amplitude in the intervals between heart beats. By constraining the firing of alpha motoneurones to brief ( approximately 400 ms) bursts we could emulate real sympathetic bursts. Individual motoneurones generated 0-7 spikes during the emulated sympathetic bursts, with firing patterns similar to those exhibited by real sympathetic neurones. Eleven motor units showed significant positive linear correlations between the number of spikes they generated within a burst and its amplitude, whereas for 17 motor units there were significant positive correlations between the number of spikes and burst duration. This indicates that burst duration is a major determinant of the number of times an alpha motoneurone will fire during a brief burst, and we suggest that the same principle may explain the firing pattern typical of human sympathetic neurones.

Behaviour of the adrenergic cardiovascular drive in atrial fibrillation and cardiac arrhythmias

AIM

Animal studies have conclusively shown that the sympathetic nervous system plays a major role not only in regulating sinus node activity but also in promoting cardiac rhythm alterations. Less univocal and often circumstantial have been the evidences collected on this issue in humans. However, the introduction of the microneurographic technique in clinical research has allowed to gain new important insights on the role of neuroadrenergic factors in the pathophysiology of cardiac arrhythmias.

METHODS

The present paper will review the results of microneurographic studies performed by our group and others in the field of cardiac rhythm disturbances by addressing three specific issues. First it will examine the relationships between heart rate and muscle sympathetic neural outflow in a variety of cardiovascular diseases characterized by sympathetic activation. This will be followed by an analysis of the behaviour of the sympathetic nerve traffic responses to paroxysmal atrial fibrillation. Finally, the sympathetic adjustments to spontaneously occurring or artificially induced pre-mature ventricular contractions will be highlighted.

The activity of single vasoconstrictor nerve units in hypertension

AIM

It has long been established from controlled experiments in anaesthetized animals that it is more accurate to quantify the mean frequency of efferent sympathetic nerve activity from single unit than from multi-unit bursts recordings. More recently, sympathetic nerve hyperactivity has been reported in patients with essential hypertension (EHT) when using microneurographic recordings from peripheral efferent nerves. This review will focus on the mean frequency of single unit of muscle sympathetic nerve activity (s-MSNA) in relation to that of multi-unit bursts (MSNA) as obtained by microneurography in EHT.

RESULTS

We have shown that the resting levels of s-MSNA and MSNA were increased in uncomplicated EHT, white coat hypertension and in EHT complicated by left ventricular hypertrophy. There was a relatively greater increase in s-MSNA than in MSNA in mild hypertension and in complicated EHT. We also found that both s-MSNA and MSNA were increased to a similar extent in conditions known to affect reflexes emanating from the heart and influencing sympathetic output, such as acute myocardial infarction. In other preliminary studies, the increase of s-MSNA in response to the discomfort of cold pressor test was greater than that of MSNA and this difference was abolished by the centrally sympatholytic agent moxonidine.

CONCLUSION

These results are consistent with the hypothesis that an increase in the mean frequency of central sympathetic discharge to the periphery (greater s-MSNA than MSNA) is involved in the pathogenesis and complications of EHT. Target organ damage may in turn lead to an increase in overall sympathetic output (excessive MSNA increase) through the operation of peripheral reflex mechanisms.

Pathological sympathoexcitation: how is it achieved?

AIM

Congestive heart failure (CHF) and obstructive sleep apnoea syndrome (OSAS) are both associated with an intense sympathoexcitation, including an increased muscle sympathetic nerve activity (MSNA). We have studied the firing characteristics of single vasoconstrictor fibres to the muscle vascular bed in CHF and OSAS patients, at rest and during transient sympathoexcitatory stimuli, to elucidate the mechanisms by which vasoconstrictor output is augmented in these conditions.

RESULTS

The main alternatives for augmenting sympathetic output are an increased firing frequency of individual nerve fibres and an increased recruitment of nerve fibres. Starting with the frequency alternative, the inherent bursting character of MSNA provides two possibilities to increase the firing of individual fibres: (1) by increasing the proportion of neural bursts in which the fibre is active (increased firing probability) and (2) by increasing the number of spikes a fibre generates per burst (increased multiple within-burst firing). At rest and in cardiac sinus rhythm, an increased firing probability is seen in both CHF and OSAS patients, whereas increased multiple within-burst firing is found in OSAS but not in CHF. In response to transient sympathoexcitatory stimuli (such as pre-mature heart beats), both patient groups show marked shifts towards multiple within-burst firing. Thus, both mechanisms for augmenting discharge frequency are operating in these two pathological conditions, but the firing characteristics at rest differ significantly. During recording sessions in sympathoexcited patients, we have encountered vasoconstrictor fibres that are active almost exclusively during periods of transient sympathoexcitation, while being virtually silent at rest. This suggests that recruitment of previously inactive vasoconstrictor fibres, the second main alternative for increasing vasoconstrictor output, contributes to transient sympathoexcitatory responses in these patients. Although it seems reasonable to assume that recruitment may also contribute to the resting level of MSNA in CHF and OSAS, this issue is difficult to resolve in microneurographic studies.

CONCLUSION

In conclusion, pathological sympathoexcitation appears to depend on both recruitment and increased firing frequency. A shift towards multiple within-burst firing, at rest or in response to transient stimuli, may constitute a risk factor per se as it entails neural volleys with high instantaneous firing frequencies and consequently higher release of neurotransmitters.

Functional organization of peripheral vasomotor pathways

AIM

In this article, we review the functional organization of the peripheral autonomic pathways regulating the vasculature.

RESULTS

The final motor neurones in vasomotor pathways tend to be smaller than neurones in other autonomic pathways. This suggests that they have relatively smaller target territories and receive fewer pre-ganglionic inputs than non-vasomotor neurones. Nevertheless, single vasomotor neurones project to large areas of the vasculature separated by up to 7 mm. Different functional pools of vasomotor neurones project to specific segments of the vasculature, allowing for the selective neural control of resistance in vessels in proximal or distal regions of the vascular bed. In many cases, each functional pool of vasomotor neurones utilizes a characteristic combination of cotransmitters. The various pools of final motor neurones in vasomotor pathways receive convergent synaptic input from different pools of pre-ganglionic neurones, many of which also contain neuropeptides which enhance the excitability of the final motor neurones. The excitability of vasomotor neurones regulating gastrointestinal and mesenteric blood flow, also can be increased by the actions of peptides such as substance P that are released from visceral nociceptors.

CONCLUSIONS

We propose that autonomic pathways regulating the vasculature are organized into 'vasomotor units'. Each vasomotor unit consists of a pre-ganglionic neurone, the final motor neurones it innervates, and the blood vessels that they regulate. The vasomotor units are likely to be grouped into functional pools that can be recruited as necessary to provide highly specific, graded control of blood flow both within and between vascular beds.

Mechanisms of sympathoexcitation: single-unit analysis of muscle vasoconstrictor neurons in awake OSAS subjects

In congestive heart failure (CHF), muscle sympathetic activity (MSNA) is greatly elevated, but our laboratory has shown that single muscle vasoconstrictor neurons primarily fire only once per cardiac interval, as in normal subjects (Elam M and Macefield VG. J Appl Physiol 91: 717-724, 2001; Macefield VG, Rundqvist B, Sverrisdottir YB, Wallin BG, and Elam M. Circulation 100: 1708-1713, 1999). In this study, we used patients with obstructive sleep apnea syndrome (OSAS) to test the hypothesis that this firing pattern is maintained in other states of sympathoexcitation. Unitary recordings were made from muscle vasoconstrictor neurons in eight awake OSAS patients. The average firing frequency of 12 units was 0.96 Hz and the firing probability 51%, similar to previous observations in CHF patients (0.98 Hz, 55%) but higher than in healthy subjects (0.40 Hz, 31%). However, the percentages of cardiac intervals in which neurons generated one, two, three, or four spikes were 59, 27, 10, and 3% in OSAS, compared with 71, 18, 7, and 2% in CHF and 73, 18, 5, and 3% in healthy subjects. Thus the firing pattern is different in OSAS and CHF, leading to rejection of the hypothesis: although in both conditions individual neurons show an increase in firing probability, in OSAS patients they also fire more often within a cardiac interval. It is likely that differences may also be apparent in other states of sympathoexcitation.

Sympathetic response to ventricular extrasystolic beats in hypertension and heart failure

Provoked premature ventricular contractions (PVCs) evoke, in concomitance with an early and late blood pressure fall and overshoot, an early sympathoexcitation and a later period of sympathoinhibition, respectively. The present study was designed to examine whether in healthy subjects this is the case for spontaneous PVCs. Because of their pathophysiological relevance for arrhythmogenesis, it was also designed to determine whether the sympathetic responses are different from those seen in essential hypertension and congestive heart failure. In 14 untreated mild essential hypertensives (EH; age, 53.8+/-2.6 years; mean+/-SEM), 20 untreated congestive heart failure patients (CHF; age, 56.7+/-2.5 years; New York Heart Association class, II or III), and 16 age-matched healthy subjects (control) in Lown class <II, we evaluated the blood pressure (Finapres), heart rate (ECG), and muscle sympathetic nerve traffic (MSNA; by microneurography) responses to isolated monofocal PVCs. MSNA, quantified as bursts/100 heart beats, was significantly increased in EH (57.8+/-3.8, P<0.05) and CHF patients (77.7+/-4.0, P<0.01) compared with controls (44.6+/-4.4). In controls, the PVC-induced blood pressure fall and overshoot were accompanied by a sympathoexcitation (144.2+/-14%), followed by a period of sympathoinhibition (average duration, 12043+/-985 ms). The responses were similar in EH but not in CHF, in whom the magnitude of the sympathoexcitation and particularly the duration of the subsequent sympathoinhibition were strikingly reduced (average reduction, -46.1 and -72.8%, respectively). The most important factor accounting for this reduction appeared to be an altered baroreflex response to the PVC-induced BP changes. These data demonstrate that the MSNA responses to spontaneous PVCs are similar in controls and EH but markedly impaired in CHF, presumably because of the baroreflex alteration. This may represent an important factor for the genesis of the life-threatening ventricular arrhythmias that characterize CHF.

Firing properties of single postganglionic sympathetic neurones recorded in awake human subjects

For over three decades, the technique of microneurography has allowed us to record sympathetic neural outflow directly from postganglionic axons in awake human subjects. But because sympathetic axons are clustered within a nerve fascicle, such recordings have been limited to the analysis of multi-unit neural activity. To improve the information content of intraneural recordings, we developed the single-unit approach, in which focal recordings can be made from a single C-fibre via a high-impedance tungsten microelectrode. In this review, we describe our methodology for analyzing unitary sympathetic activity and discuss the similarities in the firing properties of individual muscle vasoconstrictor, cutaneous vasoconstrictor and sudomotor neurones.