Advances in sympathetic nerve recording in 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.

Sensitivity to ischaemia of single sympathetic nerve fibres innervating the dorsum of the human foot

KEY POINTS

Changes in nerve conduction velocity following an impulse (i.e. velocity recovery cycles) reflect after-potentials, and can provide an indication of altered nerve membrane properties. This study used microneurography to assess the effects of ischaemia on single human sympathetic fibres innervating the dorsum of the foot. It was found that velocity recovery cycles can distinguish whether a sympathetic nerve fibre is depolarized or not. The method may be used to detect membrane depolarization of sympathetic nerve fibres in human patients when autonomic neuropathy is suspected.

ABSTRACT

The aim of this study was to determine whether velocity recovery cycles (VRCs) could detect the effects of ischaemia on sympathetic nerve fibres. VRCs of human sympathetic nerve fibres of the superficial peroneal nerve innervating the dorsum of the foot were recorded by microneurography in seven healthy volunteers. Sympathetic nerve fibres were identified by studying their response to manoeuvres increasing sympathetic outflow and by measuring activity-dependent slowing at 2 Hz stimulation. VRCs were assessed at rest, during 30 min of induced limb ischaemia and during 20 min of recovery after ischaemia. From each VRC was measured the relative refractory period (RRP), the supernormality and the time to peak supernormality (SN@). During ischaemia, RRP increased from the baseline value of 37.4 ± 8.7 ms (mean ± SEM) to 67.1 ± 12.1 ms (P < 0.01) and SN@ increased from 68.6 ± 9.8 ms to 133.8 ± 11.0 ms (P < 0.005). The difference between SN@ and RRP separated ischaemic from non-ischaemic sympathetic nerve fibres. It is concluded that these sympathetic nerve fibres are sensitive to ischaemia, and that VRCs provide a method to study changes of axonal membrane potential of human sympathetic nerve fibres in vivo.

Differential activation of renal sympathetic burst amplitude and frequency during hypoxia, stress and baroreflexes with chronic angiotensin treatment

NEW FINDINGS

What is the central question of this study? Is the elevated tonic renal nerve activity induced by chronic angiotensin administration mediated by recruitment or increased firing frequency and does this occur via stress, chemoreflex or baroreflex pathways? What is the main finding and its importance? Long-term angiotensin treatment in rabbits elevates renal sympathetic nerve activity by recruitment of previously silent fibres. This was similar to the effect of chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors. Modulation of sympathetic nerve activity involves control by the CNS of the amplitude of neural discharges, reflecting recruitment of neurons and their firing frequency. We tested whether elevated tonic renal sympathetic nerve activity (RSNA) induced by chronic angiotensin administration is mediated by recruitment or increased firing frequency and whether this is characteristic of the pattern observed with activation of stress, chemoreflex or baroreflex pathways. Conscious rabbits treated with angiotensin II for 12 weeks to increase blood pressure by 10-30% were subjected to stress (air jet), hypoxia (10% O2 + 3% CO2) and drug-induced changes in blood pressure to produce baroreflexes. Total RSNA and RSNA burst amplitude were scaled to 100 normalized units (n.u.) by the maximal response to smoke. After 12 weeks of treatment, blood pressure was 17% higher than baseline 68 ± 1 mmHg (P = 0.02). Compared with sham treatment, total RSNA and burst amplitude were +82% (P < 0.001) and 39% (P = 0.04) greater, but burst frequency was similar. Total RSNA increased during hypoxia (+38% from 4.9 ± 0.7 n.u.), owing to greater amplitude, but not frequency. Air-jet stress increased total RSNA (+44% from 4.3 ± 0.5 n.u.) and burst frequency (+21% from 5.4 ± 0.7 bursts s(-1) ), but not amplitude. Angiotensin enhanced total RSNA responses to both air jet (+33%) and hypoxia (+58%), but only increased the amplitude response to air jet. The RSNA baroreflexes reset to the higher blood pressure, but amplitude or frequency was not differentially altered. Chronic angiotensin treatment elevated RSNA by recruitment of neurons, which is similar to chemoreflex stimulation, but not to stress or baroreceptor activation, suggesting that presympathetic pathways activated by angiotensin may be common to those activated by chemoreceptors.

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 activation and endothelial dysfunction in polycystic ovary syndrome are not explained by either obesity or insulin resistance

OBJECTIVE

Polycystic ovary syndrome (PCOS) is a common endocrine condition underpinned by insulin resistance and associated with increased risk of obesity, type 2 diabetes and adverse cardiovascular risk profile. Previous data suggest autonomic imbalance [elevated sympathetic nervous system (SNS) activity and decreased heart rate variability (HRV)] as well as endothelial dysfunction in PCOS. However, it is not clear whether these abnormalities are driven by obesity and metabolic disturbance or whether they are independently related to PCOS.

PARTICIPANTS AND METHODS

We examined multiunit and single-unit muscle SNS activity (by microneurography), HRV (time and frequency domain analysis) and endothelial function [ischaemic reactive hyperaemia index (RHI) using the EndoPAT device] in 19 overweight/obese women with PCOS (BMI: 31·3 ± 1·5 kg/m(2), age: 31·3 ± 1·6 years) and compared them with 21 control overweight/obese women (BMI: 33·0 ± 1·4 kg/m(2), age: 28·2 ± 1·6 years) presenting a similar metabolic profile (fasting total, HDL and LDL cholesterol, glucose, triglycerides, insulin sensitivity and blood pressure).

RESULTS

Women with PCOS had elevated multiunit muscle SNS activity (41 ± 2 vs 33 ± 3 bursts per 100 heartbeats, P < 0·05). Single-unit analysis showed that vasoconstrictor neurons were characterized by elevated firing rate and probability and incidence of multiple spikes (P < 0·01 for all parameters). Women with PCOS also had impaired endothelial function (RHI: 1·77 ± 0·14 vs 2·18 ± 0·14, P < 0·05). HRV did not differ between the groups.

CONCLUSION

Women with PCOS have increased sympathetic drive and impaired endothelial function independent of obesity and metabolic disturbances. Sympathetic activation and endothelial dysfunction may confer greater cardiovascular risk in women with PCOS.

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.