Muscle sympathetic nerve activity during postural change in healthy young and older adults

Recent evidence suggests that during orthostatic stress the reflex increase in muscle sympathetic nerve activity may be diminished in older adults. To test this hypothesis, we measured muscle sympathetic nerve activity, plasma noradrenaline concentrations, heart rate, and arterial blood pressure in twelve young (mean, 25 years; range, 19–29 years) adults and 14 older (mean 64 years; range, 60–74 years) healthy adults, while supine and during upright sitting. Supine control levels of muscle sympathetic nerve activity were higher in the older subjects (35 ± 1 vs. 25 ± 1 bursts/min,p < 0.05), but there were no differences in plasma noradrenaline concentrations, heart rate or arterial pressure. Despite higher supine control levels in the older group, the absolute unit increases in muscle sympathetic nerve activity in response to upright sitting (p < 0.05 vs. control) were not different in the two groups (7 ± 1 vs. 7 ± 1 bursts/min), nor were the increases in plasma noradrenaline concentrations. Heart rate did not increase above supine control in response to sitting in either group. Arterial pressure increased slightly (p < 0.05, supine vs. control), but there were no age-related differences. These results indicate that, contrary to recent findings, the reflex increases in muscle sympathetic nerve activity and plasma noradrenaline concentrations and regulation of arterial pressure during this natural orthostatic stress are well preserved in older healthy men and women.     

The validity and reproducibility of the skin vasomotor test—studies in normal subjects,after spinal anaesthesia,and in diabetes mellitus

Skin sympathetic vasomotor control can be examined in the extremities by the skin vasomotor test. In this test the change in skin blood flow and skin temperature in the hand and foot in response to a cold stimulus is utilized as an index of distal sympathetic nerve fibre integrity. This is of importance in conditions such as diabetes mellitus as peripheral autonomic neuropathy is associated with orthostatic hypotension and diabetic foot complications. The validity and reproducibility of the test as a marker of distal sympathetic nerve function has been studied. The test was performed in nine healthy control subjects and in nine subjects (undergoing minor surgery) after a sympathetic nerve conduction block (L2–L3) was achieved in the lower extremities by spinal analgesia. Changes in skin temperature (p < 0.001) and skin blood flow (p < 0.005) in responses to cooling were significantly larger in the control group than in the group with spinal analgesia. Repeated skin temperature measurements on 42 occasions (test—retest period of 4 weeks) in eight healthy and 34 diabetic subjects indicated a reliability coefficient of 80%. We conclude, therefore, that the skin vasomotor test provides a valid and reproducible quantitative assessment of skin sympathetic nerve function in upper and lower extremities.     

Microneurographic analysis of muscle sympathetic nerve activity in amyotrophic lateral sclerosis

Muscle sympathetic nerve activity by (microneurograph) blood pressure and heart rate has been studied in patients with amyotrophic lateral sclerosis and in age-matched normal subjects (controls) at rest and during head-up tilt. Muscle sympathetic nerve activity in amyotrophic lateral sclerosis patients was significantly increased at rest unlike controls. There was no correlation between muscle sympathetic nerve activity and age in the patients with amyotrophic lateral sclerosis. Elevated muscle sympathetic nerve activity was present mainly in younger patients. There were no differences between blood pressure or heart rate in either group at rest or during head-up tilt in amyotrophic lateral sclerosis. The increase in muscle sympathetic nerve activity following tilt in the amyotrophic lateral sclerosis patients was less than in the controls, but they had no postural hypotension. The possible reasons for this observation of increased muscle sympathetic nerve activity at rest in amyotrophic lateral sclerosis are discussed.Corresponding Author     

Influence of resting sympathetic activity on reflex sympathetic responses in normal man

Reflex sympathetic responses to physiologic stress are known to be modulated by afferent sensory mechanisms. However, the potential influence of baseline sympathetic tone on these reflex-mediated responses is unclear. To test the hypothesis that the resting level of muscle sympathetic nerve activity (MSNA) influences reflex-mediated changes in MSNA in normal man, MSNA, blood pressure (BP), central venous pressure (CVP), and heart rate (HR) was measured in 38 normal subjects at rest and during deactivation of cardiopulmonary baroreceptors (CPBR) with lower body negative pressure (LBNP; 0 to — 15 mmHg). A cold pressor test (CPT) also was performed in 25 subjects. Incremental LBNP decreased CVP (from 5.8 ± 0.4 to 2.1 ± 0.4 mmHg) without altering BP or HR, and increased in MSNA burst frequency (from 22.5 ± 1.4 to 30.2 ± 1.4 bursts/min). There was no significant correlation between levels of MSNA and any haemodynamic parameter at rest. There was a significant inverse correlation between CPBR sympathetic gain (CPBRSG, slope of the regression line correlating percentage change in MSNA (bursts/min) per mmHg decrease in CVP during non-hypotensive LBNP) and resting MSNA (r = -0.72,p < 0.0001). A significant inverse correlation was also observed between MSNA responses to the CPT (expressed as percentage change in burst frequency from control) and the resting MSNA (r = –0.63,p = 0.008). Sixteen subjects were restudied 3 weeks to 14 months later to determine reproducibility of measurements; resting BP and CVP, HR, and MSNA levels were not different between the two sessions, as was CPBRSG. In ten of these 16 subjects, in whom the CPT was repeated the MSNA response also was not significantly different. These studies demonstrate an inverse relationship between resting MSNA and both cardiopulmonary baroreflex sensitivity and sympathetic neural responses to the nonbaroreflex mediated cold pressor stimulus. These findings suggest that resting levels of sympathetic neural activity influence reflex-mediated changes in muscle sympathetic nerve activity.     

Increase in muscle nerve sympathetic activity after glucose intake is blunted in the elderly

Muscle nerve sympathetic activity (MSA; involved in blood pressure regulation) was recorded by microneurography in the peroneal nerve for 90 min after ingestion of 100 gd-glucose in three groups of healthy subjects: young subjects (mean age 26 years) and 70-year-old men with normal and reduced insulin sensitivity as assessed by euglycaemic insulin clamp. Muscle nerve sympathetic activity at rest was lowest in the young and highest in the insulin-resistant subjects (burst frequencies 19.8±6.0, 47.7±7.0 and 55.1±11.5 bursts/min for the three groups, respectively). The young subjects responded to glucose intake with a pronounced increase in MSA, a response that was blunted in the elderly and weakest in the insulin-resistant subjects. A similar relationship was observed during a Valsalva manoeuvre, indicating that the blunted response in the elderly is a generalized phenomenon. Blood pressure remained stable in the young subjects but fell slightly and significantly in the elderly subjects. It is concluded that old subjects utilize their total capacity for MSA close to maximum at rest. Thus, the reserve for response to stimuli normally evoking a strong increase in MSA is restricted. This restriction may contribute to postprandial hypotension in the healthy elderly.     

Effect of neurovestibular stimulation on autonomic regulation

Conditions associated with nausea and vomiting, such as motion sickness or side effects of medications, are commonly associated with a clinical picture consistent with parasympathetic activation and sympathetic withdrawal. It can be postulated, therefore, that vestibular stimulation contributes to sympathetic withdrawal. To test this hypothesis five normal volunteers, 24–33 years old, were studied during caloric vestibular stimulation while monitoring muscle sympathetic nerve activity directly through a needle electrode placed in a peroneal nerve. The ear was irrigated with water at a flow rate of 450 ml/min and 37°C. The water temperature was sequentially lowered by 7°C intervals until intolerable side effects developed or a temperature of 16°C was reached. Nystagmus was induced in all subjects, but heart rate, blood pressure, muscle sympathetic nerve activity and plasma norepinephrine levels did not change significantly during or after caloric stimulation, even when the subjects felt dizzy and nauseated. No evidence of sympathetic withdrawal was observed in any subject either by muscle sympathetic nerve activity or plasma norepinephrine measurements. In conclusion, we have found that selective vestibular stimulation is not accompanied by significant changes in the sympathetic nervous system function. In particular, no sympathetic withdrawal was observed. It could be argued that lack of sympathetic stimulation is an inadequate response to the symptoms associated with caloric stimulation.     

Relative burst amplitude in human muscle sympathetic nerve activity: a sensitive indicator of altered sympathetic traffic

Microneurographically recorded sympathetic outflow to the human muscle vascular bed is traditionally quantified by identifying pulse-synchronous bursts of impulses in a mean voltage neurogram and expressing them in terms of bursts per minute (burst frequency) or bursts per 100 heart beats (burst incidence). As both these measures show large inter-individual differences in resting healthy subjects, a problem arises when comparing sympathetic traffic in cross-sectional studies, making moderate differences in muscle sympathetic nerve activity (MSA) between groups difficult to identify. Absolute measures of the strength of the sympathetic discharges (burst amplitude or area) can also be evaluated. However, as they critically depend on the proximity of the microelectrode to the recorded fibres, such measures cannot be used for inter-individual comparisons.The aim of the present study was to evaluate the use of relative burst amplitude spectra for quantification of MSA, describing the proportion of small vs large bursts in a neurogram. We recorded MSA in 18 patients with mild to moderate congestive heart failure (CHF) (New York Heat Association functional classes I–IIIA) and 18 matched healthy controls. Sympathetic activity was expressed as burst frequency, burst incidence and burst amplitude spectra. When comparing the traditional burst counts between the groups (presented as the median and 25th–75th percentiles) there was a tendency towards higher MSA in CHF patients, but the difference was not significant (42 (34–52) vs 53 (41–63) bursts/min, 62 (51–78) vs 69 (52–84) bursts/100 heart beats, both ns). Relative burst amplitude spectra, on the other hand, were clearly shifted to the right in the CHF group compared to the control group (median burst amplitudes 42 (34–45) vs 30 (28–35),P=0.0002).Relative burst amplitude spectra thus appear to provide a more sensitive indicator of altered MSA than traditional burst counts. The right-ward shift of these spectra may suggest that sympatho-excitation occurs early in the development of CHF.     

Variation of aortic blood velocity with age at rest and during exercise in normal subjects

The effect of age on aortic blood velocity signal and its derived indices of left ventricular function namely peak velocity, stroke distance (the velocity—time integral) and minute distance (stroke distance × heart rate) was studied at rest and at maximal-tolerated supine exercise using a 2-mHz continuous-wave Doppler ultrasound applied via the suprasternal notch in 66 normal healthy volunteers. The peak velocity, stroke distance and the minute distance, both at rest and at maximal exercise decreased with advancing age by 1–1.1% per annum between the ages of 20 and 70 years. Accordingly a method for age-adjustment of the peak velocity, stroke and minute distances was introduced for clinical use in assessing individual subjects. These results indicate that age is an important variable affecting aortic blood velocity and its derived variables as non-invasive measures of left ventricular function both at rest and with exercise. The relationship with age is an important consideration when assessing individual subjects.     

Attenuated 24-h heart rate variability in apparently healthy subjects,subsequently suffering sudden cardiac death

Attenuated cardiac parasympathetic activity appear to be an important risk factor contributing to sudden cardiac death in subjects with overt coronary disease but its predictive value in otherwise healthy normal subjects is not known. We have for 8 years followed 260 apparently healthy adult subjects who underwent Hotler monitoring. Twelve died, 14 developed ischaemic heart disease and four suffered sudden cardiac death. A healthy control subject was matched, along with other risk factors, for each case. In each subject 24-h heart rate variability was calculated as the deviation of all normal R—R intervals from mean R—R (SD) and the percentage of successive R—R interval differences exceeding 6% (%DIF6%)—this was used as an index of cardiac parasympathetic activity. There were no significant differences in heart rate variability between the cases developing problems and controls. In the sudden cardiac death victims, however, there was a clear trend towards lower heart rate variability. In them waketime mean SD was 73 ms versus 85 ms for cases and controls respectively (p = 0.08), and for sleeptime 61 ms versus 76 ms (p = 0.07). Compared to normal limits for heart rate variability obtained in 140 subjects that remained healthy for 8 years, figures for both SD and %DIF6% in sudden cardiac death subjects were at or below 95% confidence limits. The results indicate that altered autonomic balance may contribute to sudden cardiac death even in apparently healthy subjects. Subjects with a low 24-h heart rate variability on Holter monitoring may be predicted at an early stage of being at greater risk. This has considerable implications not only for predicting subjects at risk but for assessing physiological (such as exercise) and pharmacological interventions which may reduce such risk.     

Sympathetic activity at rest and motor brain areas: FDG-PET study

Although recent studies identified brain areas which are involved in short term activation of the sympathetic nervous system, little is known about brain mechanisms which generate the individual variability of basal autonomic activity. In this fluorodeoxyglucose positron emission tomography study (FDG-PET), we aimed to identify brain regions, which covary with function parameters of the autonomic nervous system at rest. Therefore, FDG-PET (Siemens, Germany) was performed twice in 14 healthy resting subjects (7 m, 7 f; mean age 29.5 years) while different parameters of autonomic function were assessed simultaneously: Blood pressure, heart rate, power spectra of heart rate variability (HF/LF ratio) and plasma catecholamines. In order to control for attention, subjects had to focus visual affective neutral presentations during the experiment. Correlation analysis was performed as a region of interest analysis using SPM2 software (p<0.001 uncorrected). Sympathetic activity at rest varied substantially between subjects. There were significant positive correlations between increase of regional cerebral glucose metabolism (rCGM) of the heads of caudate nuclei on both sides and the HF/LF ratio of heart rate variability. Furthermore, significant negative correlations between both heart rate and plasma catecholamines and rCGM decreases of caudate nuclei heads were found. In addition, there was a positive correlation between plasma catecholamines and primary motor cortex activation. Autonomic nervous system at rest seems to be partially interlocked with activity of motor brain regions - the caudate nuclei and the motor cortex. This might have clinical implications for the understanding of stress-related disorders, which are frequently accompanied by increased sympathetic activity as well as muscle tone.     

Muscle and skin sympathetic activities in Ross syndrome

ObjectivesRoss syndrome (RS) is a rare degenerative disorder characterized by tonic pupil, areflexia and anhydrosis. The underlying lesion affects postganglionic skin sympathetic nerve fibers whereas the postganglionic muscle sympathetic branch is thought to be spared. Microneurography explores both skin and muscle peripheral sympathetic branches and it does not usually detect peripheral sympathetic outflow in either branch in chronic autonomic failure syndromes. The aim of this study was to record sympathetic activity by microneurography for the first time in RS patients to confirm the selective involvement of skin sympathetic nerve activity (SSNA) with spared muscle sympathetic nerve activity (MSNA).MethodsWe studied seven patients (49 ± 14 years, four males) with a typical clinical picture and skin biopsy findings. Patients underwent cardiovascular reflexes and microneurography from the peroneal nerve (anhydrotic skin) to record MSNA, SSNA and the corresponding organ effector responses (skin sympathetic response-SSR and skin vasomotor response-SVR) in the same innervation field. The absence of sympathetic bursts was established after exploring at least three different corresponding nerve fascicles. Twenty age-matched healthy subjects served as controls.ResultsRS patients complained of diffuse anhydrosis and they showed tonic pupil and areflexia. Cardiovascular reflexes were normal. All patients displayed absent SSNA, SSR and SVR whereas MSNA was always recorded showing normal characteristics.ConclusionMicroneurographic study of sympathetic activity from affected skin confirmed the selective involvement of skin sympathetic activity with spared muscle sympathetic activity and it may represent the neurophysiological hallmark of the disease.SignificanceMicroneurography together with clinical and skin biopsy findings may contribute to RS diagnosis. Our data also suggest that autonomic damage in RS does not involve cardiovascular activity.     

A comparison of cardiovascular reflex tests and spectral analysis of heart rate variability in healthy subjects

Determination of whether results of cardiovascular reflex tests and spectral analysis of heart rate variability are age dependent and whether there is correlation between results of both, cardiovascular reflex tests (the Valsalva manoeuvre, deep breathing test, handgrip test, cold face stimulus test, orthostatic test) and spectral analysis of heart rate variability were performed on 83 healthy volunteers of both genders, aged 21 to 70 years. We found that results of all heart rate based tests and results of spectral analysis decreased with aging, while results of blood pressure based tests did not. Parasympathetic activity predominated in younger subjects, while in older subjects sympathetic activity was dominant. Valsalva, deep breathing, and orthostatic ratios correlated with integrals of amplitude spectra in the standing posture and deep breathing and cold face stimulus ratios with integrals of amplitude spectra in the supine posture, whereas blood pressure changes during handgrip and orthostatic test did not correlate with integrals of the amplitude spectra. These findings suggest that tests based on heart rate may be more sensitive than tests based on blood pressure changes. This study supports the use of spectral analysis as an additional clinical test of autonomic nervous system function and stresses the importance of age in the evaluation of the results of autonomic nervous system function testing.     

THE INCREASE IN SYMPATHETIC NERVE ACTIVITY AFTER GLUCOSE INGESTION IS REDUCED IN TYPE I DIABETES

Food intake is followed by an increase in baroreflex-governed sympathetic outflow to muscle vessels. It is established that insulin contributes to this stimulation; however, the increase occurs (to a lesser degree) even in the absence of enhanced insulin secretion. To further elucidate the role of insulin, muscle nerve sympathetic activity was recorded by microneurography, and the increase after an oral 100-g glucose load in eight C-peptide-negative patients with type I diabetes without any signs of neuropathy was compared with that in 16 healthy control subjects. The level of sympathetic activity at rest was similar in the two groups (type I diabetes patients, 19.5 +/− 2.4 bursts/min; controls, 20.4 +/− 4.8 bursts/min; means +/− S.D.). Following glucose intake there was a significant increase in activity in both groups, with maximum values at 30 min of 24.3 +/− 3.7 bursts/min for type I diabetes patients and 34.4 +/− 9.1 bursts/min for controls. The summarized response (during 90 min) of the diabetic patients was less than half that of the control subjects (P = 0.0003). It is concluded that the response of muscle nerve sympathetic activity to glucose ingestion is reduced to about half of its normal strength in the absence of insulin, and that there is no difference in sympathetic outflow at rest between healthy subjects and diabetic patients without polyneuropathy.     

Assessment of sympathetic cardiovascular influences in man: Haemodynamic and humoral markers versus microneurography

Evaluation of sympathetic cardiovascular influences has important physiological, pathophysiological and clinical implications. This paper reviews some of the methods employed to measure these influences in man, along with their advantages and disadvantages. The most useful methods appear to be the measurement of plasma noradrenaline (particularly when modified to calculate spillover rate of noradrenaline) and direct recording of sympathetic nerve traffic. With the former, despite the technological advances in measurement, certain methodological problems remain, such as the separation of noradrenaline secretion from clearance. With the latter technique peripheral muscle and skin sympathetic activity can be measured separately but the question of regional vascular variability has still to be resolved. A combination of these two methods may represent the ideal approach. This review considers the complex problems associated with attempts to precisely quantify sympathetic cardiovascular influences in man.     

Reproducibility of methods for assessing baroreflex sensitivity and temporal QT variability in end-stage renal disease and healthy subjects

Spontaneous baroreflex sensitivity (BRS), the reflex heart rate modulation in response to blood pressure changes (predominantly an index of cardiac vagal activity) and temporal QT variability (an index of myocardial repolarization) have been demonstrated to convey important prognostic information. The information about reproducibility of BRS and temporal QT variability is limited and there is lack of information regarding patients with cardiovascular diseases. We investigated reproducibility of spontaneous BRS using the sequence technique and temporal QT variability index (QTVI) in terms of intra-, interexaminer and within-subject variability in end-stage renal disease patients (ESRD, n=17, age 55+/-14 years) and healthy subjects (HS, n=29, age 32+/-12 years, P<0.01). ECG and blood pressure (Portapres) were recorded on two separate days and BRS and QTVI were evaluated by two independent examiners. The mean heart rate was similar in ESRD patients in comparison to healthy controls, whereas the mean arterial pressure was 13 % higher in ESRD patients (P<0.01). Spontaneous BRS was 62% lower (P<0.01) and QTVI was 41% higher in ESRD patients (P<0.01) compared to healthy subjects, respectively. Coefficient of variation (CV) of within-subject reproducibility of BRS and QTVI measurements was moderate (BRS: 33 % for ESRD, 27% for HS; QTVI: 40% for ESRD, 18% for HS). The 95% limit of within-subject reproducibility of BRS measurements was 3.8 ms/mm Hg for ESRD patients and 8.1 ms/mm Hg for healthy subjects; whereas the 95% limit of reproducibility of within-subject reproducibility of QTVI measurements was 0.73 for ESRD patients and 0.55 for healthy subjects. Concordance correlation coefficients of within-subject variability of BRS and QTVI were between 0.74 and 0.83 in both groups. CV of intra- and inter-examiner reproducibility of BRS and QTVI measurements in both groups ranged between 1 and 11%. In conclusion, the intra- and inter-examiner reproducibility/agreement of BRS and QTVI were high, whereas the within-subject reproducibility of these two methods was moderate, in both ESRD patients and healthy subjects. Thus, small differences in BRS and QTVI in longitudinal/interventional studies should be interpreted with caution.     

The biochemical assessment of sympathoadrenal activity in man

Sympathoadrenal activity in man can be assessed by measuring catecholamines in plasma or by recording impulses in sympathetic nerves to skin and muscles by microneurography. Several studies have indicated that forearm venous plasma noradrenaline concentration and muscle sympathetic nerve activity are closely correlated in normal subjects at rest as well as during various conditions with increased or decreased sympathetic activity. Both parameters are influenced by baroreceptors and increase with age. Plasma adrenaline should preferably be measured in arterial blood because the extraction of adrenaline in organs and tissues may increase considerably when plasma adrenaline increases. The problem of studying the metabolic clearance rate of noradrenaline but not of adrenaline is discussed. It is emphasized that sympathetic activity is highly differentiated and it should therefore be measured in specific organs and tissues. Sympathetic activity in internal organs can be studied by measuring the release of noradrenaline from these organs. Imaging technique may, however, prove useful in future studies. The significance of microdialysis, measurements of plasma catecholamine metabolites, dopa and dopamine, plasma neuropeptide Y, catecholamines in urine and in the cerebrospinal fluid is discussed. Furthermore, it is emphasized that adrenergic agonist and antagonist drugs are important tools to study sensitivity and responsiveness to catecholamines preferably in specific organs and tissues. Finally, a few examples are given of the values in human research of the techniques described.     

Spectral analysis of heart rate variability in bronchial asthma

Sympathetic and parasympathetic activity was evaluated in ten healthy controls, nine asymptomatic, untreated asthmatic subjects and ten asthmatic patients during treatment for acute asthma, by measurement of the variation in resting heart rate using frequency spectrum analysis. Heart rate was recorded by ECG and respiratory rate by impedance plethysmography. Spectral density of the beat-to-beat heart rate was measured within the low frequency band 0.04 to 0.10 Hz (low frequency power) modulated by sympathetic and parasympathetic activity, and within a 0.12 Hz band width at the respiratory frequency mode (respiratory frequency power) modulated by parasympathetic activity. Acute asthmatics had higher heart rates than either of the other two groups; this was probably related to the effects of beta-adrenoceptor agonist medication. Sympathetically mediated heart rate variability (normalized low frequency power) was significantly lower in both asymptomatic (p < 0.002) and acute (p < 0.02) asthma subjects compared to controls. This is consistent with altered sympathetic/parasympathetic regulation of heart rate in subjects with bronchial asthma.     

Effect of sensory stimulation (acupuncture) on sympathetic and parasympathetic activities in healthy subjects

It has been postulated that sensory stimulation (acupuncture) affects the cardiovascular system via the autonomic nervous system. Previously, skin temperature, thermography, plethysmography and blood pressure changes have been used in evaluation of sympathetic nerve activity following acupuncture. By using power spectral analysis, the low frequency and high frequency components of heart rate variability can be calculated reflecting the sympathetic and parasympathetic activity. The purpose of this study was to investigate to what extent acupuncture applied into the thenar muscle and into the cavum concha of the ear induced changes in the sympathetic and/or parasympathetic nervous system in healthy subjects. MATERIALS AND METHODS: Twelve healthy volunteers, six men and six women, mean age 34.4 (range 23-48) participated in three balanced, randomly distributed sessions. At an individual initial visit the 12 volunteers were introduced to the needle sensation by having a needle inserted into the point LI 11. The needle sensation was evoked and the subject was trained to identify the characteristic needle sensation. The introduction was followed by three test sessions. SESSION A: A short acupuncture needle, (Seirin no 3, ∅0.20x15 mm) was inserted perpendicular into the earpoint, Lu 1, in the left inferior hemi-conchae. SESSION B: An acupuncture needle (Hwato, ∅0.30x30 mm) was inserted perpendicular into the Hegu point (LI 4) in the middle of the right dorsal thenar muscle. SESSION C: An acupuncture needle (Hwato, ∅0.30x30 mm) was inserted perpendicular superficially into the skin overlying the Hegu point on the left hand. Results. Stimulation of the ear induced a significant increase in the parasympathetic activity during the stimulation period of 25 min (P<0.05) and during the post-stimulation period of 60 min (P<0.05). No significant changes were observed in either the sympathetic activity, blood pressure or heart rate. Stimulation of the thenar muscle resulted in a significant increase in the sympathetic and the parasympathetic activity during the stimulation period (P<0.01) and during the post-stimulation period (P<0.01 and P<0.001, respectively). A significant decrease in the heart rate frequency (P<0.05) at the end of the post-stimulation period was also demonstrated. The superficial needle insertion into the skin overlaying the right thenar muscle caused a pronounced balanced increase in both the sympathetic and parasympathetic activity during the post stimulation period of 60 min (P<0.01) while no changes were observed during the stimulation period. CONCLUSION: It is indicated that sensory stimulation (acupunctura) in healthy persons is associated with changed activity in the sympathetic and parasympathetic nervous system depending on site of stimulation and period of observation.     

Sympathetic neural control of integrated cardiovascular function: insights from measurement of human sympathetic nerve activity

Sympathetic neural control of cardiovascular function is essential for normal regulation of blood pressure and tissue perfusion. In the present review we discuss sympathetic neural mechanisms in human cardiovascular physiology and pathophysiology, with a focus on evidence from direct recordings of sympathetic nerve activity using microneurography. Measurements of sympathetic nerve activity to skeletal muscle have provided extensive information regarding reflex control of blood pressure and blood flow in conditions ranging from rest to postural changes, exercise, and mental stress in populations ranging from healthy controls to patients with hypertension and heart failure. Measurements of skin sympathetic nerve activity have also provided important insights into neural control, but are often more difficult to interpret since the activity contains several types of nerve impulses with different functions. Although most studies have focused on group mean differences, we provide evidence that individual variability in sympathetic nerve activity is important to the ultimate understanding of these integrated physiological mechanisms.     

Inhibition of muscle sympathetic nerve activity during yawning

Yawning is a complex event that depends largely on the autonomic nervous system. Microneurographic techniques were used to study the mechanism involved in yawning. A series of spontaneous yawns displayed by a healthy 39-year-old male offered us the opportunity to study the muscle sympathetic nerve activity (MSNA) during this phenomenon. It was found that 2 s of yawning inhibited the MSNA recorded at the right peroneal nerve in the lateral knee area, while 3 s of slow expiration succeeding a yawn provoked an MSNA discharge. Blood pressure decreased with each slow expiration by 5–6 mmHg, and increased again with the renewed MSNA discharge. We conclude that yawning is associated with a sympathetic suppression that favours a parasympathetic dominance, as indicated by the MSNA and the decrease in blood pressure. The slow expiration following a yawn is associated with a sympathetic activation marked by an MSNA discharge and an increase in blood pressure.