Muscle sympathetic single-unit responses during rhythmic handgrip exercise and isocapnic hypoxia in males: the role of sympathoexcitation magnitude

Author(s):  
Anthony V. Incognito ◽  
André L. Teixeira ◽  
Brooke M. Shafer ◽  
Massimo Nardone ◽  
Tyler D. Vermeulen ◽  
...  

A small proportion of postganglionic muscle sympathetic single units can be inhibited during sympathoexcitatory stressors in humans. However, whether these responses are dependent on the specific stressor or the level of sympathoexcitation remains unclear. We hypothesize that, when matched by sympathoexcitatory magnitude, different stressors can evoke similar proportions of inhibited single units. Multiunit and single-unit muscle sympathetic nerve activity (MSNA) were recorded in seven healthy, young males at baseline and during: 1) rhythmic handgrip exercise (40% of maximum voluntary contraction) and 2) acute isocapnic hypoxia (partial pressure of end-tidal O2: 47±3 mmHg). Single units were classified as activated, nonresponsive, or inhibited if the spike frequency was above, within, or below the baseline variability, respectively. By design, rhythmic handgrip and isocapnic hypoxia similarly increased multiunit total MSNA (D273±208 vs. D254±193 AU, P=0.84) and single-unit spike frequency (D8±10 vs. D12±13 spikes/min, P=0.12). Among 19 identified single units, the proportion of activated (47% vs. 68%) non-responsive (32% vs. 16%) and inhibited (21% vs. 16%) single units were not different between rhythmic handgrip and isocapnic hypoxia (P=0.42). However, only 9 (47%) single units behaved with concordant response patterns across both stressors (7 activated, 1 non-responsive, and 1 inhibited during both stressors). During the 1-min epoch with the highest increase in total MSNA during hypoxia (D595±282 AU, P<0.01) only 1 single unit was inhibited. These findings suggest that the proportion of muscle sympathetic single units inhibited during stress are associated with the level of sympathoexcitation and not the stressor per se in healthy young males.

2006 ◽  
Vol 290 (2) ◽  
pp. H853-H860 ◽  
Author(s):  
Hisayoshi Murai ◽  
Shigeo Takata ◽  
Michiro Maruyama ◽  
Manabu Nakano ◽  
Daisuke Kobayashi ◽  
...  

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.


1983 ◽  
Vol 54 (6) ◽  
pp. 1457-1462 ◽  
Author(s):  
S. R. Muza ◽  
L. Y. Lee ◽  
R. L. Wiley ◽  
S. McDonald ◽  
F. W. Zechman

Previous research indicates that fatiguing static exercise causes hyperventilation and a decrease of end-tidal CO2 partial pressure PETCO2. The objectives of this study were 1) to examine the changes in pattern of breathing during static exercise, and 2) to define the isocapnic ventilatory response. Six healthy males were studied once a week at one of three levels of static handgrip exercise: 15, 25, or 30% maximum voluntary contraction (MVC) was sustained for 5 min while holding PETCO2 constant or allowing it to run free. During 25 and 30% MVC, we observed 1) progressive increases in mean tidal volume (VT), inspiratory ventilation (VI), VT/TI, heart rate (HR), and arterial BP, 2) increased breath-to-breath variability of VT, 3) no significant changes in respiratory frequency (f), and 4) progressive decreases in PETCO2. Keeping PETCO2 constant at preexercise levels did not change the pattern or magnitude of the ventilatory response to exercise. The time course and magnitude of the subjects' perceived effort resembled the time course and magnitude of the ventilatory response. The variability of VT during the response to static exercise suggests an element of control instability. The identical ventilatory responses during hypocapnic and isocapnic conditions may result from the slow response of the central chemoreceptors; an overriding influence of muscle afferents; and/or increased central command arising with fatigue.


2019 ◽  
Vol 316 (1) ◽  
pp. H13-H23 ◽  
Author(s):  
Anthony V. Incognito ◽  
Connor J. Doherty ◽  
Massimo Nardone ◽  
Jordan B. Lee ◽  
Karambir Notay ◽  
...  

Two subpopulations of muscle sympathetic single units with opposite discharge characteristics have been identified during low-level cardiopulmonary baroreflex loading and unloading in middle-aged adults and patients with heart failure. The present study sought to determine whether similar subpopulations are present in young healthy adults during cardiopulmonary baroreflex unloading ( study 1) and rhythmic handgrip exercise ( study 2). Continuous hemodynamic and multiunit and single unit muscle sympathetic nerve activity (MSNA) data were collected at baseline and during nonhypotensive lower body negative pressure (LBNP; n = 12) and 40% maximal voluntary contraction rhythmic handgrip exercise (RHG; n = 24). Single unit MSNA responses were classified as anticipated or paradoxical based on whether changes were concordant or discordant with the multiunit MSNA response, respectively. LBNP and RHG both increased multiunit MSNA burst frequency (∆5 ± 3 bursts/min, P < 0.001; ∆5 ± 8 bursts/min, P = 0.005), burst amplitude (∆5 ± 7%, P = 0.04; ∆13 ± 14%, P < 0.001), and total MSNA (∆302 ± 191 AU/min, P = 0.001; ∆585 ± 556 AU/min, P < 0.001). During LBNP and RHG, 43 and 64 muscle single units were identified, respectively, which increased spike frequency (∆9 ± 11 spikes/min, P < 0.001; ∆10 ± 19 spikes/min, P < 0.001) and the probability of multiple spike firing (∆10 ± 12%, P < 0.001; ∆11 ± 26%, P = 0.001). During LBNP and RHG, 36 (84%) and 39 (61%) single units possessed anticipated firing responses (∆12 ± 10 spikes/min, P < 0.001; ∆19 ± 19 spikes/min, P < 0.001), whereas 7 (16%) and 25 (39%) single units exhibited paradoxical reductions (∆−3 ± 1 spikes/min, P = 0.003; ∆−4 ± 5 spikes/min, P < 0.001). The observation of divergent subpopulations of muscle sympathetic single units in healthy young humans during two mild sympathoexcitatory stressors supports differential control at the fiber level as a fundamental characteristic of human sympathetic regulation. NEW & NOTEWORTHY The activity of muscle sympathetic single units was recorded during cardiopulmonary baroreceptor unloading and rhythmic handgrip exercise in young healthy humans. During both stressors, the majority of single units (84% and 61%) exhibited anticipated behavior concordant with the integrated muscle sympathetic response, whereas a smaller proportion (16% and 39%) exhibited paradoxical sympathoinhibition. These results support differential control of postganglionic muscle sympathetic fibers as a characteristic of human sympathetic regulation during mild sympathoexcitatory stress. Listen to this article's corresponding podcast at https://ajpheart.podbean.com/e/differential-control-of-sympathetic-outflow-in-young-humans/ .


2018 ◽  
Vol 314 (1) ◽  
pp. R114-R121 ◽  
Author(s):  
Anthony V. Incognito ◽  
Connor J. Doherty ◽  
Jordan B. Lee ◽  
Matthew J. Burns ◽  
Philip J. Millar

Negative and positive muscle sympathetic nerve activity (MSNA) responders have been observed during mental stress. We hypothesized that similar MSNA response patterns could be identified during the first minute of static handgrip and contribute to the interindividual variability throughout exercise. Supine measurements of multiunit MSNA (microneurography) and continuous blood pressure (Finometer) were recorded in 29 young healthy men during the first (HG1) and second (HG2) minute of static handgrip (30% maximal voluntary contraction) and subsequent postexercise circulatory occlusion (PECO). Responders were identified on the basis of differences from the typical error of baseline total MSNA: 7 negative, 12 positive, and 10 nonresponse patterns. Positive responders demonstrated larger total MSNA responses during HG1 ( P < 0.01) and HG2 ( P < 0.0001); however, the increases in blood pressure throughout handgrip exercise were similar between all groups, as were the changes in heart rate, stroke volume, cardiac output, total vascular conductance, and respiration (all P > 0.05). Comparing negative and positive responders, total MSNA responses were similar during PECO ( P = 0.17) but opposite from HG2 to PECO (∆40 ± 46 vs. ∆-21 ± 62%, P = 0.04). Negative responders also had a shorter time-to-peak diastolic blood pressure during HG1 (20 ± 20 vs. 44 ± 14 s, P < 0.001). Total MSNA responses during HG1 were associated with responses to PECO ( r = 0.39, P < 0.05), the change from HG2 to PECO ( r = −0.49, P < 0.01), and diastolic blood pressure time to peak ( r = 0.50, P < 0.01). Overall, MSNA response patterns during the first minute of static handgrip contribute to interindividual variability and appear to be influenced by differences in central command, muscle metaboreflex activation, and rate of loading of the arterial baroreflex.


2013 ◽  
Vol 305 (6) ◽  
pp. H931-H938 ◽  
Author(s):  
Philip J. Millar ◽  
Hisayoshi Murai ◽  
Beverley L. Morris ◽  
John S. Floras

Atrial mechanoreceptors, stimulated by increased pressure or volume, elicit in healthy humans a net sympathoinhibitory response. The co-existence of an atrial reflex eliciting muscle sympathoexcitation has been postulated but undetected by conventional multi-unit muscle sympathetic nerve activity (MSNA). We hypothesized that in response to a selective increase in atrial pressure, single-unit MSNA would reveal a subpopulation of efferent sympathetic neurons with firing patterns opposite to the integrated multi-unit MSNA envelope. Multi- and single-unit MSNA recordings were acquired in eight healthy middle-aged subjects (age, 57 ± 8 years; body mass index, 25 ± 2 kg/m2) submitted to selective decreases or increases in atrial pressure by nonhypotensive lower body negative pressure (LBNP; −10 mmHg) or nonhypertensive lower body positive pressure (LBPP; +10 mmHg), respectively. Single-unit MSNA firing responses were classified as anticipated if spike frequency and incidence increased with LBNP or decreased with LBPP and paradoxical if they decreased with LBNP or increased with LBPP. LBNP decreased (3.2 ± 2.8 to 1.4 ± 3.1 mmHg, P < 0.01) and LBPP increased (3.3 ± 2.7 to 4.9 ± 2.8 mmHg, P < 0.01) estimated central venous pressure without affecting stroke volume, systemic pressure, or resistance. Multi-unit MSNA increased with LBNP (31 ± 17 to 38 ± 19 bursts/min, P < 0.01) and diminished with LBPP (33 ± 15 to 28 ± 15 bursts/min, P < 0.01). Of 21 single-units identified, 76% exhibited firing responses to both LBNP and LBPP concordant with multi-unit MSNA, whereas 24% demonstrated discordant or paradoxical responses. The detection of two subpopulations of single-units within the multi-unit MSNA recording, exhibiting opposite firing characteristics, establishes the first evidence in humans for the existence of an excitatory cardiac-muscle sympathetic reflex activated by increasing atrial pressure.


1989 ◽  
Vol 66 (4) ◽  
pp. 1586-1592 ◽  
Author(s):  
W. F. Taylor ◽  
J. M. Johnson ◽  
W. A. Kosiba ◽  
C. M. Kwan

Cutaneous vascular responses to dynamic exercise have been well characterized, but it is not known whether that response pattern applies to isometric handgrip exercise. We examined cutaneous vascular responses to isometric handgrip and dynamic leg exercise in five supine men. Skin blood flow was measured by laser-Doppler velocimetry and expressed as laser-Doppler flow (LDF). Arterial blood pressure was measured noninvasively once each minute. Cutaneous vascular conductance (CVC) was calculated as LDF/mean arterial pressure. LDF and CVC responses were measured at the forearm and chest during two 3-min periods of isometric handgrip at 30% of maximum voluntary contraction and expressed as percent changes from the preexercise levels. The skin was normothermic (32 degrees C) for the first period of handgrip and was locally warmed to 39 degrees C for the second handgrip. Finally, responses were observed during 5 min of dynamic two-leg bicycle exercise (150–175 W) at a local skin temperature of 39 degrees C. Arm LDF increased 24.5 +/- 18.9% during isometric handgrip in normothermia and 64.8 +/- 14.1% during isometric handgrip at 39 degrees C (P less than 0.05). Arm CVC did not significantly change at 32 degrees C but significantly increased 18.1 +/- 6.5% during isometric handgrip at 39 degrees C (P less than 0.05). Arm LDF decreased 12.2 +/- 7.9% during dynamic exercise at 39 degrees C, whereas arm CVC fell by 35.3 +/- 4.6% (in each case P less than 0.05). Chest LDF and CVC showed similar responses.(ABSTRACT TRUNCATED AT 250 WORDS)


2001 ◽  
Vol 280 (3) ◽  
pp. H969-H976 ◽  
Author(s):  
Catherine F. Notarius ◽  
Deborah J. Atchison ◽  
John S. Floras

Peak oxygen uptake (V˙o 2 peak) in patients with heart failure (HF) is inversely related to muscle sympathetic nerve activity (MSNA) at rest. We hypothesized that the MSNA response to handgrip exercise is augmented in HF patients and is greatest in those with lowV˙o 2 peak. We studied 14 HF patients and 10 age-matched normal subjects during isometric [30% of maximal voluntary contraction (MVC)] and isotonic (10%, 30%, and 50% MVC) handgrip exercise that was followed by 2 min of posthandgrip ischemia (PHGI). MSNA was significantly increased during exercise in HF but not normal subjects. Both MSNA and HF levels remained significantly elevated during PHGI after 30% isometric and 50% isotonic handgrip in HF but not normal subjects. HF patients with lower V˙o 2 peak (<56% predicted; n = 8) had significantly higher MSNA during rest and exercise than patients withV˙o 2 peak > 56% predicted ( n = 6) and normal subjects. The muscle metaboreflex contributes to the greater reflex increase in MSNA during ischemic or intense nonischemic exercise in HF. This occurs at a lower threshold than normal and is a function ofV˙o 2 peak.


2001 ◽  
Vol 281 (3) ◽  
pp. H1312-H1318 ◽  
Author(s):  
C. F. Notarius ◽  
D. J. Atchison ◽  
G. A. Rongen ◽  
J. S. Floras

Adenosine (Ado) increases muscle sympathetic nerve activity (MSNA) reflexively. Plasma Ado and MSNA are elevated in heart failure (HF). We tested the hypothesis that Ado receptor blockade by caffeine would attenuate reflex MSNA responses to handgrip (HG) and posthandgrip ischemia (PHGI) and that this action would be more prominent in HF subjects than in normal subjects. We studied 12 HF subjects and 10 age-matched normal subjects after either saline or caffeine (4 mg/kg) infusion during isometric [30% of maximal voluntary contraction (MVC)] and isotonic (10%, 30%, and 50%) HG exercise, followed by 2 min of PHGI. In normal subjects, caffeine did not block increases in MSNA during PHGI after 50% HG. In HF subjects, caffeine abolished MSNA responses to PHGI after both isometric and 50% isotonic exercise ( P < 0.05) but MSNA responses during HG were unaffected. These findings are consistent with muscle metaboreflex stimulation by endogenous Ado during ischemic or intense nonischemic HG in HF and suggest an important sympathoexcitatory role for endogenous Ado during exercise in this condition.


2021 ◽  
pp. 1-12
Author(s):  
Henry H. Hunter ◽  
Graeme G. Sorbie ◽  
Fergal M. Grace ◽  
Yaodong Gu ◽  
Wing-Kai Lam ◽  
...  

BACKGROUND: Electromyographic systems are widely used in scientific and clinical practice. The reproducibility and reliability of these measures are crucial when conducting scientific research and collecting experimental data.


2009 ◽  
Vol 587 (11) ◽  
pp. 2613-2622 ◽  
Author(s):  
Hisayoshi Murai ◽  
Masayuki Takamura ◽  
Michirou Maruyama ◽  
Manabu Nakano ◽  
Tatsunori Ikeda ◽  
...  

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