Effect of ageing on muscle sympathetic nerve activity in man. Basic activity in supine position and responsiveness in head-up tilting

Beat-to-beat variability of the QT interval (QTV) is sought to provide an indirect noninvasive measure of sympathetic nerve activity, but a formal quantification of this relationship has not been provided. In this study we used power contribution analysis to study the relationship between QTV and muscle sympathetic nerve activity (MSNA). ECG and MSNA were recorded in 10 healthy subjects in the supine position and after 40° head-up tilt. Power spectrum analysis was performed using a linear autoregressive model with two external inputs: heart period (RR interval) variability (RRV) and MSNA. Total and low-frequency power of QTV was decomposed into contributions by RRV, MSNA, and sources independent of RRV and MSNA. Results show that the percentage of MSNA power contribution to QT is very small and does not change with tilt. RRV power contribution to QT power is notable and decreases with tilt, while the greatest percentage of QTV is independent of RRV and MSNA in the supine position and after 40° head-up tilt. In conclusion, beat-to-beat QTV in normal subjects does not appear to be significantly affected by the rhythmic modulations in MSNA following low to moderate orthostatic stimulation. Therefore, MSNA oscillations may not represent a useful surrogate for cardiac sympathetic nerve activity at moderate levels of activation, or, alternatively, sympathetic influences on QTV are complex and not quantifiable with linear shift-invariant autoregressive models.

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Superentrainment of muscle sympathetic nerve activity during sinusoidal galvanic vestibular stimulation

Journal of Neurophysiology ◽

10.1152/jn.00036.2016 ◽

2016 ◽

Vol 116 (6) ◽

pp. 2689-2694 ◽

Cited By ~ 7

Author(s):

Vaughan G. Macefield ◽

Cheree James

Keyword(s):

Supine Position ◽

Sympathetic Nerve ◽

Sympathetic Nerve Activity ◽

Muscle Sympathetic Nerve Activity ◽

Vestibular Stimulation ◽

Galvanic Vestibular Stimulation ◽

Upright Position ◽

Nerve Activity ◽

Sinusoidal Stimulus ◽

Cross Correlation Analysis

Sinusoidal galvanic vestibular stimulation (sGVS), delivered at frequencies ranging from 0.08 to 2.0 Hz, induces vestibular illusions of side-to-side motion and robust modulation of muscle sympathetic nerve activity (MSNA) to the lower legs. We have previously documented, in seated subjects, de novo synthesis of bursts of MSNA that are temporally locked to the sinusoidal stimulus rather than to the cardiac-related rhythm. Here we tested the hypothesis that this vestibular entrainment of MSNA is higher in the upright than in the supine position. MSNA was recorded from the common peroneal nerve in 10 subjects lying on a tilt table. Bipolar binaural sGVS (±2 mA, 200 cycles) was applied to the mastoid processes at 0.2, 0.8, and 1.4 Hz in the supine and upright (75°) positions. In four subjects, “superentrainment” of MSNA occurred during sGVS, with strong bursts locked to one phase of the sinusoidal stimulus. This occurred more prominently in the upright position. On average, cross-correlation analysis revealed comparable vestibular modulation of MSNA in both positions at 0.2 Hz (84.9 ± 3.6% and 78.7 ± 5.7%), 0.8 Hz (77.4 ± 3.9% and 74.4 ± 8.9%), and 1.4 Hz (69.8 ± 4.6% and 80.2 ± 7.4%). However, in the supine position there was a significant linear fall in the magnitude of vestibular modulation with increasing frequency, whereas this was not present in the upright position. We conclude that vestibular contributions to the control of blood pressure are higher in the upright position.

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