Muscle sympathetic nerve activity responses to dynamic passive muscle stretch in humans

Abstract

It is suggested that mechanoreceptors in muscle play an important role in the exercise pressor reflex. However, it has not been verified whether isolated stimulation of the mechanoreceptors can induce responses in muscle sympathetic nerve activity (MSNA) in young healthy individuals. We tested the hypothesis that passive stretch of muscle can evoke an increase in MSNA in healthy individuals. In 12 young subjects, leg calf muscles were passively stretched, or actively contracted for 5 s followed by a 15-25 s (random length) relaxation period. Stretch and contraction were each repeated 25 times. MSNA, heart rate and blood pressure were analysed, and averaged according to the onset of the force on a beat-by-beat basis. At the 1st to the 3rd heart beat from the onset of stretch, MSNA (199 +/- 30%, P < 0.05) as well as heart rate (102.5 +/- 0.7%, P < 0.05) increased transiently but significantly from the prior stretch baseline (100%), followed (from 3rd to 7th beat from the onset of stretch) by a transient increase in mean blood pressure (101.9 +/- 0.3%, P < 0.05) from the baseline. Similar response patterns were observed during active muscle contractions. The present data show that MSNA responses to isolated stimulation of mechanoreceptors are measurable. Because of baroreflex engagement, the magnitude of the response is small and transient, and the haemodynamic consequences using this protocol may be limited.

Figure 1. Representative tracing of stretch force, ECG, muscle sympathetic nerve activity (MSNA), heart rate (HR), arterial blood pressure (BP), and respiratory excursion (Resp.) during two passive stretch bouts

The stretch bouts were repeated 25 times in each trial, and the intervals between 2 bouts were of random length (15–25 s). The filled circles and squares represent the processed beat-by-beat data of MSNA and force, respectively. The arrows indicate the beat of the onset stretch, and were used to align the data segments of the beat-by-beat data in signal averaging.

Figure 2. Example of signal averaging in one representative subject

A, raw tracing and the averaged signals of force and neurogram during stretch in one subject. The averaging was performed directly with recorded signals. It should note that averaged neurogram tracing was not low in the later portion of the stretch, while MSNA was clearly suppressed in the period. B, averaged force and responses of heart rate (HR), MSNA, mean blood pressure (MAP) by stretch with the processed beat-by-beat data in the same subject. The responses were expressed as a percentage of the respective mean values of 10-beats data prior to the stretch. There were transient increases in heart rate, MSNA and blood pressures after onset of the stretch. The values of the peak responses (indicated with arrows) and the delay were selected from each subject, and the mean peak responses and the delays are shown in Fig. 4 and Table 2, respectively.

Figure 3. Mean dynamic responses of heart rate (HR), MSNA and mean blood pressure (MAP) by passive stretch with spontaneous (A) and controlled (B) breathing in all subjects

The responses were expressed as a percentage of the respective mean values of 10-beats data prior to the stretch. Subject number = 12.

Figure 4. Peak responses of heart rate (HR), MSNA and mean blood pressure (MAP) to passive stretch and active contraction

The responses were expressed as a percentage of the mean values of 10-beats data prior to the stretch or contraction (100%). Subject numbers for the respective condition are shown in Table 1. There was no significant difference in the peak responses between the spontaneous and controlled breathing conditions. *Significantly different from the prior stretch/contraction baseline (P < 0.05). #Significantly different from the stretch with the same breathing condition (P < 0.05).

Figure 5. Mean dynamic responses of heart rate (HR), MSNA and mean blood pressure (MAP) by contraction with spontaneous (A) and controlled (B) breathing

The responses were expressed as a percentage of the respective mean values of 10-beats data prior to the contraction (100%). Subject numbers for the respective conditions are shown in Table 1.

Figure 6. Mean dynamic responses of heart rate (HR), MSNA and mean blood pressure (MAP) to arousal stimulation by low pressure on an ankle

The responses were expressed as percentage of the respective mean values of 10-beats data prior to the contraction (100%). Subject number = 4.