Limb congestion enhances the synchronization of sympathetic outflow with muscle contraction

In this report, we examined if the synchronization of muscle sympathetic ne rve activity (MSNA) with muscle contraction is enhanced by limb congestion. To explore this relationship, we applied signal-averaging techniques to th e MSNA signal obtained during short bouts of forearm contraction (2-s contr action/ 3-s rest cycle) at 40% maximal voluntary contraction for 5 min. We performed this analysis before and after forearm venous congestion; an inte rvention that augments the autonomic response to sustained static muscle co ntractions via a local effect on muscle afferents. There was an increased p ercentage of the MSNA noted during second 2 of the 5-s contraction/ rest cy cles. The percentage of total MSNA seen during this particular second incre ased from minute 1 to 5 of contraction and was increased further by limb co ngestion (control minute 1 = 25.6 +/- 2.0%, minute 5 = 32.8 +/- 2.2%; limb congestion minute 1 = 29.3 +/- 2.1%, minute 5 = 37.8 +/- 3.9%; exercise mai n effect <0.005; limb congestion main effect P = 0.054). These changes in t he distribution of signal-averaged MSNA were seen despite the fact that the mean number of sympathetic discharges did not increase over baseline. We c onclude that synchronization of contraction and MSNA is seen during short r epetitive bouts of handgrip. The sensitizing effect of contraction time and limb congestion are apparently due to feedback from muscle afferents withi n the exercising muscle.