Sympathetic co-activation of skin blood vessels and sweat glands

Abstract.  Skin blood vessels and sweat glands are both innervated bysympathetic C fibers. We investigated whether during diverserespiratory maneuvers the vasomotor responses (VRs) and thesympathetic skin responses (SSRs) were frequently oroccasionally co-activated. We simultaneously recorded theamplitude of the vasomotor responses and the sympathetic skinresponses, the ECG and the respiratory movements in 30 healthysubjects during natural breathing at rest, rhythmic respirationsat 6 per minute, sudden deep inspiration and Valsalva maneuver.We found: 1) The SSR habituates with all respiratory maneuverswhereas the VRs do not habituate. 2) There was slightco-activation between the SSRs and VRs during natural defaultbreathing (56 percent). 3) During rhythmic breathing at 6 perminute the VRs and the SSRs were frequently co-activated (97percent). The SSR appeared at the end of the inspirationcoinciding with the end of the decreased blood flow. However theSSR habituated after few rhythmic respirations. 4) During suddendeep inspiration one hundred percent of co-activations werebetween the initial phase of the VRs and the SSR. The SSR islarge in amplitude and longer in duration than during rhythmicbreathing. 5) During the Valsalva maneuver there was a strongco-activation (100 percent) particularly during the phases IIand III that are characterized by vaso-constriction but alsoduring phase IV. The SSR is the longest of duration in all ofthe maneuvers. The sympathetic innervation to the sweat glandsof the palm of the hand and to the skin blood vessels of thefingertips is differentiated. Under normothermic conditionssudden deep inspiration and Valsalva maneuver induced a largesympathetic simultaneous outflow to the skin blood vessels andsweat glands. The simultaneous recording of skin blood flow andthe SSRs provides a more complete assessment of the sympatheticoutflow to the skin than either one alone.