Neural regulation of blood flow in nonworking muscles. Part III

To characterize the threshold behavior of the blood shunting response in nonstressed tissues, vasoconstrictor fiber activity to nonworking muscles was analyzed in healthy human volunteers. Multifiber sympathetic vasoconstrictor activity in fascicles of the peroneal nerves supplying the muscles was recorded using micrographic techniques during rhythmic and sustained handgrip exercise of low to maximum intensity. The results indicate a low threshold of exercise intensity during rhythmic exercise below which the blood shunting response in nonstressed tissues is not activated. Increased sympathetic fiber activity occurs first as recruitment of additional fibers and later as increased frequency of response to pulsatile blood pressure fluctuations. Analysis of vasoconstrictor fiber behavior at the time of initiation and cessation of exercise indicates that changes in activity are caused by reflex responses to exercise rather than by coactivation with skeletal muscle contraction.