Muscle sympathetic nerve responses to graded leg cycling

Abstract

The aim of this study was to clarify the relationship between sympathetic outflow to skeletal muscle and oxygen uptake during dynamic exercise. Muscle sympathetic nerve activity (MSNA) was recorded from the right median nerve microneurographically in eight healthy volunteers during leg cycling at four different intensities in a seated position for a 16-min bout. Work loads selected were 20, 40, 60, and 75% of maximal oxygen uptake (VO2max). Heart rate and blood pressure were measured during each exercise test. MSNA burst frequency was suppressed by 28% during cycling at 20% VO2max (23 vs. 33 bursts/min for control). Thereafter, it increased in a linear fashion with increasing work rate, with a significantly higher burst frequency during 60% VO2max than the control value. Both heart rate and mean blood pressure rose significantly during 20% VO2max from the control value and increased linearly with increased exercise intensity. During light exercise, MSNA was suppressed by arterial and cardiopulmonary baroreceptors as a result of the hemodynamic changes associated with leg muscle pumping. The baroreflex inhibition may overcome the muscle metaboreflex excitation to induce MSNA suppression during light exercise. These results suggest that during light exercise MSNA is inhibited, perhaps due to loading of the cardiopulmonary and arterial baroreflexes, and that during heavier exercise the increase in MSNA occurs as muscle metaboreflexes are activated.

Pressor Response to Static and Dynamic Knee Extensions at Equivalent Workload in Humans

“…Saito et al [23] reported that light-intensity cycling suppresses the muscle SNA because of the great activation of cardiopulmonary and arterial baroreflexes caused by the perfusion increases from active muscles, which are produced by a muscle pump. The muscle pump must have occurred in the repetitive contractions in the present study because it is produced by a repetition of muscle contractions and relaxations.…”

Post-exercise changes in blood pressure, heart rate and rate pressure product at different exercise intensities in normotensive humans

“…It has been well established that the magnitude of neural and hemodynamic responses during exercise is directly related to exercise intensity (22,23). Thus, it is possible that different exercise intensities have also distinct effects on cardiovascular changes after exercise.…”

Denervated Sympathetic Nerve Distributed to Motor Muscle as a Possible Cause of Enhanced BP Response to Exercise in Patients with Heart Disease

“…[5][6][7] The pathway from exercise stress to elevated BP may be influenced by both the excitability of sympathetic nerves, and the sensitivity of resistance vessels. Accordingly, we sought to relate the phenomenon of denervation supersensitivity to enhanced BP responses during exercise.…”

“…7,8) While this method is thought to provide reliable data and to accurately reflect sympathetic vasomotor tone, the method is invasive and is accompanied by technical difficulties. Furthermore, MSNA is able to directly reflect the only activity of neurons, but not the function of sympathetic terminal function in contact with adrenoreceptors.…”