Role of the sympathetic nervous system in the generation of one-minute wave in body fluid volume during upright standing.

In clarifying the role of sympathetic nerve outflow to the muscles in the genesis of one-minute waves in body fluid volume, power spectral analysis of muscle sympathetic nerve activity (MSNA) was performed. MSNA was recorded by microneurography in an upright standing position simultaneously with the measurements of body circumference, venous pressure at the dorsum pedis veins, intrathoracic fluid volume, soleus EMG, and calf fluid volume. Subjects were seven healthy male volunteers aged 18 to 32 years. Spectral analysis revealed the one-minute oscillation in MSNA as well as in other variables. Cross spectral analysis between the soleus EMG and each variable revealed a phase order of the one-minute oscillations. It was as follows: 1) an increase in calf fluid volume; 2) a decrease in intrathoracic fluid volume; 3) an increase in venous pressure at the dorsum pedis veins; 4) an increase in the soleus EMG activation; 5) an activation of MSNA; 6) a decrease in calf fluid volume; and 7) an increase in intrathoracic fluid volume. It is concluded that sympathetic nerve activity as well as cardiovascular variables have a cyclic rhythm with a duration of one minute to maintain hemodynamic homeostasis in humans in an upright standing position.