Central motor command activates sympathetic outflow to the cutaneous circulation in humans.

1. The aim of this study was to provide direct evidence that sympathetic outflow to the skin in humans is governed by central neural mechanisms. 2. Microneurographic measurements of skin sympathetic nerve activity (SNA) from the personal nerve was performed in nine subjects during: (1) static hand grip at 10, 20 and 30% maximal voluntary contraction (MVC); and (2) attempted static hand grip during partial neuromuscular blockade produced by injection of vecuronium. 3. Two minutes of static hand grip at 20 and 30% MVC (force output, 9.6 +/- 0.2 and 14.4 +/- 0.3 kg, respectively) evoked significant increase in skin SNA that were graded to the intensity of the exercise. Static hand grip at 10% MVC (force output, 4.8 +/- 0.1 kg) caused a small but insignificant increase in skin SNA. 4. During vecuronium-induced neuromuscular blockade, subjects failed to maintain a force output equivalent to the output produced during 10% MVC before vecuronium (force output: 1st min, 4.4 +/- 0.6 kg; 2nd min, 2.1 +/- 0.4 kg), in spite of maximal effort being applied. This attempted hand grip exercise consistently evoked considerable increase in skin SNA that did not significantly differ from the responses produced by hand grip at 30% MVC; total skin SNA increased by 246 +/- 93% during 2 min of attempted hand grip and increased by 243 +/- 77% during 2 min of static hand grip at 30% MVC (means +/- S.E.M., P < or = 0.05). These increase in skin SNA were not due to activation of resting muscles because measurements of surface electromyography showed no activity in resting forearm muscles during static or attempted hand grip exercise. 5. This study provides direct neurophysiological evidence that central motor command can activate sympathetic outflow. During static hand grip, central motor command is the primary mechanism that stimulates sympathetic outflow to skin.