Abstract
It is known that muscle sympathetic nerve activity (MSNA) increases during respiratory apnoea, cold pressor test, handgrip exercise, and mental arithmetic task in humans. We hypothesized that increased MSNA may modify the sensitivity of muscle spindle afferents. To test this, we evaluated tonic vibration reflex (TVR) in the condition with enhanced MSNA. Eight subjects with no neurologic impairment performed four maneuvers accompanying sustained activation of MSNA: 1) respiratory apnoea, 2) cold pressor test for 1 min, 3) mental arithmetic task for 2 min, and 4) static handgrip exercise at 30-40% of maximal voluntary contraction for 1.5 min followed by post-handgrip ischemia for 1 min. Arterial blood pressure (AP) and ECG were noninvasively monitored and cardiac output and total peripheral resistance (TPR) were estimated from the AP waveform using the Modelflow method. TPR was considered as an index of peripheral sympathetic nerve activity. TVR tended to increase as TPR increased during respiratory apnoea, cold pressor test, and mental arithmetic task. TVR tended to increase when AP increased during static handgrip exercise. However, TVR changed little, when MAP didn’t change during post-handgrip ischemia, mental arithmetic task, or cold pressor test in some subjects. TVR decreased in one subject, when TPR decreased during mental arithmetic task. The correlation between TVR and TPR indicates that increased sympathetic outflow may facilitate TVR in support of the concept that MSNA can influence the sensitivity of human muscle spindles. [Jpn J Physiol 55 Suppl:S118 (2005)]
