A study on the role of muscle sympathetic nerve activity during movements in humans

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The present study was aimed to clarify the role of sympathetic outflow to skeletal muscles in humans in the mechanisms to control voluntary and involuntary movements. Muscle sympathetic nerve activity (MSNA) was recorded microneuro-graphically from the tibial nerve which innervated the triceps surae muscle, during voluntary and involuntary movements, different kinds of physiological and pathological tremors, simultaneously with systemic blood pressure (BP), electromyo-grams (EMGs), muscle tension, local fatigue sensation (LFS) and venous pooling of the contracting muscles.1.During voluntary static exercise of hand gripping and tonic flexion of the arm in supine position, MSNA was enhanced in accordance with the increase in BP,EMGs, muscle tension and LFS.2.MSNA in standing position was enhanced during voluntary tonic contraction of both the triceps surae muscle and the tibialis anterior muscle, but was suppressed by voluntary rhythmic contraction of the same muscles concomitantly with the reduction in the venous pooling in the leg.3.Both physiological tremors (orthostatic tremor in antigravity muscle and cold-induced shivering) and pathological tremors (Parkinsonian tremor and essential tremor) were apparent when MSNA was strongly discharged. The increase of these involuntary rhythmic movements reduced venous pooling in the leg and elevated BP,inducing transient suppression of MSNA.In conclusion, MSNA is enhanced during voluntary static contraction of the skeletal muscles. This enhancement elevates systemic blood pressure to maintain the blood flow in the contracting muscle and the central nervous system. MSNA is rather suppressed by voluntary and involuntary rhythmic muscle contraction depending on baroreflex mechanisms due presumably to muscle pumping effects induced by rhythmic movements.