The cutaneous silent period is mediated by spinal inhibitory reflex

High-intensity cutaneous stimuli inhibit tonically firing motor neurons res ulting in a silent period (CSP) in EMG activity. To determine the central n ervous system (CNS) circuitry of this inhibitory reflex, soleus H reflexes evoked by tibial nerve stimuli were conditioned by high-intensity sural sti muli in 5 normal men and 5 men with complete, traumatic cervical myelopathy . The sural-tibial interstimulus interval (ISI) was varied between 0 and 20 0 ms, In normals, the CSP in the tonically contracted soleus muscle began 9 0-100 ms after sural stimuli and had a duration of 60-80 ms. In the relaxed soleus, the conditioned soleus H-reflex amplitude was correspondingly redu ced at ISIs of 60-120 ms. In patients, conditioned H-reflex amplitude was a lso reduced over the same ISI range, but the degree of inhibition was signi ficantly less than in normals, These data support the hypothesis that the C SP is mediated by a spinal inhibitory reflex that is subject to supraspinal descending control. (C) 1999 John Wiley & Sons, Inc.