The cutaneous silent period is mediated by spinal inhibitory reflex

High-intensity cutaneous stimuli inhibit tonically firing motor neurons resulting in a silent period (CSP) in EMG activity. To determine the central nervous system (CNS) circuitry of this inhibitory reflex, soleus H reflexes evoked by tibial nerve stimuli were conditioned by high-intensity sural stimuli in 5 normal men and 5 men with complete, traumatic cervical myelopathy. The sural-tibial interstimulus interval (ISI) was varied between 0 and 200 ms. In normals, the CSP in the tonically contracted soleus muscle began 90-100 ms after sural stimuli and had a duration of 60-80 ms. In the relaxed soleus, the conditioned soleus H-reflex amplitude was correspondingly reduced at ISIs of 60-120 ms. In patients, conditioned H-reflex amplitude was also reduced over the same ISI range, but the degree of inhibition was significantly less than in normals. These data support the hypothesis that the CSP is mediated by a spinal inhibitory reflex that is subject to supraspinal descending control.