The sensitivity of muscle spindles in gastrocnemius-soleus (GS) muscles following dorsal root (DR) antidromic stimulation (ASt) was investigated in anesthetized rats, in which the hindleg was totally denervated except the GS muscle, ventral roots from L3 to S1 and DRs from L4 to L6, which were severed after laminectomy. The peripheral end of a teased DR filament with single unit tonic discharges from a constantly stretched muscle spindle were recorded and antidromically stimulated after identification. In response to ASt with various frequencies, two different types of ending were found: (1) type I, its tonic discharge was not altered following ASt at 100 Hz for 10 s, while type II was significantly inhibited even at a frequency of 50 Hz; (2) at 300 Hz, both types of endings were inhibited, however, type I recovered in less than 4 s, while type II took more than 15 s to return to control level, and a silent period (1-5 s) was found in type II but not in type I; (3) the effective minimum inhibitory frequency for type I and type II was 200 and 50 Hz, respectively. These results suggest that type I and type II endings may correspond to the primary and the secondary endings of the muscle spindle whose sensitivity may be modulated antidromically by the dorsal root reflex.
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