THE ROLE OF REFLEX ACTIVITY IN THE REGULATION OF MUSCLE TONE IN RATS

The aim of the study was to assess the contribution of reflex activity to the regulation of muscle tone in rats. The experiment was carried out on young Wistar male and female rats. The hindfoot of a rat was fl exed or extended at the ankle joint by 25 deg over 250 ms. The resista nce of the foot to passive movements, as well as the electromyographic (EMG) activity in the gastrocnemius and the tibialis anterior muscles , were recorded simultaneously. During passive movements, reflex EMG a ctivity developed simultaneously in both antagonistic muscles of the f oot. Three components were distinguished: a short-latency EMG-A (withi n the first 0-20 ms of a movement), long-latency EMG-B (within 60-160 ms), and EMG-C (within 220-340 ms). When the amplitudes of EMG-B and E MG-C components of the gastocnemius muscle reflex response were greate r than 50 mu V, a significant correlation was found between them and t he maximum resistance of the hindfoot (MMG(max)) during flexion, where as no such correlation was observed for the tibialis anterior muscle. No correlation was found when the amplitudes of the long-latency compo nents of the gastrocnemius muscle were less than 50 mu V. Moreover, no correlation was observed between the EMG-A and the MMG(max). The abov e results suggest that: (1) the muscle tone of the gastrocnemius muscl e in rats seems to be regulated by long-latency (supraspinal) reflexes only when the level of EMG activity exceeds a critical threshold of c a 50 mu V; (2) when the level of EMG activity is lower, a major role i n the resistance of hindlimb muscles is played by some non-neuronal fa ctors; and (3) the proposed animal model emphasizes new aspects of the reflex which may be useful in a search for basic mechanisms underlyin g changes in the muscle tone.