[Long-lasting changes in motoneuron discharges following 50 Hz tetanization of afferent nerves in cats]

The effects of 50 Hz tetanization on motoneuron discharges were studied in non-immobilized, non-anesthetized, decerebrate cats. Motoneuron discharges were monitored by recording either muscle (hamstring muscles or foot extensors) action potentials or ventral root (L7 or S1) discharges. In the latter case both ventral and dorsal roots were cut and corresponding dorsal roots were stimulated. It was found that in 13 out of 32 cases the tetanization of the sciatic nerve at 4.0T for 5 s produced enhancement of monosynaptically evoked muscle action potentials (H-wave) for 20-25 min. Three categories of potentials were distinguished in ventral root discharge according to latencies and thresholds: monosynaptic, polysynaptic and late potentials. When the dorsal root was tetanized at 2.5-5.0T for 5 s, all of these 3 categories of potentials exhibited an enhancement of the potential lasting for more than 30 min. The late potential, which was the longest-latency polysynaptic potential (4.7-6.5 ms), was the most sensitive to the tetanization. Its enhancement could develop slowly, yet the degree of the enhancement was the greatest, as compared with other two categories of potentials. In contrast, the enhancement of these potentials were always immediate. In 15 out of 46 cases at least one of the 3 categories of potentials exhibited the enhancement. Since the long-lasting changes were induced only with the strength enough to activate polysynaptic pathways, interneurons were considered to be responsible for these plastic changes. The monosynaptic potential could show a long-lasting depression following the tetanization.