Afferent control of cortically evoked reactions of spinal cord ventral horn interneurons

The reactions of interneurons of the ventral horn (L6,7) of the spinal cord in response to stimulation of the motor zone of the cortex and of afferents of the hind limbs were investigated intra- and extracellularly in experiments on cats. It was demonstrated that cortically evoked reactions of the interneurons are modulated by the preceding afferent wave. Three types of effects were distinguished: intensification, suppression, and biphasic changes. Facilitation is brief (not more than 30-35 msec); the duration of suppression varied from 60-100 msec to 300-400 msec. A correlation between the effect of afferentation and the latent period of the cortically evoked reaction was established: facilitation was observed among the short-latency reactions; long-latency reactions were inhibited as a rule. Inhibition was more clearly manifested: for the responses from the ipsilateral than the contralateral hemisphere; during the conditioning of the contralateral afferents, than during the conditioning of the ipsilateral; of the high-threshold afferents than of the low-threshold. The mechanisms of afferent control of descending activity and the role of "polysynaptic loops" in the processes of the afferentation as a supplementary modulator of the excitability of motoneurons are discussed. It is proposed, on the basis of the analysis of the data obtained (the absence of IPSP in the interneurons investigated, etc.), that a principal role in the suppression of descending activity belongs to the mechanism of presynaptic inhibition, as well as to the disfacilitation caused by it (the decrease in depolarization and the increase in the critical discharge level) in the interneurons themselves due to the blocking of the background afferent influx from other inputs.