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.

[The afferent control of the cortically evoked reactions of the interneurons in the spinal cord ventral horn]. / Afferentnyi kontrol' kortikal'no vyzvannykh reaktsii interneironov ventral'nogo roga spinnogo mozga.

Responses of spinal interneurons of the ventral horn (L6, 7) to stimulation of motor cortex and hindlimbs' afferents, were studied in cats. The cortically induced responses were shown to be modulated by a preceding afferent wave. Three types of effects were revealed: facilitation, depression, and biphasic changes. The facilitation was rather short (not over 30-35 msec) whereas the depression varied from 60-100 to 300-400 msec. The facilitation was commonly observed for the short-latency responses whereas the long-latency ones were rather depressed. The depression was more obvious for ipsilateral responses, when conditioning contralateral afferents rather than ipsilateral ones, high-threshold afferents rather than low-threshold ones. The data obtained suggest the mechanism of presynaptic inhibition to play a major part in the depression of descending activity, as well as the defacilitation in the interneurons themselves due to a blocking of the background afferent inflow from other inputs.

[Corticofugal effects on interneuron segmental reactions of the ventral horn of the lumbar segments of the spinal cord in the cat]. / Kortikofugal'nye vliianiia na segmentarnye reaktsii interneironov ventral'nogo roga poiasnichnykh segmentov spinnogo mozga koshki.

Impulse activity of the lumbar ventral horn interneurons evoked by stimulation of afferent nerves of both hindlimbs is shown to be modulated (facilitated or inhibited) by descending volleys from the cerebral motor cortex. Facilitation was of short duration and did not exceed 35-40 ms. Duration of inhibition varied from 125 to 350 ms; in a few cases it lasted up to 500 ms. Polysynaptic reactions evoked in one and the same neuron by activation of different inputs were modulated differently: some of them could be depressed, while others--enhanced or unchanged. It is suggested that such differentiation is due to specific organization of these afferent inputs. Central latency of the test response was another main factor determining the type of its cortical modulation. Polysynaptic responses with a central delay less than 5 ms were facilitated or unchanged (in a few cases--depressed) while responses with central delay more than 5 ms were always depressed. Reactions with a latency more than 10 ms were depressed most deeply. Possible mechanisms and ways of transmission of corticofugal inhibition and facilitation are discussed.

Bilateral influences from the motor cortex on lumbar ventral horn interneurons.

Microelectrode recordings were made of discharges of ventral horn interneurons (VHIN) in segments L6-7 of the spinal cord during bilateral stimulation of the motor cortex in cats. The overwhelming majority of VHIN was shown to be activated by influences from the contralateral cortex, and about half of them also by ipsilateral influences. Clear correlation was established between convergence of afferent and cortical influences: Neurons with inputs from ipsilateral afferents only were activated, irrespective of their other characteristics, by descending influences from the contralateral cortex only, whereas bilateral cortical influences converged on cells with bilateral afferent connections. It is suggested that VHIN with bilateral segmental and supraspinal connections are important integrative elements in the mechanism of bilateral coordination of motor responses of different degrees of complexity.

[Bilateral motor cortex effects on interneurons of the ventral horn of the lumbar segments]. / Bilateral'nye vliianiia s motornoi kory na promezhutochnye neirony ventral'nogo roga poiasnichnykh segmentov.

The ventral horn interneurons in the spinal cord L6-7 segments were recorded in anesthetized cats during stimulation of the pericruciate cortex of both cerebral hemispheres. The majority of ventral interneurons are activated by volleys from the contralateral motor cortex and nearly half of them--by volleys from the ipsilateral cortex as well. Two kinds of firing patterns occurred: separate spikes and bursts of spikes. The latency of the 1st type firing was significantly greater than that of the 2nd one. There was a close relation of convergence of the afferent and corticospinal volleys on ventral horn interneurons: neurons activated by ipsilateral afferents only received synaptic influence only from the contralateral motor cortex whereas neurons with ipsilateral as well as contralateral inputs were activated by corticospinal volleys from both hemispheres. The interneurons with bilateral segmental and supraspinal connections seem to be involved in integrative action as important elements of the neuronal mechanism of bilateral motor coordination.

[The interaction of symmetrical cortical descending influences on lumbar motoneurons]. / Vzaimodeistvie simmetrichnykh kortikal'nykh niskhodiashchikh vliianii v poiasnichnykh motoneironakh

Postsynaptic potentials (PSPs) of the cat motoneurons elicited by stimulation of contralateral and ipsilateral motor cortex, and their changes during combination of both kinds of stimulation were studied with use of the intracellular recording technique. For elicitation of ipsilateral PSPs, the cortex stimulation of greater intensity was needed as compared with contralateral activation. The ipsilateral synaptic reactions had a longer latency and time of segmental delay than the contralateral ones. Conditioning stimulation of the ipsilateral cortex exerted an inhibitory effect upon contralateral cortically evoked EPSP and IPSP of motoneurons and upon potential recorded at the spinal cord surface. Suppression of conditioned PSPs occurred without changes of their sign or pattern in most cases. The data obtained suggest that inhibitory interaction of cortically evoked descending activities involves segmentary inhibitory interneurons and is realized at an interneuronal level and/or at the membrane of the motoneurons.