Supraspinal control of transmission in the polysynaptic reflex pathway to montoneurones during sleep.

Experiments were performed to find out whether supraspinal inhibitory influences affect spinal interneurones during physiological sleep. Single shock stimulation of the tibial nerve elicits a direct motor action potential from the intrinsic plantar muscles, followed by two reflex action potentials. The early response is due to monosynaptie excitation of the spinal motoneurones, while the late response is referred to polysynaptic excitation of the same pool of motoneurones. I t was then possible to study the modulation of monosynaptie and polysynaptic reflexes involving the same motoneuronal population during the same episodes of sleep. In unrestrained, unanaesthetized cats both reflexes may be slightly depressed during desynchronized sleep, while a prominent phasic depression occurs at the time of the bursts of rapid eye movements (REM). There is, however, a striking difference in the amount of depression of the two reflexes during REM, since the polysynaptic reflex is more strongly depressed than the monosynaptie. I t has been previously shown that the phasic depression of the spinal reflexes is due to presynaptic inhibition of the primary afferents. The greater depression of the polysynaptie reflex during I~EM can be referred to additional events possibly leading to postsynaptic inhibition of the interneurones of the polysynaptic reflex pathway to spinal motoneurones.