[Participation of midbrain central gray neurons in the organization of defensive conditioned reflexes in the cat].

Neuronal reactions of the periaqueductal gray matter were studied in cats during conditioning defence reflex to auditory stimuli. Phasic and tonic spike responses lasting up to 3s developed in the recorded neurons in response to conditional stimulus. These reactions had developed long before the elaboration of the conditioned reflex and disappeared significantly later than the conditioned reflex became extinct. The generation of the spike response after elaboration of the conditioned reflex took place 100-200 ms before the appearance of the movement evoked by the conditional stimulus. An increase of the tonic discharge of the recorded neurons also preceded the voluntary movements by 100-500 ms. Neuronal responses in the periaqueductal gray matter to differential stimuli could be elaborated with great difficulty; they had the form of group discharges 150-200 ms long. These responses were unstable. On the basis of the obtained data the conditioned reflex is considered as a result of a multilevel hierarchic process of neuronal activity change which begins in unspecific structures of the mesencephalon.