The effects of the associated mediodorsal nucleus of the thalamus on spike activity of respiratory neurons in the medulla oblongata and on respiration were studied in normal conditions and in oxygen insufficiency. At normal atmospheric pressure, before animals were elevated to low pressures, electrical stimulation of the mediodorsal nucleus of the thalamus had predominantly inhibitory effects. At the initial phase of hypoxia, at a "height" of 4,000-5,000 m, hypoxic activation of neuron discharge frequency occurred. with an increase in the frequency of respiration. In these conditions, the inhibitory effect of stimulation of the mediodorsal nucleus of the thalamus was less marked than in normoxic conditions. The opposite effect occurred at the second phase of hypoxia (7,500-8,000 m)-inhibition of activity in the medulla oblongata and thalamic center. In severe hypoxia, there was inhibition of neuron spike activity and a decrease in the frequency of respiration, which became superficial; in these conditions, the inhibitory effect of the thalamus was insignificant.
The EEG and impulse activity of the neurons of the cerebral cortex and other structures of the brain were studied in the dynamics of hypoxic influence. In the initial phase of hypoxia (2000-6000 m), activation of the EEG and impulse discharge of neurons set in; in this case EEG activation arose earlier and was more pronounced. In the second phase of hypoxia (7500-10,000 m), the EEG changed in the direction of a reorganization of the frequency spectrum from one rhythm to another - from fast to slow activity of the type of delta waves. At this time the impulse activity was gradually suppressed, and the cortical neurons exhibited higher sensitivity to hypoxia and were inhibited earlier than the cells of the hypothalamus and medulla oblongata.
Dynamic studies were performed in conditions of hypoxia on the effects of stimulation of the ventral, lateral, and medial nuclei of the septum on the spike activity of bulbar respiratory neurons and respiration. The various phases of hypoxia provided a model experiment over which the overall effects of the septal neurons were summed. Electrical stimulation of these septal nuclei in conditions of normal atmospheric pressure had both facilitatory and inhibitory effects on the spike activity of respiratory neurons in the respiratory center of the medulla oblongata, inhibitory effects being predominant. The ventral nucleus had the most effective inhibitory effect on the activity of respiratory neurons. Electrical stimulation of the septal nuclei in the initial phase (4000-5000 m) of hypoxia, on the background of activation, had a predominantly inhibitory influence on the activity of respiratory neurons. During the phase of severe hypoxia (7500-8000 m), on the background of marked hypoxic suppression of respiratory neuron activity, stimulation of the septal nuclei produced no characteristic changes in the activity of these neurons.
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