The effects of the mediodorsal nucleus of the thalamus on respiratory neurons of the medulla oblongata and respiration in rats in conditions of hypoxia.

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.

Studies of the role of the central nucleus of the amygdala in controlling cardiovascular functions.

Studies on cats anesthetized with a mixture of chloralose and Nembutal addressed the effects of high-frequency stimulation (100 impulses/sec) of the central nucleus of the amygdala on bioelectrical activity in two postganglionic sympathetic nerves-the inferior cardiac nerve and the vertebral branch of the stellate ganglion, which innervate the coronary vessels and the vessels of the anterior thorax respectively. The central nucleus of the amygdala was found to have differential, selective effects, in most experiments producing increases in the amplitude of integrated activity in the inferior cardiac nerve and decreases in the amplitude of biopotentials in the vertebral nerve. In a few experiments, a second type of modulation of the activities of these two postganglionic nerves was seen, with selective inhibition of activity in the inferior cardiac nerve and an accompanying increase in activity in the vertebral nerve. Stimulation of the central nucleus of the amygdala induced significant increases in systemic arterial blood pressure. The role of the central nucleus of the amygdala in the development of experimental neurogenic hypertension was studied in a series of chronic experiments on rats; these established that rats subjected to bilateral electrolytic lesioning of the central nucleus of the amygdala prevented the development of neurogenic hypertension induced by daily imposition of stress for four weeks for induction of operant aversive conditioned reflexes, which was not the case in control rats. The role of the central nucleus of the amygdala in the regulation of vascular tone is discussed.

Responses of hypothalamic neurons to stimulation of the vestibular nerve and lateral vestibular nucleus in the rabbit.

Acute experiments were performed on rabbits to study the responses of neurons in the anterior, ventromedial, and posterior nuclei of the hypothalamus to single, paired, and rhythmic stimulation of the vestibular nerve and lateral vestibular nucleus of Deiters. The data obtained showed that neurons of the posterior nucleus of the hypothalamus were the most sensitive. Three types of response were seen from hypothalamic neurons, with short, long, and intermediate latent periods. This provides evidence that ascending afferent spike activity from the lateral vestibular nucleus of Deiters to the hypothalamus is mediated by mono-, oligo-, and polysynaptic pathways.

An investigation of certain physiological mechanisms of the neurohumoral regulation of vascular tone.

The neuronal organization of the hypothalamobulbar system of regulation of vascular tone was studied in experiments on cats anesthetized with chloralose and nembutal and immobilized by dithylin [suxamethonium iodide]. It was established that the descending influence of the posterior, tuberal, and paraventricular structures of the hypothalamus on the activity of antidromically identified sympathicoactive reticulospinal neurons of the ventrolateral region of the medulla oblongata is realized by mono-, oligo-, and polysynaptic mechanisms. It was shown in a series of experiments carried out in rats that animals subjected to chemical desympathization, by contrast with the controls, do not develop chronic neurogenic hypertension during a six-week stressor influence on higher nervous activity. The central and peripheral neurohumoral mechanisms of the regulation of vascular tone are discussed.

The role of the specific and nonspecific afferent systems in the mechanism of changes in evoked cortical responses in the presence of vibration.

The interrelationship of the activity of specific and nonspecific afferent systems in the mechanism of changes in the electrical activity of the cortex under the influence of vibration was studied under chronic experimental conditions. The character of the postvibrational shifts in the vestibulo- and reticulocortical evoked responses in the presence of the isolated and combined disengagement of the specific (vestibular) and nonspecific afferent systems of the brain was investigated. It was established that vestibular afferentation plays a significant role in the mechanism of postvibrational shifts in the vestibulocortical evoked responses, whereas nonspecific, as well as proprioceptive, somatic, and visceral as well as other inputs play the dominant role for the reticulocortical system of integration.

Neuronal and neurochemical mechanisms of hypothalamic inhibition of the nociceptive reflex.

Using cats anesthetized with chloralose-Nembutal we have studied the effect of high frequency stimulation of the medial and lateral structures of the posterior, tuberal, and anterior hypothalamus, and also of the central gray matter of the midbrain on the nociceptive jaw-opening reflex induced by tooth pulp stimulation. We recorded the EMG response of the digastric muscle as the index of the nociceptive reflex. We have shown that the EMG response of the nociceptive reflex is effectively suppressed on stimulation of all hypothalamic structures and the central gray matter, the threshold of the suppressive action being lower on stimulation of the central gray matter. The effects of the hypothalamic suppression of the nociceptive reflex were eliminated almost completely after naloxone administration, with the exception of the central gray matter, which is slightly more resistant to the action of this agent. After bilateral electrolytic destruction of the central gray matter the antinociceptive effect of the hypothalamus was retained, decreasing only insignificantly, The effect of complete suppression of the amplitude of the EMG response, similar to the effect of stimulation of the hypothalamus and the central gray matter, was also observed after intravenous administration of phenapidine, an opiate agonist with a marked central analgesic action. The neuronal and neurochemical mechanisms of hypothalamic suppression of the nociceptive jaw-opening reflex are discussed.

Electrophysiological study of reticulo-limbic interrelationships during prolonged action of vibration.

Phasic changes in the bioelectrical activity of the dorsal hippocampus (field CA3), the mesencephalic reticular formation, and several regions of the neocortex and the reticulo-cortical evoked potentials were measured under conditions of the action of prolonged vibration (3 months) in electrophysiological experiments with rabbits. Daily three-hour vibration during the first month of the experiment evoked an activation reaction in the EEG, characterized by a desynchronization effect in the neocortex and hippocampus and by the stabilization of the v rhythm in the mesencephalic reticular formation. Noted against this background was a certain facilitation in the reticulo-cortical evoked potentials, more pronounced in the neocortex, and a decline in the ascending activational influence of the mesencephalic reticular formation. The three-month action of vibration exerted an inhibitory influence on reticulo-cortical interrelationships, expressed in a decline in the compound bioelectrical activity of the cortex and subcortical formations, the excitability of the mesencephalic reticular formation, and the suppression of reticulo-cortical evoked potentials. At the same time an elongation of the latent periods of the positive phases of the evoked potentials, a decline in their amplitude, and a reduction of the negative phase in limbic structures was noted. The question of the physiological mechanism of development of vibrational pathology is discussed.

Activity of neurons in limbic cortex during stimulation of somatosensory zones.

The effect of high-frequency and burst stimulation of the first and second somatosensory zones on the activity of identical neurons in the anterior limbic cortex was studied comparatively in acute experiments with cats. A histogramic analysis of neuronal responses in the limbic cortex showed that most background-active cells responded to stimulation of both the first and the second somatosensory zones. Both zones variously adjusted the activity of neurons in the limbic cortex. It was found that the responses in the limbic cortex with the maximum density of the potentials of long-latent reactions are recorded during stimulation of the first somatosensory zone. It was established that the first and second somatosensory zones exert a preferentially activating influence on neurons in the anterior limbic cortex (51.5 and 66.6%, respectively), inhibitory responses comprised 33.3 and 20.0%, while mixed responses were recorded in 15.5 and 13.3% of the neurons, respectively. It was shown that the somatosensory zones exert a modulating influence on the activity of neurons, thus participating in the regulation and processing of information entering the limbic cortex.

Effects of acute hypoxia on the EEG and impulse activity of the neurons of various brain structures in rats.

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.

Evoked responses of pyramidal tract neurons to stimulation of the lateral hypothalamus.

The evoked responses of identified pyramidal tract neurons of the pericruciate region of the cortex arising on stimulation of the posterior, tuberal, and anterior sections of the lateral hypothalamus were investigated in acute experiments on cats anesthetized with chloralose. THe pyramidal tract neurons recorded were situated at a depth of 0.75-2.5 mm and on stimulation of the hypothalamus discharged with a latent period of 1.6-52.5 msec. They discharged most efficiently in the case of stimulation of the caudal half of the lateral hypothalamus. The pyramidal tract neurons activated by the hypothalamus were also tested by electrocutaneous stimulation of the four limbs. Of the neurons 75% responded to stimulation of several limbs, i.e., they had a broad bilateral receptive field, while 15% reacted to stimulation of one contralateral limb (either the anterior limb or the posterior limb), i.e., they had a small contralateral receptive field. Comparison of the latent periods of the anti- and orthodromic responses of the pyramidal tract neurons did not reveal any relationship between the magnitude of the latent period of the orthodromic response (on hypothalamic or electrocutaneous stimulation) and the type of pyramidal tract neuron (according to the axonal transmission velocity). No relationship between the latent period of hypothalamic stimulation and the magnitude of the latent period of the response to a peripheral stimulus was detected either.

Spontaneous and evoked hypothalamic unit activity in kittens during development.

Spontaneous and evoked hypothalamic unit activity in response to sensory stimuli was studied in acute experiments on anesthetized kittens. After the second day of postnatal development, spontaneous unit activity can be recorded in the posterior hypothalamus. Evoked responses to sciatic nerve stimulation were recorded on the fourth to fifth day after birth. In the early stages of development phasic and tonic types of unit responses are present in the hypothalamus, with some predominance of phasic. At the end of the third week responses of hypothalamic neurons in kittens have still not reached their definitive formation.

Analysis of neocortical electrical responses evoked by stimulation of various hypothalamic structures.

Evoked potentials (EPs) and extracellular unit activity were recorded in various parts of the neocortex of unanesthetized cats immobilized with succinylcholine. During hippocampal stimulation EPs of maximal amplitude are recorded in the anterior sigmoid gyrus with a latent period (LP) of 0.5-2.5 msec. The first component is a fast positive spike with a duration of about 1.5 msec. In the middle suprasylvian gyrus, hypothalamo-cortical (HC) EPs have a monophasic surface-negative configuration and are recorded after an LP of 2-6 msec. HC EPS in the anterior sigmoid gyrus can bind a rhythm of stimulation of up to 200 pulses/sec and are characterized not only by high functional mobility, but also by the property of posttetanic potentiation. The main positive wave appears to a testing stimulus with an interval of 7-10 msec. Complete recovery of all components of HC EP takes place at an interval of 50-150 msec. In response to the conditioning stimulus a biphasic change in excitability of the neurons is observed in the association areas of the anterior sigmoid and middle suprasylvian gyri of the neocortex. The maximal number of responding neurons in the deep layers of the cortex discharges in the period of development of the initial spike and the main positive wave of the HC EP.