FUNDAMENTAL DIFFERENCES IN THE EXCITABILITY OF SOMATIC AND AUTONOMIC CENTERS IN RESPONSE TO ANOXIA

Abstract

The effect of anoxia produced by inhalation of O2-N2 mixtures varying between 4.5% and 8.1% O2 was studied on autonomic and somatic responses elicited by stimulation of hypothalamus, medulla and spinal cord in narcotized cats. Anoxia increased the contractions of the n.m. produced by stimulation of the hypothalamus and this effect persisted after the denervation of the carotid sinuses and bilateral vagotomy. Since anoxia exerted no effect on the contraction of the n.m. elicited by stimulation of the cephalic end of the cervical sympathetic, the increased response to hypothalamic stimulation was attributed to an increased excitability of the hypothalamic sympathetic centers. Somatic movements elicited from the same area, however, declined or disappeared under conditions of anoxia. The blood pressure rise resulting from hypothalamic stimulation increased during anoxia in the normal animal but decreased in the cat deprived of the carotid sinuses and vagi. Similar results were obtained in expts. on direct stimulation of the vasomotor center in the medulla. They suggest that the increased excitability of the vasomotor center to indirect (supramedullary) as well as direct stimuli (applied to the vasomotor center itself) depends on the presence of afferent impulses from the chemoreceptors of the sino-aortic area. Further studies on the effect of anoxia on the contraction of the n.m. and the erection of hairs elicited by medullary and spinal stimulation indicate an increased excitability of these centers in anoxia. Somatic movements induced by stimulation of the spinal cord declined during anoxia. These results were obtained in narcotized as well as in decerebrate cats.