Hypothalamic Stimulation and the Inhibition of the Activity of Cardiac Sympathetic and Parasympathetic Nerves

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Abstract
The electrical activity from the left sympathetic and parasympathetic cardiac nerves was recorded simultaneously with the activity from bulbar neurones. Electrodes for stimulating or recording were placed stereotaxically in the posterior hypothalamus, in the medulla and in the sygmoid gyrus of a dog. During the hypothalamic stimulation the sympathetic discharge increased but it was strongly reduced immediately after the cessation of the stimuli. In all experiments the spontaneous sympathetic activity was invariably reduced after the hypothalamic stimulation. In most experiments the parasympathetic discharge was also reduced but in some cases the vagal activity was increased simultaneously with the reduction in sympathetic discharge. The reduction in both sympathetic and parasympathetic discharges appeared independently of the values in systemic blood pressure. Several experimental observations indicate that the reduction in the sympathetic activity can not be attributed to fatigue but rather to an active process of inhibition. The sympathetic inhibition occurring after the hypothalamic stimulation can be overcome by several influences such as, the selfsustained cortical activity from the sigmoid gyri, the afferent impulses originated in saphenous nerve fibers or the impulses arising in the carotid presso-receptors. The relevance of the sympathetic inhibition is discussed in relation with the changes in blood pressure and heart rate occurring during and after the activation of the posterior hypothalamus and also in relation to the parasympathetic modifications appearing simultaneously with the sympathetic inhibition.