Inhibition of the baroreceptor reflex on stimulation in the brain stem defence centre

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Abstract
In anesthetized cats, the pattern of cardiovascular response characteristic of the defense reaction was elicited by localized electrical stimulation within the appropriate region of the hypothalamus. The baroreceptor reflex response has been elicited by raising the pressure in a blind sac preparation of the carotid sinus or by electrical stimulation of the sinus nerve. In addition to arterial blood pressure, heart rate and regional blood flows, activity was recorded in cardiac and renal sympathetic nerves, to assess more precisely the cardiomotor and vasomotor changes during interactions between brain stem stimulation and baroreceptor activation. The sympatho-inhibitory and depressor effects of carotid sinus stimulation or electrical stimulation of the sinus nerve could be completely suppressed by stimulation within the hypothalamic defense area, as could the reflex bradycardia. This suppression is effected through the CNS. Stimulation at points in the hypothalamus close to, but outside, the defense area, which elicited increases in arterial pressure and sympathetic activity of similar magnitude to those from the defense area itself, did not abolish the sympatho-inhibitory or depressor effects of baroreceptor activation, though the reflex bradycardia was usually inhibited. This less localized change probably results from augmentation of the central inspiratory drive which inhibits the vagal outflow to the heart.