| Abstract: | Stimulation of sites in the midbrain reticular formation and in the posterior hypothalamus of the cat resulted in a large to modest rise of arterial pressure and the induction of cardiac dysrhythmias. Most frequently, these arrhythmias developed upon cessation of brain stem stimulation but also occurred during the stimulus period in 5 of 23 cats studied. The arrhythmias disappeared upon cooling and reappeared upon rewarming the vagus nerves. The ventricular dysrhythmias also were abolished by methylscopolamine, by bilateral vagatomy, or by extirpation of the stellate ganglia. Simultaneous stimulation of both distal end of the cut right vagus nerve and the decentralized right stellate ganglion caused arrhythmias similar to those observed after diencephalic stimulation. These data are interpreted to indicate that the cardiac arrhythmias evoked by brain stem stimulation result from the interplay of both sympathetic and parasympathetic influences on the heart. The response patterns of a population of medullary neurons activated by carotid sinus nerve stimulation were modified by condition stimuli to posterior hypothalamic sites. From studies on unit activity of brain stem areas known to participate in cardiovascular adjustment, a schema is proposed of hypothalamic-medulla interaction as a central mechanism that may account for the development of ventricular arrhythmias. |
