Bradycardia during stimulation of the septum and somatic afferents in the rabbit.

Experiments were done in paralyzed rabbits anesthetized with either pentobarbital sodium or alpha-chloralose to test the possibility that the septum may alter the cardiovascular responses elicited by stimulation of somatic afferent fibers. Electrical stimulation of the lesser saphenous nerve (LSN), a branch of the sciatic nerve, at certain parameters elicited bradycardia, which could be abolished by bilateral vagotomy or intravenous injection of atropine methylbromide. Distinct and characteristic changes in mean arterial pressure and heart rate were elicited by electrical stimulation of histologically localized sites in five septal areas. Septal sites from which stimulation elicited bradycardia were chosen for the study of interaction between the septum and LSN. The cardiac slowing elicited by combined stimulation of the lateral septum and LSN was significantly greater than the sum of the responses elicited by separate stimulation of the two different structures. In contrast, the bradycardia elicited by combined stimulation of medial septal structures and LSN was significantly smaller than the sum of the individual responses. These experiments demonstrate that the magnitude of heart rate responses elicited by stimulation of somatic afferent fibers may be modified by the septum.