Integration of the cardiovagal mechanism in the medulla oblongata of the cat

The central cardiovagal mechanism of the medulla oblongata was explored by stimulation and ablation techniques in the anesthetized cat. Insertion of an electrode into the nucleus solitarius (NS) occassionally evoked slight and transient bradycardia, but similar mechanical irritation to the nucleus ambiguus (NA) usually evoked prolonged and intense bradycardia. Electrical stimulation of the dorsal motor nucleus of the vagus (DNV) produced no or little bradycardia. Stimulation of NS and NA consistently produced cardiac slowing with a latency of less than 2 s and the effect was more prominent in the NA. Contralateral vagotomy did not significantly affect the bradycardia on the NS and NA stimulation but ipsilateral vagotomy caused a complete abolition. Lesions of the NA or DNV largely or completely abolished the bradycardia consequent to NS stimulation. Extensive destruction of the NS and/or DNV did not affect the bradycardia resulting from NA stimulation. Destruction of the ventral midline area partially reduced th bradycardia on NS STIMULATION BY 36-54%. The results suggest that the sequence of the three vagal nuclei for cardiac inhibition runs in the following order: NS, DNV, and NA. Synaptic connections are probably scanty in the DNV. Part of the vagal pathway passes through the ventral midline area before it reaches the NA. A scheme of the neural pathway for reflex bradycardia of vagal origin has been proposed.