Organization and reflex control of vagal cardiomotor neurons

The anatomical and functional organization of cardiac vagal preganglionic somata and the organization of ascending spinal pathways regulating the cell bodies were studies in the dog and cat. Horseradish peroxidase was injected subepicardially in cats after left or right cervical vagotomy to retrogradely label cardiomotor neurons. Somata were ipsilateral to the intact vagus in the dorsal motor nucleus (DMN) of the vagus, the nucleus ambiguus (NA) and an intermediate zone (IZ) between the DMN and NA. The NA contained the maximum number of cell bodies (72%) while successively fewer somata were located in the DMN (19%) and IZ (9%). NA somata were heterogeneously distributed along the longitudinal neuroaxis while DMN and IA somata were homogeneously distributed. Cell bodies of the NA were larger than those of the DMN and IZ. To investigate the possibility that the NA and DMN regulate different cardiac functions, ventricular contractility (LV dP/dt) and heart rate (HR) were recorded in beta-blocked cats during NA and DMN stimulation with and without cardiac pacing. DMN stimulation produced decreases in LV dP/dt but no HR changes, while NA stimulation produced decreases in HR and increases in LV dP/dt. Since cardiac pacing eliminated the LV dP/dt increase, the contractility response was secondary to the bradycardia. Thus, cardiac vagal somata are organized for differential cardiac control. Ascending spinal pathways affecting vagal control of the heart were identified in beta-blocked cats by comparing cardiac responses to nerve stimulation before and after making spinal lesions. The carotid sinus nerve was stimulated to elevate cardiac vagal activity. Somatic afferent stimulation inhibited the negative chronotropic and inotropic responses produced by carotid sinus stimulation. Bilateral lesions placed in the dorsolateral sulcus area abolished the effects of somatic afferent stimulation. Thus, an ascending pathway regulating somata of the DMN and NA is located bilaterally in the dorsolateral sulcus. Similar studies in the dog suggested the pathway extends into the dorsolateral funiculus as well.