Classification of raphe neurons with cardiac-related activity

The cardiac-related component of sympathetic nerve discharge (SND) results from entrainment of a rhythm of brain stem origin by the baroreceptor reflexes [Gebber, Am. J. Physiol. 239 (Heart Circ. Physiol. 8): H143-H155, 1980]. As a consequence, changes in heart rate are accompanied by shifts in the phase relations between SND and the arterial pulse wave. This observation presented us with the opportunity of distinguishing brain stem neurons whose discharges are locked to SND from other neuronal types that exhibit cardiac-related activity. Raphe neurons in the cat medulla with cardiac-related activity were identified using post-R wave interval analysis. The discharges of one group of these neurons remained locked to inferior cardiac SND during changes in ventricular rate which shifted the phase relations between SND and the arterial pulse wave. Most importantly, the lag between unit spike occurrence and the peak of the cardiac-related component of SND was unchanged during ventricular pacing (as demonstrated with cross-correlation analysis). It is suggested that these neurons were contained in brain stem networks which control SND. The discharges of two additional types of raphe neurons remained locked to the arterial pulse wave during ventricular pacing. Neurons of the first type exhibited an increase in discharge probability in late diastole and early systole. Neurons of the second type exhibited a period of reduced activity time-locked to systole.