The Effects of Brief Vagal and Sympathetic Stimulation on Rate and Rhythm Changes in the Sinus Node

How the parasympathetic and sympathetic nervous system influence the rate of pacemaker generation and conduction within the sinus node is a subject that has fascinated electrophysiologists for many years. Our early studies in the dog using brief vagal and brief sympathetic nerve stimulation demonstrated that there was a considerable difference in the time course between the effects of the parasympathetic and sympathetic nervous system (Spear and Moore, 1973). The acceleration of sinus node rate caused by brief stellate stimulation was observed to be a gradual acceleration followed by a gradual slowing. The effects of vagal stimulation on sinus rhythm were more brief and more complex. Brief vagal stimulation resulted in an initial slowing of sinus rhythm followed by a transient period of acceleration in which the spontaneous rate sometimes actually exceeded the control sinus rate followed by a second period of sinus slowing. This phenomena of anomalous acceleration following brief vagal stimulation was shown first by Brown and Eccles in 1934 in studies on the cat sinus node. Since Brown and Eccles’ early studies it has been shown that this acceleratory component is not due to a simultaneous stimulation of sympathetic fibers since beta adrenergic blockade does not eliminate the anomalous acceleratory component observed following brief vagal stimulation (Levy et al., 1970; Spear and Moore, 1973).