Transmission of impulses in the parasympathetic cardiomotor pathway to the sino-atrial node

Abstract

In mammals and other vertebrates the pacemaker cells of the sino-atrial (SA) node are under powerful inhibitory control of parasympathetic postganglionic cardiomotor neurones. The preganglionic neurones are located in the nucleus ambiguus; some are also located in the dorsal motor nucleus of the vagus although their function in the regulation of pacemaker activity is unknown (Jones, 2001). Activation of these preganglionic cardiomotor neurones leads to activation of cholinergic postganglionic neurones in cardiac ganglia, which in turn results in inhibition of the pacemaker cells in the SA node and subsequently in a decrease of heart rate. It is universally propagated in textbook chapters and elsewhere that the synaptic transmission from parasympathetic preganglionic neurones to postganglionic neurones in the cardiac ganglia occurs with a high safety factor without modulation and that the inhibition of pacemaker cells by activation of cholinergic cardiomotor neurones is mainly generated by the opening of potassium channels via the second-messenger adenylate cyclase/cAMP pathway and by suppression of hyperpolarization-activated Na+/K+-channels. These assumptions need to be corrected in view of experimental data published previously and recently.