Control of the heart in the mudpuppy, Necturus maculosus

The cardiac innervation of the mudpuppy, Necturus maculosus, was investigated using in vivo recording of blood pressure and heart rate, a perfused heart preparation, isolated paced ventricular and atrial strip preparations, Falck-Hillarp fluorescence histochemistry and measurements of plasma concentrations of catecholamines. The heart of Necturus receives a 'classical' vagal inhibitory innervation by cholinergic fibres which act via muscarinic cholinoceptors. In addition, there appears to be an atropine-resistant inhibitory component in the vagal innervation of the heart. This component is best seen at higher frequencies of vagal stimulation (10-20 Hz), but the mechanism behind this response is not known. There is no direct adrenergic innervation of the ventricle or atria, but by use of Falck-Hillarp fluorescence histochemistry, adrenergic fibres have been demonstrated at the atrio-ventricular border, particularly within the intracardiac ganglion. The histochemical evidence is consistent with a modulation of the ganglionic transmission in the vagal pathways by adrenergic neurons and small intensely fluorescent (SIF) cells, but physiological evidence is wanting. An adrenergic control of the heart via circulating adrenaline may occur during 'stress', when the plasma level of adrenaline is high enough to affect cardiac performance.