Myocardial muscle stretch is also capable of inducing spontaneous activity. The upper panel of this figure, adapted from the work of Kaufman and Theophile (Pfleugers Arch. 297:174,1967), shows 3 electrically stimulated action potentials recorded from a papillary muscle followed by a quiescent interval after stopping stimulation. As the fiber is gradually stretched by the amount shown in the lower panel, there is gradual depolarization of the resting membrane to approximately -60 mV and then a burst of spontaneous activity which slows and then ceases when the fiber is returned to its original length (arrow). The last 4 action potentials are again in response to electrical stimulation.
The figure illustrates that even fibers not usually thought of as subsidiary pacemakers, such as papillary muscles, may, in the appropriate set of circumstances, develop pacemaker characteristics. It also suggests a possible mechanism responsible for arrhythmias in patients with dilated hearts.