Fig. 3. Effects of brief stretches on the heartbeat rhythm. (A) Example records showing phase advance (i) or phase delay (ii) of the ganglionic burst cycle induced by a brief stretch (duration, 200 ms; amplitude, 0.7 mm). In each record: upper trace, membrane potential of the heart muscle showing rhythmical bursts of excitatory junctional potentials (EJPs); lower trace, applied stretch shown by displacement of the unfixed stage. A brief stretch was given at an earlier phase (i) or at a later phase in the burst cycle (ii). Dots above each record indicate the expected timing of the EJP bursts in the absence of perturbation. The 0.7 mm stretch was approximately 1.4x larger than the estimated amplitude of the circumferential dimension change in a given heart. (B) The relationship between changes in the burst interval and phases of the burst cycle in which the brief stretches were presented. The control burst interval was determined as the average interval of 10 bursts prior to the stretch. Data were obtained from three preparations, and each symbol represents data from one specimen. Free-run burst frequencies were 1.5 Hz (circle), 1.1 Hz (square) and 0.9 Hz (triangle). The stretches were given at a duration of 200 ms (circles and triangles) or 300 ms (squares). The amplitudes of the stretches (circles, 0.7 mm; squares, 0.5 mm; triangles, 0.8 mm) were approximately 1.4x larger than the estimated amplitude of the circumferential dimension change in the given hearts.