. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MYOGENIC HEARTBEAT IN TRIOPS 357 B. Figure 11. Effects on myocardial activity of current injection at two sites of the heart. Intracellular activity of the heart muscle was recorded simultaneously at two sites in the heart: one near the anterior end (upper traces), and one near the posterior end (lower traces). (A) Hyperpolarizing DC current (lOOnA) was injected for 5 s through the electrode inserted at the anterior region, using a bridge circuit. (B) Hyperpolarizing DC current (lOOnA) was injected for 5 s through the electr
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. MYOGENIC HEARTBEAT IN TRIOPS 357 B. Figure 11. Effects on myocardial activity of current injection at two sites of the heart. Intracellular activity of the heart muscle was recorded simultaneously at two sites in the heart: one near the anterior end (upper traces), and one near the posterior end (lower traces). (A) Hyperpolarizing DC current (lOOnA) was injected for 5 s through the electrode inserted at the anterior region, using a bridge circuit. (B) Hyperpolarizing DC current (lOOnA) was injected for 5 s through the electrode inserted at the posterior region, using a bridge circuit. beat independently with their own rhythms; subsequently their rhythms were gradually resynchronized (Fig. 4). This suggests that the heart muscle cells synchronize their activities through interaction. The high sensitivity of the Triops heart to mechanical stimuli suggests some stretch- mediated contribution to the synchronization of the mus- cle cells. However, spontaneous activity was rapidly syn- chronized (Fig. 3A). and the heart muscle cells appear to be electrically coupled (Fig. 5). We therefore conclude that the heart muscle cells are synchronized through electrical interactions, as in the myogenic hearts of other invertebrate species (reviewed by Ebara, 1993). The heart of Triops can be said to have a diffuse myogenic pacemaker with the properties of a functional syncytium. These characteristics of the heart are very similar to those of the myogenic hearts of em- bryos and early juveniles of the isopod Ligin (Yamagishi and Hirose, 1997) and of most molluscs (reviewed by Hill and Welsh. 1966; Irisawa, 1978; Jones, 1983). The rhythm of the muscle activity could be phase- shifted by a brief current pulse (Figs. 6 and 7) and could be entrained to the frequency of repeated, brief current pulses injected into the muscle cell (Fig. 8). In addition, the frequency of the muscle activity changed linearly with the inte
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