Intracellular electrical records from muscle cells in intact dorsal blood vessels of the earthworm, Lumbricus terrestris, showed a polarization rhythm which consisted of : (a) a prolonged ramp of depolarization, (b) one or a series of several non-overshooting spike potentials, (c) a sustained, hump-like depolarization, and (d) a complete repolarization. The polarization rhythm of the intact vessel was reset by intracellular current injection. Simultaneous recordings of intracellular electrical

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... nd mechanical activities during rhythmic pulsation waves of the intact vessel indicated that the ramp-like depolarization corresponded temporally to a gradual filling of the vessel with blood. Spike potentials occurred during the later portion of filling, and constriction began near the peak of the slow, sustained depolarization. Simultaneous intracellular and extracellular electrical records during rhythmic pulsations of the intact vessel indicated a close temporal correspondence between spike potentials in both electrical records. The repolarization phase in intracellular records corresponded to a small, slow wave in extracellular records. A polarization rhythm was usually absent in isolated preparations of the dorsal blood vessel, but in a few preparations rhythmic activity persisted for up to 1 h. In quiescent isolated preparations membrane potentials were stable (mean resting potential = −66·7 mV), but active membrane responses (non-overshooting spikes and sustained depolarizations), were evoked by intracellular current injection or by stretching of the vessel. Evidence from dual intracellular recordings in isolated dorsal vessels suggest that muscle cells are electrically coupled. The intracellular activity patterns in intact and isolated vessels, and the responsiveness of cells to current injection and stretching, suggest that both initiation and conduction of pulsation waves in the dorsal blood vessel are myogenic.