The initiation, continuation, and termination of spontaneous episodes of circus movements in the pregnant myometrium of the rat

OBJECTIVES: The objectives of this study were to determine the mechanisms r esponsible for the initiation and the termination of episodes of circus mov ements and to describe the characteristics of these circuits in rat myometr ium. STUDY DESIGN: By means of simultaneous recordings from 240 sites in Wistar rats at 17, 19, or 21 days' gestation, a spatial and temporal analysis was performed on the electrical activation from the beginning until the end of 22 episodes of circus movements. RESULTS: The initiation of reentry was caused by (1) transient unexcitable areas at the beginning of the burst or (2) lines of conduction blocks. Circ us movements were terminated by (1) reversal of recruitment (decruitment) a t the end of the burst, (2) occurrence of lines of conduction blocks, or (3 ) interruption of the circuit. On average circuits rotated for 6 revolution s during 2 seconds. In most cases circuits remained stable; sometimes circu its drifted along the tissue and occasionally multiple wavelets were seen i nstead of a single circuit. CONCLUSIONS: Spatial nonhomogeneities in conduction caused by transient une xcitable areas play a major role in both the initiation and the termination of episodes of circus movements in the pregnant rat myometrium. In additio n, most circuits remained in a single location, although sometimes meanderi ng or even fibrillatory activity was seen.