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 responsible for the initiation and the termination of episodes of circus movements and to describe the characteristics of these circuits in rat myometrium.

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. Circus movements were terminated by (1) reversal of recruitment (decruitment) at the end of the burst, (2) occurrence of lines of conduction blocks, or (3) interruption of the circuit. On average circuits rotated for 6 revolutions during 2 seconds. In most cases circuits remained stable; sometimes circuits drifted along the tissue and occasionally multiple wavelets were seen instead of a single circuit.

CONCLUSIONS

Spatial nonhomogeneities in conduction caused by transient unexcitable areas play a major role in both the initiation and the termination of episodes of circus movements in the pregnant rat myo-metrium. In addition, most circuits remained in a single location, although sometimes meandering or even fibrillatory activity was seen.