Co-ordination of electrical activity in the myometrium of pregnant ewes

Abstract

Summary. The co-ordination of spontaneous and evoked electromyographic (EMG) activity was examined in the uterus of ewes from Day 100 of pregnancy and during labour. During pregnancy the onset of bursts of EMG activity was synchronous to within 2\p=n-\3min when recorded by electrodes at any recording site. Individual EMG spikes, whether occurring spontaneously or evoked by electrical stimulation of the myometrium, appeared to propagate between recording electrodes that were 1\p=n-\3cm apart, in the longitudinal or circular orientation of the uterus, but not when the electrodes were separated by more than 3 cm. Apparent conduction velocity in the longitudinal orientation was 7 cm sec\m=-\1 during pregnancy and increased to 13 cm sec\m=-\1 during labour. No consistent pattern could be distinguished in the sites at which EMG activity was first observed at any stage of pregnancy studied. The nature of propagation of EMG activity remains unresolved but it is clear that it is not initiated at a single site in the myometrium.

Patterns of electrical propagation in the intact pregnant guinea pig uterus

“…This phenomenon explains an anomaly (26) as to why spikes that propagate quite fast (in our case 7 cm/s) can take so long to travel relatively short distances (Figs. 8 and 9).…”

“…Another potential complication is the fact that gap junctions that electrically connect myometrial cells may form spontaneously in vitro, which may improve electrical coupling and affect electrical propagation (14,29). Finally, even if a stable pacemaker had been found, there is evidence to suggest that this may not be stable during longer recording periods (26). Nevertheless, this study provides information as to the electrical behavior of the pregnant uterus at term.…”

“…Most of these studies were performed on isolated segments of myometrium, but a few studies have recorded from a limited number of electrodes in the whole intact myometrium in vivo. Harding et al (15), with four implanted electrodes in the pregnant sheep, could not detect propagation and suggested that there did not seem to be a specific pacemaker area, while Parkington et al (26), also in pregnant sheep, discovered that propagation could only be detected if the recording electrodes were closer to each other than 3 cm.…”

The location of pacemakers in the uteri of pregnant guinea pigs and rats

“…Although suggestive of the presence of cells generating spontaneous potentials, this characteristic is not present in all impalements within the same preparation and tissue location (24). Parkington et al (28), in the ewe, could not find a consistent site for the initiation of myometrial bursts, even during labor. In the rat, several studies did find a tendency for the pacemaker to occur more frequently at the ovarial end of the horn (4, 9), which was also seen in the current study.…”

“…The electrical activity in the uterus is characterized by bursts of spiking activity interspaced with periods of quiescence (6,16,19,25). Recordings with several to large numbers of extracellular electrodes have shown that these spikes (action potentials) propagate through the myometrium from cell to cell through the presence of gap junctions (10,11,26,28,35). There is also increasing evidence that electrical and contractile activity is quite limited during gestation but increases when term approaches to prepare for delivery (13,41).…”