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

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 2-3 min 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 1-3 cm 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-1 during pregnancy and increased to 13 cm sec-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.