Electrical activity of the pregnant uterus in the cynomolgus monkey.

Electromyographic activity in six pregnant cynomolgus monkeys was recorded in vivo by means of bipolar electrodes individually attached to the surface of the uterine corpus. These data were compared with recordings of intra-amniotic pressure obtained simultaneously by means of an open-ended, fluid-filled catheter placed in the amniotic cavity. During the last third of pregnancy, the electromyograms showed recurrent electrical complexes (trains of potentials, TOP) which lasted for about 2 minutes and whose rate of occurrence varied during the nyctohemeral period. The initiation of electrical activity in various uterine areas was always synchronous with and was related to mechanical contraction. The complex, polyphasic contractions that were observed can be explained, largely, by the variations of potentials within a train of potentials. However, a simple quantitative relationship between the mean frequency of the potentials within the TOP and the mean amplitude of the change in pressure could not be demonstrated. In contrast to data available on the woman, the increase in contractile strength at the onset of labor in the macaque appears to be due to a higher frequency of excitation of individual cells rather than the result of an improved functional coordination between the active muscular elements.