Introduction

Since reported uterine contraction curves are uniform in intramyometrial pressure and external tocodynamometry, and regular preterm labor contraction has been unable to be alleviated by medicinal treatment to prolong the pregnancy duration until full term birth. The clinical difficulty will be caused by no understanding of the process to develop labor contraction, but thinking only to suppress uterine myometrial activity, namely, labor contraction will develop by the coordination of a positive feed-back system composed of the uterus, uterus-brain nerve, hypothalamus and hypophysis which produce oxytocin to enhance uterine contraction until physiological oscillation of whole feed-back system. 

As the myometrium is simple compound of myomertrial cells which has no particular center of contraction rhythm, but there is close similarity of contraction interval and the contraction curves of intramyometrial pressure reported by Roberto Caldeyro-Barcia 1958 in Monte-Video [1] and external tocodynamometry recorded in 1980s in Tottori. Although there was no communication among two studies, they were highly similar.

As there was close similarity between labor contraction recorded by Caldeyro-Barcia and enlarged ultrasound oscillation (Figure 1), the author understood the mechanism of developing labor contraction, namely, both curves are the result of oscillation, which is produced by the presence of positive feed-back system, both in electric and physiologic oscillations.

Electric signal is enlarged in an amplifier, of which output is fed back to the input of amplifier, and further enlarged until saturation, where the frequency is constant according to the frequency constant of the feed-back loop, and its amplitude is also.

Methods and Results

The electrical oscillation developed in a positive feed-back loop composed of an amplifier, of which output is fed back to the input of amplifier through the feed-back loop, of which time constant determines output wave frequency. Signal amplitude is limited by the saturation of amplifier system, then constant amplitude of constant frequency signal is produced after the saturation of the feed-back system.


Figure 1: Uterine contractions are close to electric oscillation. A: Uterine contraction, Caldeyro-Barcia, 1960 [1]. B: Electric oscillation, 2MHz ultrasound, 1975.

The　labor　will　be　initiated　by　local　and　weak　uterine　contraction, which　is　distributed whole uterus through the gap junction, the contraction signal is conducted hypothalamus through the uterus-brain nerve [2-5] which amplifies the signal then the hypophysis is stimulated to discharge oxytocin, which further stimulates uterine contraction, and the positive feed-back loop is further stimulated to saturate uterine contraction to constant intensity and frequency, that is the physiologic oscillation (Figure 2).


Figure 2: Regular labor is physiologic oscillation caused by positive feed-back through uterus.

Discussion

Braxton-Hicks contraction is after 30 weeks of pregnancy, while it does not proceed to the vaginal delivery, is an example of incomplete uterine oscillation, namely, local early contraction appears, while the positive feed-back system does not work to make the contraction to regular labor contraction. That should be discussed in the treatment of preterm labor,

Tocolysis is common in the preterm labor, while it is unable to stop the contraction to prolong the pregnancy to full-term delivery. The author thinks that the preterm labor is a complete uterine oscillation, which is difficult to stop with common tocolysis, Therefore, it will be necessary to suppress the formation of positive feed-back loop with any anesthesia of uterus-brain nerve, or to detect local uterine contraction and suppress it by tocolysis, and so on, namely, more early stage suppression will be effective, after detection of the early weak contraction and tocolyzer is administered.

Conclusion

Labor contraction of uterus is developed by a positive feed-back system including brain until physiologic oscillation. As preterm labor contraction develops by the same process, preterm labor treatment will be started from the understanding of the oscillation of positive feed-back system.

References

1.           Caldeyro-Barcia R, Poseiro JJ, Mendez-Bauer C and Gulin LO. Effect of abnormal uterine contractions on fetal heart rate in labor (1967) 9-27.

2.           Wiesel O, Toth IE, Boldokoi Z, Homyak A. Bokor V, et al. Comparison of transsynaptic viral labeling of central nervous system structures rom the uterine horn in virgin, pregnant, and lactating rats (2004) Microsc Res Tech 63: 244-252. https://doi.org/10.1002/jemt.20034

3.           Yellon SM, Grushan LA, Rambau GM, Leshuga TJ and Kriby MA. Pregnancy-related changes in connections from the cervix to forebrain and　hypothalamus in mice (2010) Reproduction 140: 155-164. https://doi.org/10.1530/REP-10-0002

4.           Gnanamanickam GJ and Liewellyn-Smith IJ. Innervation of the rat uterus at esterus: a study in full-thickness, immunoperoxidase-stained whole-mount preparations (2011) J Comp Neurol 519: 621-643. https://doi.org/10.1002/cne.22515

5.           Poletini MO, McKee DT, Szawa RE, Bertham Richard, Helena CVV, et al. Cervical stimulation activates A1 and locus coeruleus neurons that project to the paraventricular nucleus of hypothalamus (2012) Brain Res Bull 88: 566-573.https://dx.doi.org/10.1016%2Fj.brainresbull.2012.06.004

Keywords

Uterus; Labor contraction; Preterm labor; Oscillation; Electric; Physiologic.