Uterine Contractions in Normal Labor Developed by a Positive Feed-back and Oscillation

Regular uterine contractions in normal labor are expulsive power to deliver the fetus. The intrauterine pressure changes were recorded and studied in the measurement of labor contractions of uterus, while external tocodynamometry is common in clinical fetal monitoring [1]. The duration of contraction was about 1 min and interval was also 1 min, and the peak intrauterine pressure was 30 to 50 mmHg in normal labor contractions, which was initiated at the pace-maker located at the corner of uterine fundus [1], it was conducted by the gap-junction [2] from the fundus to the cervix [1]. In addition, myometrial action potential was studied in details [3-5]. However, no mechanism of regular uterine contractions has been clarified, though the regular and constant myometrial contraction intensity would be important feature to push the fetus downwards and dilate the cervix. First step of studies was the detection of regular contraction, second the simulation of the repeated contraction comparing to electric oscillation in this report.


Introduction
Regular uterine contractions in normal labor are expulsive power to deliver the fetus. The intrauterine pressure changes were recorded and studied in the measurement of labor contractions of uterus, while external tocodynamometry is common in clinical fetal monitoring [1]. The duration of contraction was about 1 min and interval was also 1 min, and the peak intrauterine pressure was 30 to 50 mmHg in normal labor contractions, which was initiated at the pace-maker located at the corner of uterine fundus [1], it was conducted by the gap-junction [2] from the fundus to the cervix [1]. In addition, myometrial action potential was studied in details [3][4][5]. However, no mechanism of regular uterine contractions has been clarified, though the regular and constant myometrial contraction intensity would be important feature to push the fetus downwards and dilate the cervix. First step of studies was the detection of regular contraction, second the simulation of the repeated contraction comparing to electric oscillation in this report.

Methods
A typical uterine contraction was reported by Caldeyro-Barcia, where repeated uterine contractions were found in amniotic pressure, of which intensity were approx. 40 mmHg, duration was approx. one min, and interval also1 min [1]. External tocodynamometry was objectively studied comparing to intrauterine pressure changes by us [3] (Figure 1).
The contraction curve was compared to the electronic oscillation in an experimental 2 MHz ultrasound ( Figure 2). The regularity of uterine contraction curve resembled the ultrasound waves; however, the difference was the presence of minus deflection of ultrasound waves, which was lost in uterine contraction. This is natural because uterine contraction is a biological uterine muscle action, but not the electrical information. Therefore, the uterine contraction curve was conversed upside down to make it minus deflection, and then attached to the base of original contraction curve. The combined curve showed a sine wave-like oscillatory change, where it was confirmed that the uterine contraction curve was the part of a biological oscillation ( Figure 3). The result made it possible to analyze the labor contraction as an oscillation.

Comments
The developmental mechanism of electrical oscillation is a positive feed-back process in an electronic circuit composed of an amplifier and a positive feedback loop (Figure 4), where the electrical output is Since a labor contraction of uterus is a biological oscillation, a positive feed-back system is adopted to produce regular oscillation, where the amplifier is composed of the contraction promoting system, including hypothalamic nerve center, hypophysis and oxytocin secretion. The contraction amplifying system output is uterine contraction, and the contraction information is fed back to the input of the contraction promoting system through the innervations between the uterus and hypothalamus [6][7][8][9] (Figure 5). The frequency of oscillation developed by the biological positive feed-back system is approx. 0.008 Hz (the wave length is 2 min, due to slow distribution of contraction in the uterus), and positive amplitude is approx. 40 mmHg. The labor contractions develop regularly with fixed frequency and amplitude in the oscillation, until the delivery of fetus.
The most important role of uterus-brain innervation will be to maintain the labor contraction to the most appropriate condition for the delivery of fetus, while the preeclampsia will be caused by the stimulation of hypothalamic sympathetic center continuously by enlarged uterus [10].
Abnormal labor contraction of uterus may be treated by the stimulation or suppression of a component of the positive feed-back system, e.g. in the repeated uterine contraction of a premature labor, the nerves between the uterus and brain will be deeply sedated to interrupt the positive feed-back loop by the paralysis of nerves. In contrast, a week labor pain will be treated by some stimulation of the uterus-brain nerves to develop normal positive feed-back. An anesthetic or analgesic medicine would be used to sedate the uterus-brain innervations in the treatment of a premature labor, while the innervations between the uterus and brain would be exited in the weak labor pain. New therapeutic strategies will be created after the studies on the positive feed-back system.

Conclusion
The development of regular labor contraction of uterus is provided by the oscillation of positive feed-back system in the loop composed of center of hypothalamus-hypophysis-oxitocin secretion, uterine contraction and the positive feed-back nerves of informing uterine contractions to the hypothalamus. New managements of abnormal labor contraction of uterus will be open after understanding the presence of positive feed-back and oscillation of uterine contraction in the labor with new pharmaceutical strategies.