Uterine contraction

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Uterine contractions are muscle contractions of the uterine smooth muscle that can occur at various intensities in both the non-pregnant and pregnant uterine state. The non-pregnant uterus undergoes small, spontaneous contractions in addition to stronger, coordinated contractions during the menstrual cycle and orgasm. Throughout gestation, [1] the uterus enters a state of uterine quiescence due to various neural and hormonal changes. During this state, the uterus undergoes little to no contractions, though spontaneous contractions still occur for the uterine myocyte cells to experience hypertrophy. [1] The pregnant uterus only contracts strongly during orgasms, labour, and in the postpartum stage to return to its natural size. [2]

Contents

Throughout menstrual cycle

Uterine contractions that occur throughout the menstrual cycle, also termed endometrial waves or contractile waves, [3] appear to involve only the sub-endometrial layer of the myometrium. [3]

Follicular and luteal phase

In the early follicular phase, uterine contractions in the non-pregnant woman occur 1–2 times per minute and last 10–15 seconds with a low intensity of usually 30 mmHg or less. This sub-endometrial layer is rich in estrogen and progesterone receptors. [3] The frequency of contractions increases to 3–4 per minute towards ovulation. During the luteal phase, the frequency and intensity decrease, possibly to facilitate any implantation.

Menstruation

If implantation does not occur, the frequency of contractions remains low; but at menstruation the intensity increases dramatically to between 50 and 200 mmHg producing labor-like contractions. [3] These contractions are sometimes termed menstrual cramps, [4] although that term is also used for menstrual pain in general. These contractions may be uncomfortable or even painful, [5] but they are generally significantly less painful than contractions during labour. Painful contractions are called dysmenorrhea.

Directionality of contractions

A shift in the myosin expression of the uterine smooth muscle has been hypothesized as arising for changes in the directions of uterine contractions during the menstrual cycle. [3]

Labour and pregnancy

Uterine contractions are a vital part of natural childbirth, [6] which occur during the process of labour and delivery, (typically this excludes caesarean section). These labour contractions are characterized by their rhythmic tightening and relaxation of the myometrium, the most prominent uterine muscle. Labour contractions primarily serve the purpose of opening and dilating the cervix, [7] which leads to the assisting of the passage of the baby through the vaginal canal during the first stage of labour.

Throughout pregnancy, the uterus experiences motor denervation, thus inhibiting spontaneous contractions. The remaining contractions are predominantly hormonally controlled. The decrease in the coordination of uterine smooth muscles cells reduces the effectiveness of contractions, causing the uterus to enter a state of uterine quiescence. [8] During the beginning of labour, contractions may initially be intermittent and irregular, [7] but will transition into a more coordinated pattern as the labour progresses. [7] This transition is governed by various myogenic, neurogenic, and hormonal factors working together. [8] As labour progresses, contractions will typically increase in frequency and intensity, which leads to a significant rise in intrauterine pressure. [7]

Otherwise, not all contractions experienced by pregnant individuals are indications of the beginning of labour. Some women experience what are commonly called Braxton Hicks contractions before their initial due date, which are characterized as “false labour." Though similar to labour uterine contractions, these contractions do not play a prominent role in cervical dilation or the progression of childbirth.

Oxytocin

The hormone oxytocin has been identified as inducing uterine contractions, and labour in general. [9] Oxytocin is produced by the body naturally and since the 1950s has also been available in synthetic pharmaceutical form. [10] [11] In either form, oxytocin stimulates uterine contractions to accelerate the process of childbirth. Production and secretion of oxytocin is controlled by a positive feedback mechanism, where its initial release, either naturally or in pharmaceutical form, stimulates production and release of further oxytocin. For example, when oxytocin is released during a contraction of the uterus at the start of childbirth, this stimulates production and release of more oxytocin and an increase in the duration, intensity and frequency of contractions. This process compounds in intensity and frequency and continues until the triggering activity ceases.

Prostaglandins

The concentration of prostaglandins in the blood plasma and amniotic fluid increases during labor. [5] These inflammatory mediators encourage myometrial contractions to induce labor. [5] Prostaglandins are also related to the changes in gap junction formation and connexin-43 expression during labor. [9]

In orgasm

Uterine and vaginal contractions usually take place during female sexual stimulation, including sexual arousal, and orgasm. [12]

Monitors

Knitted Bellyband with conductive thread and RFID chip to monitor contractions Courtesy Bellyband with chip Shima Seiki Haute Technology Laboratory DSCF2178.jpg
Knitted Bellyband with conductive thread and RFID chip to monitor contractions

Uterine contractions can be monitored by cardiotocography, in which a device is affixed to the skin of the mother or directly to the fetal scalp. The pressure required to flatten a section of the uterine wall correlates with the internal pressure, thereby providing an estimate of it. [13]

A type of monitoring technology under development at Drexel University embeds conductive threads in the knitted fabric of a bellyband. When the fibers stretch in response to a contraction, the threads function like an antenna, and send the signals they pick up to an embedded RFID (radio-frequency identification device) chip that reports the data. [14]

Mechanism

Resting state

The resting membrane potential (Vrest) of uterine smooth muscle has been recorded to be between −35 and −80 mV. [3] As with the resting membrane potential of other cell types, it is maintained by a Na+/K+ pump that causes a higher concentration of Na+ ions in the extracellular space than in the intracellular space, and a higher concentration of K+ ions in the intracellular space than in the extracellular space. Subsequently, having K+ channels open to a higher degree than Na+ channels results in an overall efflux of positive ions, resulting in a negative potential.

This resting potential undergoes rhythmic oscillations, which have been termed slow waves, and reflect intrinsic activity of slow wave potentials. [3] These slow waves are caused by changes in the distribution of Ca2+, Na+, K+ and Cl− ions between the intracellular and extracellular spaces, which, in turn, reflects the permeability of the plasma membrane to each of those ions. [3] K+ is the major ion responsible for such changes in ion flux, reflecting changes in various K+ channels. [3]

Excitation-contraction

As the uterus becomes essentially denervated during gestation, it is unlikely that any coordinated nervous regulation of the myometrium is centrally orchestrated. [15]

Excitation

The excitation-contraction coupling of the uterine smooth muscle is also very similar to that of other smooth muscles in general, with intracellular increase in calcium (Ca2+) leading to contraction.

Nitric oxide (NO) is particularly effective in relaxing the myometrium and in fact has a lower inhibitory concentration 50% (Ki) in human than guinea pig or non-human primate myometrium. [15]

Restoration to resting state

Uterine smooth muscle mechanisms of relaxation differ significantly from those of other human smooth muscles. [15] Removal of Ca2+ after contraction induces relaxation of the smooth muscle, and restores the molecular structure of the sarcoplasmic reticulum for the next contractile stimulus. [3]

Measuring uterine contractility ex vivo

Ethically donated human uterine tissues can be used to measure uterine contractility ex vivo . In these experiments, sections of myometrium are set up in an organ bath system that to measure changes in isometric force production. Following functional checks to ensure the tissue is physiologically active, compounds can be added to the organ bath in increasing concentrations to create a cumulative concentration-response curve (CCRC).

A key advantage of measuring uterine contractility ex vivo is the ability to eliminate species differences. For example, while magnesium reduces myometrial contractility in animal studies and in vitro , it does not demonstrate the same effect in clinical studies. [16] And while the peptide hormone relaxin has been shown to inhibit uterine contractility in rats, mice, and pigs, it does not prevent uterine contractility in humans. [17]

See also

Related Research Articles

<span class="mw-page-title-main">Cervix</span> Lower part of the uterus in the female reproductive system

The cervix or cervix uteri is a dynamic fibromuscular organ of the female reproductive system that connects the vagina with the uterine cavity. The human cervix has been documented anatomically since at least the time of Hippocrates, over 2,000 years ago. The cervix is approximately 4 cm long with a diameter of approximately 3 cm and tends to be described as a cylindrical shape, although the front and back walls of the cervix are contiguous. The size of the cervix changes throughout a women's life cycle. For example, during their fertile years of the reproductive cycle, females tend to have a larger cervix vis á vis postmenopausal females; likewise, females who have produced offspring have a larger sized cervix than females who have not produced offspring.

<span class="mw-page-title-main">Endometrium</span> Inner mucous membrane of the mammalian uterus

The endometrium is the inner epithelial layer, along with its mucous membrane, of the mammalian uterus. It has a basal layer and a functional layer: the basal layer contains stem cells which regenerate the functional layer. The functional layer thickens and then is shed during menstruation in humans and some other mammals, including other apes, Old World monkeys, some species of bat, the elephant shrew and the Cairo spiny mouse. In most other mammals, the endometrium is reabsorbed in the estrous cycle. During pregnancy, the glands and blood vessels in the endometrium further increase in size and number. Vascular spaces fuse and become interconnected, forming the placenta, which supplies oxygen and nutrition to the embryo and fetus. The speculated presence of an endometrial microbiota has been argued against.

<span class="mw-page-title-main">Uterus</span> Female sex organ in mammals

The uterus or womb is the organ in the reproductive system of most female mammals, including humans, that accommodates the embryonic and fetal development of one or more embryos until birth. The uterus is a hormone-responsive sex organ that contains glands in its lining that secrete uterine milk for embryonic nourishment.

<span class="mw-page-title-main">Atosiban</span> Chemical compound

Atosiban, sold under the brand name Tractocile among others, is an inhibitor of the hormones oxytocin and vasopressin. It is used as an intravenous medication as a labour repressant (tocolytic) to halt premature labor. It was developed by Ferring Pharmaceuticals in Sweden and first reported in the literature in 1985. Originally marketed by Ferring Pharmaceuticals, it is licensed in proprietary and generic forms for the delay of imminent preterm birth in pregnant adult women.

<span class="mw-page-title-main">Adenomyosis</span> Extension of endometrial tissue into the myometrium

Adenomyosis is a medical condition characterized by the growth of cells that proliferate on the inside of the uterus (endometrium) atypically located among the cells of the uterine wall (myometrium), as a result, thickening of the uterus occurs. As well as being misplaced in patients with this condition, endometrial tissue is completely functional. The tissue thickens, sheds and bleeds during every menstrual cycle.

<span class="mw-page-title-main">Vaginal bleeding</span> Medical condition

Vaginal bleeding is any expulsion of blood from the vagina. This bleeding may originate from the uterus, vaginal wall, or cervix. Generally, it is either part of a normal menstrual cycle or is caused by hormonal or other problems of the reproductive system, such as abnormal uterine bleeding.

Bloody show or show is the passage of a small amount of blood or blood-tinged mucus through the vagina near the end of pregnancy. It is caused by thinning and dilation of the cervix, leading to detachment of the cervical mucus plug that seals the cervix during pregnancy and tearing of small cervical blood vessels, and is one of the signs that labor may be imminent. The bloody show may be expelled from the vagina in pieces or altogether and often appears as a jelly-like piece of mucus stained with blood. Although the bloody show may be alarming at first, it is not a concern of patient health after 37 weeks gestation.

<span class="mw-page-title-main">Myometrium</span> Smooth muscle coat of the uterus

The myometrium is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle cells but also of supporting stromal and vascular tissue. Its main function is to induce uterine contractions.

<span class="mw-page-title-main">Cervical effacement</span> Thinning and shortening of the cervix

Cervical effacement or cervical ripening refers to the thinning and shortening of the cervix. This process occurs during labor to prepare the cervix for dilation to allow the fetus to pass through the vagina. While this a normal, physiological process that occurs at the later end of pregnancy, it can also be induced through medications and procedures.

Luteolysis is the structural and functional degradation of the corpus luteum, which occurs at the end of the luteal phase of both the estrous and menstrual cycles in the absence of pregnancy.

Adenomyoma is a tumor (-oma) including components derived from glands (adeno-) and muscle (-my-). It is a type of complex and mixed tumor, and several variants have been described in the medical literature. Uterine adenomyoma, the localized form of uterine adenomyosis, is a tumor composed of endometrial gland tissue and smooth muscle in the myometrium. Adenomyomas containing endometrial glands are also found outside of the uterus, most commonly on the uterine adnexa but can also develop at distant sites outside of the pelvis. Gallbladder adenomyoma, the localized form of adenomyomatosis, is a polypoid tumor in the gallbladder composed of hyperplastic mucosal epithelium and muscularis propria.

<span class="mw-page-title-main">Endometrial ablation</span> Medical procedure

Endometrial ablation is a surgical procedure that is used to remove (ablate) or destroy the endometrial lining of the uterus. The goal of the procedure is to decrease the amount of blood loss during menstrual periods. Endometrial ablation is most often employed in people with excessive menstrual bleeding, who do not wish to undergo a hysterectomy, following unsuccessful medical therapy.

<span class="mw-page-title-main">Uterine atony</span> Loss of tone in the uterine musculature

Uterine atony is the failure of the uterus to contract adequately following delivery. Contraction of the uterine muscles during labor compresses the blood vessels and slows flow, which helps prevent hemorrhage and facilitates coagulation. Therefore, a lack of uterine muscle contraction can lead to an acute hemorrhage, as the vasculature is not being sufficiently compressed. Uterine atony is the most common cause of postpartum hemorrhage, which is an emergency and potential cause of fatality. Across the globe, postpartum hemorrhage is among the top five causes of maternal death. Recognition of the warning signs of uterine atony in the setting of extensive postpartum bleeding should initiate interventions aimed at regaining stable uterine contraction.

Couvelaire uterus is a rare but not a life-threatening condition in which loosening of the placenta causes bleeding that penetrates into the uterine myometrium forcing its way into the peritoneal cavity. This condition makes the uterus very tense and rigid.

<span class="mw-page-title-main">Carbetocin</span> Medication used for preventing postpartum bleeding

Carbetocin, sold under the brand names Pabal among others, is a medication used to prevent excessive bleeding after childbirth, particularly following Cesarean section. It appears to work as well as oxytocin. Due to it being less economical than other options, use is not recommended by NHS Scotland. It is given by injection into a vein or muscle.

Arcuate arteries located in the uterus branch out and supply blood to different layers of the uterus. These arteries meet the myometrial-endometrial junction and lead to straight and endometrial arteries. The endometrium receives blood from endometrial arteries which are also called spiral arteries. Endometrial arteries proliferate rapidly and react to different hormones released. These hormones are progesterone and estrogen released by the ovaries and produced by the endocrine system. The endometrial arteries not only supply blood to the endometrium but are also important during pregnancy. They are the initial site of transportation of blood from the mother to the baby.

<span class="mw-page-title-main">Prostaglandin F2alpha</span> Chemical compound

Prostaglandin F2α, pharmaceutically termed dinoprost is a naturally occurring prostaglandin used in medicine to induce labor and as an abortifacient. Prostaglandins are lipids throughout the entire body that have a hormone-like function. In pregnancy, PGF2 is medically used to sustain contracture and provoke myometrial ischemia to accelerate labor and prevent significant blood loss in labor. Additionally, PGF2 has been linked to being naturally involved in the process of labor. It has been seen that there are higher levels of PGF2 in maternal fluid during labor when compared to at term. This signifies that there is likely a biological use and significance to the production and secretion of PGF2 in labor. Prostaglandin is also used to treat uterine infections in domestic animals.

The Fergusson reflex is the neuroendocrine reflex comprising the self-sustaining cycle of uterine contractions initiated by pressure at the cervix, more precisely, the internal end of cervix, or vaginal walls. It is an example of positive feedback in biology. The Ferguson reflex occurs in mammals.

A uterotonic, also known as an oxytocic or ecbolic, is a type of medication used to induce contraction or greater tonicity of the uterus. Uterotonics are used both to induce labor and to reduce postpartum hemorrhage.

<span class="mw-page-title-main">Placental expulsion</span>

Placental expulsion occurs when the placenta comes out of the birth canal after childbirth. The period from just after the baby is expelled until just after the placenta is expelled is called the third stage of labor.

References

  1. 1 2 Rosen, Hadar; Yogev, Yariv (3 March 2023). "Assessment of uterine contractions in labor and delivery". American Journal of Obstetrics and Gynecology. 228 (5): S1209–S1221. doi:10.1016/j.ajog.2022.09.003. PMID   37164494 . Retrieved 17 March 2024.
  2. ↑ Tingåker, Berith K; Irestedt, Lars (June 2010). "Changes in uterine innervation in pregnancy and during labour". Current Opinion in Anesthesiology. 23 (3): 300–303. doi:10.1097/ACO.0b013e328337c881. ISSN   0952-7907. PMID   20216064. S2CID   35342557.
  3. 1 2 3 4 5 6 7 8 9 10 Aguilar, H. N.; Mitchell, S.; Knoll, A. H.; Yuan, X. (2010). "Physiological pathways and molecular mechanisms regulating uterine contractility". Human Reproduction Update. 16 (6): 725–744. doi: 10.1093/humupd/dmq016 . PMID   20551073.
  4. ↑ medicinenet.com > Menstrual Cramps Retrieved January 2011
  5. 1 2 3 Porter, Misty Blanchette; Goldstein, Steven (2019-01-01), Strauss, Jerome F.; Barbieri, Robert L. (eds.), "Chapter 35 - Pelvic Imaging in Reproductive Endocrinology", Yen and Jaffe's Reproductive Endocrinology (Eighth Edition), Philadelphia: Elsevier, pp. 916–961.e5, ISBN   978-0-323-47912-7 , retrieved 2022-09-28
  6. ↑ Uterine Contraction. U.S. National Library of Medicine Medical Subject Headings.
  7. 1 2 3 4 Netter, Frank (2016). Cervix. Elsevier. ISBN   978-0-323-32108-2.
  8. 1 2 Jain, V.; Saade, G. R.; Garfield, R. E. (1999). Uterine contraction. Encyclopedia of Reproduction. 4. 932-942.
  9. 1 2 Niebyl, Jennifer R.; Simpson, Joe Leigh; Galan, Henry L. (2016). Obstetrics: Normal and Problem Pregnancies | ScienceDirect. Elsevier. ISBN   978-0-323-32108-2 . Retrieved 2022-09-28.
  10. ↑ du Vigneaud V, Ressler C, Swan JM, Roberts CW, Katsoyannis PG, Gordon S (1953). "The synthesis of an octapeptide amide with the hormonal activity of oxytocin". J. Am. Chem. Soc. 75 (19): 4879–80. doi:10.1021/ja01115a553.
  11. ↑ du Vigneaud V, Ressler C, Swan JM, Roberts CW, Katsoyannis PG (June 1954). "The synthesis of oxytocin". J. Am. Chem. Soc. 76 (12): 3115–21. doi:10.1021/ja01641a004.
  12. ↑ Komisaruk BR, Wise N, Frangos E, Liu WC, Allen K, Brody S (2011). "Women's Clitoris, Vagina, and Cervix Mapped on the Sensory Cortex: fMRI Evidence". The Journal of Sexual Medicine . 8 (10): 2822–2830. doi:10.1111/j.1743-6109.2011.02388.x. PMC   3186818 . PMID   21797981.
  13. ↑ Tocodynamometer. Dr. Malcolm C Brown. Copyright 2000
  14. ↑ Reyes, Juliana (August 21, 2014). "Drexel's wearable-tech lab is making 'a radio out of fabric' for pregnant women". Technically Philly. Retrieved 10 May 2017.
  15. 1 2 3 Iain L O Buxton; Nathanael Heyman; Yi-ying Wu; Scott Barnett; Craig Ulrich (2011). "A Role of Stretch-Activated Potassium Currents in the Regulation of Uterine Smooth Muscle Contraction". Acta Pharmacol Sin. 32 (6): 758–764. doi: 10.1038/aps.2011.62 . PMC   4009969 . PMID   21642947.
  16. ↑ Cuppett, Courtney D.; Caritis, Steve N. (2013-01-01), Mattison, Donald R. (ed.), "19 - Uterine Contraction Agents and Tocolytics", Clinical Pharmacology During Pregnancy, Academic Press, pp. 307–330, ISBN   978-0-12-386007-1 , retrieved 2022-09-28
  17. ↑ "Encyclopedia of Reproduction | ScienceDirect". www.sciencedirect.com. Retrieved 2022-09-28.
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