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Uterine contractions are
muscle contraction Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such a ...
s of the uterine smooth muscle that occur during the
menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs ...
and labour. Uterine contractions occur throughout the menstrual cycle in the non-pregnant state and throughout
gestation Gestation is the period of development during the carrying of an embryo, and later fetus, inside viviparous animals (the embryo develops within the parent). It is typical for mammals, but also occurs for some non-mammals. Mammals during pr ...
.


Throughout menstrual cycle

Uterine contractions that occur throughout the
menstrual cycle The menstrual cycle is a series of natural changes in hormone production and the structures of the uterus and ovaries of the female reproductive system that make pregnancy possible. The ovarian cycle controls the production and release of eggs ...
, also termed ''endometrial waves'' or ''contractile waves'', appear to involve only the sub- endometrial layer of the
myometrium The myometrium is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle cells (also called uterine myocytes) but also of supporting stromal and vascular tissue. Its main function is to induce uterine contractions. Struc ...
.


Follicular and luteal phase

In the early
follicular phase The follicular phase, also known as the preovulatory phase or proliferative phase, is the phase of the estrous cycle (or, in primates for example, the menstrual cycle) during which follicles in the ovary mature from primary follicle to a fully ...
, 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 Estrogen or oestrogen is a category of sex hormone responsible for the development and regulation of the female reproductive system and secondary sex characteristics. There are three major endogenous estrogens that have estrogenic hormonal ac ...
and
progesterone Progesterone (P4) is an endogenous steroid and progestogen sex hormone involved in the menstrual cycle, pregnancy, and embryogenesis of humans and other species. It belongs to a group of steroid hormones called the progestogens and is the majo ...
receptors. The frequency of contractions increases to 3–4 per minute towards
ovulation Ovulation is the release of eggs from the ovaries. In women, this event occurs when the ovarian follicles rupture and release the secondary oocyte ovarian cells. After ovulation, during the luteal phase, the egg will be available to be fertilize ...
. During the
luteal phase The corpus luteum (Latin for "yellow body"; plural corpora lutea) is a temporary endocrine structure in female ovaries involved in the production of relatively high levels of progesterone, and moderate levels of estradiol, and inhibin A. It is th ...
, 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 Menstruation (also known as a period, among other colloquial terms) is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The menstrual cycle is characterized by the rise and fall of ...
the intensity increases dramatically to between 50 and 200 mmHg producing labor-like contractions. These contractions are sometimes termed ''menstrual cramps'',medicinenet.com > Menstrual Cramps
Retrieved January 2011
although that term is also used for menstrual pain in general. These contractions may be uncomfortable or even painful, but they are generally significantly less painful than contractions during labour. Painful contractions are called
dysmenorrhea Dysmenorrhea, also known as period pain, painful periods or menstrual cramps, is pain during menstruation. Its usual onset occurs around the time that menstruation begins. Symptoms typically last less than three days. The pain is usually in th ...
.


Directionality of contractions

A shift in the
myosin Myosins () are a superfamily of motor proteins best known for their roles in muscle contraction and in a wide range of other motility processes in eukaryotes. They are ATP-dependent and responsible for actin-based motility. The first myosin (M ...
expression of the uterine smooth muscle has been hypothesized as arising for changes in the directions of uterine contractions during the menstrual cycle.


Labour

Uterine contractions are part of the process of natural
childbirth Childbirth, also known as labour and delivery, is the ending of pregnancy where one or more babies exits the internal environment of the mother via vaginal delivery or caesarean section. In 2019, there were about 140.11 million births glob ...
(i.e., not by
Caesarean section Caesarean section, also known as C-section or caesarean delivery, is the surgical procedure by which one or more babies are delivered through an incision in the mother's abdomen, often performed because vaginal delivery would put the baby or m ...
). During labor, uterine contractions changes from episodic and uncoordinated to highly-coordinated. This change typically occurs at night, suggesting neural control, and significantly increases intrauterine pressure. Some women may experience contractions before labour is due. These are
Braxton Hicks contractions Braxton Hicks contractions, also known as practice contractions or false labor, are sporadic uterine contractions that may start around six weeks into a pregnancy. However, they are usually felt in the second or third trimester of pregnancy. ...
, which are sometimes referred to as "false labour." There are several endogenous compounds involved in coordinating uterine contractility in labour, including
oxytocin Oxytocin (Oxt or OT) is a peptide hormone and neuropeptide normally produced in the hypothalamus and released by the posterior pituitary. It plays a role in social bonding, reproduction, childbirth, and the period after childbirth. Oxytoc ...
and
prostaglandins The prostaglandins (PG) are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are deri ...
.


Oxytocin

The
hormone A hormone (from the Greek participle , "setting in motion") is a class of signaling molecules in multicellular organisms that are sent to distant organs by complex biological processes to regulate physiology and behavior. Hormones are required ...
oxytocin has been identified as inducing uterine contractions, and labour in general. Oxytocin is produced by the body naturally and since the 1950s has also been available in
synthetic Synthetic things are composed of multiple parts, often with the implication that they are artificial. In particular, 'synthetic' may refer to: Science * Synthetic chemical or compound, produced by the process of chemical synthesis * Synthetic ...
pharmaceutical form Dosage forms (also called unit doses) are pharmaceutical drug products in the form in which they are marketed for use, with a specific mixture of active ingredients and inactive components (excipients), in a particular configuration (such as a cap ...
. In either form, oxytocin stimulates uterine contractions to accelerate the process of
childbirth Childbirth, also known as labour and delivery, is the ending of pregnancy where one or more babies exits the internal environment of the mother via vaginal delivery or caesarean section. In 2019, there were about 140.11 million births glob ...
. Production and secretion of oxytocin is controlled by a
positive feedback Positive feedback (exacerbating feedback, self-reinforcing feedback) is a process that occurs in a feedback loop which exacerbates the effects of a small disturbance. That is, the effects of a perturbation on a system include an increase in th ...
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 Blood plasma is a light amber-colored liquid component of blood in which blood cells are absent, but contains proteins and other constituents of whole blood in suspension. It makes up about 55% of the body's total blood volume. It is the int ...
and
amniotic fluid The amniotic fluid is the protective liquid contained by the amniotic sac of a gravid amniote. This fluid serves as a cushion for the growing fetus, but also serves to facilitate the exchange of nutrients, water, and biochemical products between ...
increases during labor. These inflammatory mediators encourage myometrial contractions to induce labor. Prostaglandins are also related to the changes in
gap junction Gap junctions are specialized intercellular connections between a multitude of animal cell-types. They directly connect the cytoplasm of two cells, which allows various molecules, ions and electrical impulses to directly pass through a regula ...
formation and connexin-43 expression during labor.


In orgasm

Uterine and
vaginal contraction Vaginal contractions are contractions of the pelvic muscles surrounding the vagina, especially the pubococcygeus muscle. Vaginal contractions are generally an involuntary muscular response to orgasm. Though usually an involuntary response, some wo ...
s usually take place during female
sexual stimulation Sexual stimulation is any stimulus (including bodily contact) that leads to, enhances and maintains sexual arousal, and may lead to orgasm. Although sexual arousal may arise without physical stimulation, achieving orgasm usually requires physica ...
, including sexual arousal, and
orgasm Orgasm (from Greek , ; "excitement, swelling") or sexual climax is the sudden discharge of accumulated sexual excitement during the sexual response cycle, resulting in rhythmic, involuntary muscular contractions in the pelvic region chara ...
.


Monitors

Uterine contractions can be monitored by
cardiotocography Cardiotocography (CTG) is a technique used to monitor the fetal heartbeat and the uterine contractions during pregnancy and labour. The machine used to perform the monitoring is called a cardiotocograph. Fetal heart sounds was described as earl ...
, 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. 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.


Mechanism


Resting state

The
resting membrane potential A relatively static membrane potential which is usually referred to as the ground value for trans-membrane voltage. The relatively static membrane potential of quiescent cells is called the resting membrane potential (or resting voltage), as oppo ...
(Vrest) of uterine smooth muscle has been recorded to be between −35 and −80 mV. 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 Intracellular space is the interior space of the plasma membrane. It contains about two-thirds of TBW. Cellular rupture may occur if the intracellular space becomes dehydrated, or if the opposite happens, where it becomes too bloated. Thus it i ...
, 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. 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. K+ is the major ion responsible for such changes in ion flux, reflecting changes in various K+ channels.


Excitation-contraction

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


Excitation

The
excitation-contraction coupling Muscle contraction is the activation of tension-generating sites within muscle cells. In physiology, muscle contraction does not necessarily mean muscle shortening because muscle tension can be produced without changes in muscle length, such as ...
of uterine smooth muscle is also very similar to that of other smooth muscle in general, with intracellular increase in
calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...
(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.


Restoration to resting state

Uterine smooth muscle mechanisms of relaxation differ significantly from those of other human smooth muscles. Removal of Ca2+ after contraction induces relaxation of the smooth muscle, and restores the molecular structure of the
sarcoplasmic reticulum The sarcoplasmic reticulum (SR) is a membrane-bound structure found within muscle cells that is similar to the smooth endoplasmic reticulum in other cells. The main function of the SR is to store calcium ions (Ca2+). Calcium ion levels are ke ...
for the next contractile stimulus.


Measuring uterine contractility ''ex vivo''

Ethically-donated human uterine tissues can be used to measure uterine contractility ''
ex vivo ''Ex vivo'' (Latin: "out of the living") literally means that which takes place outside an organism. In science, ''ex vivo'' refers to experimentation or measurements done in or on tissue from an organism in an external environment with minimal ...
''. In these experiments, sections of
myometrium The myometrium is the middle layer of the uterine wall, consisting mainly of uterine smooth muscle cells (also called uterine myocytes) but also of supporting stromal and vascular tissue. Its main function is to induce uterine contractions. Struc ...
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 Animal studies is a recently recognised field in which animals are studied in a variety of cross-disciplinary ways. Scholars who engage in animal studies may be formally trained in a number of diverse fields, including geography, art history, ant ...
and ''
in vitro ''In vitro'' (meaning in glass, or ''in the glass'') studies are performed with microorganisms, cells, or biological molecules outside their normal biological context. Colloquially called " test-tube experiments", these studies in biology ...
'', it does not demonstrate the same effect in
clinical studies Clinical trials are prospective biomedical or behavioral research studies on human participants designed to answer specific questions about biomedical or behavioral interventions, including new treatments (such as novel vaccines, drugs, dieta ...
. And while the peptide hormone
relaxin Relaxin is a protein hormone of about 6000 Da first described in 1926 by Frederick Hisaw. The relaxin family peptide hormones belong to the insulin superfamily and consists of seven peptides of high structural but low sequence similarity; rela ...
has been shown to inhibit uterine contractility in rats, mice, and pigs, it does not prevent uterine contractility in humans.


See also

*
Involution (medicine) Involution is the shrinking or return of an organ to a former size. At a cellular level, involution is characterized by the process of proteolysis of the basement membrane (basal lamina), leading to epithelial regression and apoptosis, with accompan ...


References

{{Authority control Obstetrics Midwifery Uterus