The myometrium is the middle layer of the

uterine The uterus (from Latin ''uterus'', plural ''uteri'') 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 ...

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 contraction Uterine contractions are muscle contractions of the uterine smooth muscle that occur during the menstrual cycle and labour. Uterine contractions occur throughout the menstrual cycle in the non-pregnant state and throughout gestation. Throughout ...

Structure

The myometrium is located between the endometrium (the inner layer of the uterine wall) and the serosa or perimetrium (the outer uterine layer). The inner one-third of the myometrium (termed the ''junctional'' or ''sub-endometrial'' layer) appears to be derived from the

Müllerian duct Paramesonephric ducts (or Müllerian ducts) are paired ducts of the embryo that run down the lateral sides of the genital ridge and terminate at the sinus tubercle in the primitive urogenital sinus. In the female, they will develop to form the fal ...

, while the outer, more predominant layer of the myometrium appears to originate from non-Müllerian tissue and is the major contractile tissue during parturition and abortion. The junctional layer appears to function like a circular muscle layer, capable of peristaltic and anti-peristaltic activity, equivalent to the

muscular layer The muscular layer (muscular coat, muscular fibers, muscularis propria, muscularis externa) is a region of muscle in many organs in the vertebrate body, adjacent to the submucosa. It is responsible for gut movement such as peristalsis. The Latin, ...

of the intestines.

Muscular structure

The molecular structure of the smooth muscle of myometrium is very similar to that of smooth muscle in other sites of the body, with myosin and

actin Actin is a family of globular multi-functional proteins that form microfilaments in the cytoskeleton, and the thin filaments in muscle fibrils. It is found in essentially all eukaryotic cells, where it may be present at a concentration of ov ...

being the predominant proteins. In uterine smooth muscle, there is approximately 6-fold more actin than myosin. A shift in the myosin expression of the uterine smooth muscle may be responsible for changes in the directions of uterine contractions during the menstrual cycle.

Function

Contraction

The myometrium stretches (the smooth muscle cells expand in both size and numberSteven's and Lowe Histology p352) during

pregnancy Pregnancy is the time during which one or more offspring develops (gestation, gestates) inside a woman, woman's uterus (womb). A multiple birth, multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occur ...

to allow for the uterus to become several times its non-gravid size, and contracts in a coordinated fashion, via a positive feedback effect on the "

Ferguson reflex The Ferguson reflex (also called the fetal ejection 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 wall ...

", during the process of

labor Labour or labor may refer to: * Childbirth, the delivery of a baby * Labour (human activity), or work ** Manual labour, physical work ** Wage labour, a socioeconomic relationship between a worker and an employer ** Organized labour and the la ...

. After delivery, the myometrium contracts to expel the

placenta The placenta is a temporary embryonic and later fetal organ that begins developing from the blastocyst shortly after implantation. It plays critical roles in facilitating nutrient, gas and waste exchange between the physically separate mate ...

, and crisscrossing fibres of middle layer compress the blood vessels to minimize blood loss. A positive benefit to early breastfeeding is a stimulation of this reflex to reduce further blood loss and facilitate a swift return to prepregnancy uterine and abdominal muscle tone. Uterine smooth muscle has a phasic pattern, shifting between a contractile pattern and maintenance of a resting tone with discrete, intermittent contractions of varying frequency, amplitude and duration. As noted for the macrostructure of uterine smooth muscle, the junctional layer appears to be capable of both peristaltic and anti-peristaltic activity.

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 ...

(V_rest) of uterine smooth muscle has been recorded to be between -35 and -80 millivolt, mV. As with the resting membrane potential of other cell types, it is maintained by a Na+/K+-ATPase, Na⁺/K⁺ pump that causes a higher concentration of sodium, Na⁺ ions in the extracellular space than in the intracellular space, and a higher concentration of potassium, K⁺ ions in the intracellular space than in the extracellular space. Subsequently, having potassium channel, K⁺ channels open to a higher degree than sodium channel, Na⁺ channels results in an overall efflux (microbiology), 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 calcium, Ca²⁺, Na⁺, K⁺ and chloride, 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

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

Restoration to resting state

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.

References

{{Authority control Pelvis Mammal female reproductive system