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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 ...
s.


Structure

The myometrium is located between the
endometrium 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 ...
(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 Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...
and anti-peristaltic activity, equivalent to the muscular layer of the intestines.


Muscular structure

The molecular structure of the
smooth muscle Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
of myometrium is very similar to that of smooth muscle in other sites of the body, with
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 ...
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 ( gestates) inside a woman's uterus (womb). A multiple pregnancy involves more than one offspring, such as with twins. Pregnancy usually occurs by sexual intercourse, but ...
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", 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 Peristalsis ( , ) is a radially symmetrical contraction and relaxation of muscles that propagate in a wave down a tube, in an anterograde direction. Peristalsis is progression of coordinated contraction of involuntary circular muscles, whic ...
and anti-peristaltic activity.


Resting state

The resting membrane potential (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, 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 Flux describes any effect that appears to pass or travel (whether it actually moves or not) through a surface or substance. Flux is a concept in applied mathematics and vector calculus which has many applications to physics. For transport ph ...
, reflecting changes in various K+ channels.


Excitation-contraction

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 Smooth muscle is an involuntary non- striated muscle, so-called because it has no sarcomeres and therefore no striations (''bands'' or ''stripes''). It is divided into two subgroups, single-unit and multiunit smooth muscle. Within single-unit ...
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 ...
(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 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.


References

{{Authority control Pelvis Mammal female reproductive system