Myometrium

The uterus, often known as the womb, has the shape of an inverted pear. It is a hollow, muscular organ with thick walls and it is lined with a glandular lining known as the endometrium, which helps to regulate the flow of blood. In an adult, the uterus measures 7.5 cm (3 inches) in length, 5 cm (2 inches) in width, and 2.5 cm (1 inch) in thickness; however, during pregnancy, the uterus expands to four to five times its normal size. The cervix is the narrower, lower end of the uterus, which extends into the vaginal canal. Because it is composed of fibrous connective tissue, the cervix has a stiffer consistency than the rest of the uterus’s body. The two fallopian tubes enter the uterus from opposing sides, towards the top of the uterus’s cavity. The fundus is the portion of the uterus that is located above the entrances of the tubes; the body is the portion of the uterus that is located below the tubes. The body narrows as it approaches the cervix, and a tiny outward constriction marks the point where the body and the cervix meet in the middle.

Structure

The myometrium is a layer of tissue that lies between the endometrium (the inner layer of the uterine wall) and the serosa or perimetrium (the outer layer of the uterine wall) (the outer uterine layer).

Myometrium appears to be derived from the Müllerian duct in its inner third (known as the junctional or sub-endometrial layer), while the outer and more predominant layer of myometrium appears to be derived from non-Müllerian tissue and serves as the primary contractile tissues during parturition and abortion.

 The junctional layer appears to function similarly to a circular muscle layer, capable of both peristaltic and anti-peristaltic activity, and is analogous to the muscular layer of the intestines in structure and function. 

The musculoskeletal system

The smooth muscle of the myometrium has a molecular structure that is remarkably similar to that of smooth muscle found in other parts of the body, with myosin and actin being the most prominent proteins in the muscle.

 Actin predominates over myosin in uterine smooth muscle by a factor of around 6-to-1. During the menstrual cycle, it is possible that a shift in the expression of the myosin protein in the uterine smooth muscle is responsible for variations in the direction of uterine contraction. 

Function 

Contraction

Uterine contraction is the focus of this essay.

A positive feedback effect on the “Ferguson reflex” causes the myometrium to stretch (the smooth muscle cells increase in size and number during pregnancy, allowing the uterus to expand to several times its non-gravid size. The myometrium then contracts in a coordinated fashion during labour, via a positive feedback effect on the “Ferguson reflex.” In the hours following delivery, the myometrium contracts in order to remove the placenta, and the blood vessels are compressed by the crisscrossing fibres of the middle layer in order to reduce blood loss. This reaction is stimulated by early breastfeeding, which helps to decrease future blood loss while also allowing the recovery of uterine and abdominal muscle tone to pre-pregnancy levels more quickly and efficiently.

Uterine smooth muscle has a phasic pattern, alternating between a contractile pattern and the maintenance of a resting tone with discrete, intermittent contractions of varied frequency, amplitude, and duration. Uterine smooth muscle is composed of two types of fibres: smooth muscle and adipose tissue.

Although the junctional layer appears to have both peristaltic and anti-peristaltic action, this has not been demonstrated for the macrostructure of uterine smooth muscle.

In a state of repose

Uterine smooth muscle has been found to have a resting membrane potential (Vrest) ranging between -35 and -80 mV, depending on the study.

In the same way that the resting membrane potential of other cell types is maintained by a Na+/K+ pump, the intracellular membrane potential is maintained by a higher concentration of Na+ ions in the extracellular space than in the intracellular space, and the extracellular membrane potential is maintained by a higher concentration of K+ ions in the intracellular space than in the extracellular space. Due to the fact that K+ channels are more readily available for opening than Na+ channels, there is more outflow of positive ions from the cell, resulting in a net negative potential.

Rhythmic oscillations in this resting potential, which have been dubbed “slow waves,” are observed and are thought to reflect the intrinsic activity of slow wave potentials in the brain.

 There are changes in the distribution of Ca2+, Na+, potassium, and chlorine ions between the intracellular and extracellular regions that cause these slow waves to occur. The permeability of the plasma membrane to each of these ions is reflected in the frequency of these slow waves to appear.  As a result of alterations in various K+ channels, K+ is the most important ion responsible for changes in ion flux. 

Excitation-contraction

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

Relaxation

Smooth muscle relaxation occurs as a result of Ca2+ removal following contraction, which also helps to rebuild the molecular structure of the sarcoplasmic reticulum in preparation for the next contractile stimulus.

Conclusion

The uterus is made up of three layers of connective tissue. On the outside, there is a serous coat of peritoneum (a membrane that exudes a fluid similar to blood but without the cells and the clotting ingredient fibrinogen), which partially covers the organ and serves as a protective layer. In the front, it merely covers the body of the cervix; in the rear, it covers the body of the cervix as well as the part of the cervix that is above the vagina and is continued over the posterior vaginal wall, after which it is folded back to the rectum. Towards the side, the peritoneal layers extend from the border of the uterus to the side walls of each pelvis, resulting in the formation of the uterus’s two broad ligaments.

The muscular middle layer of tissue (myometrium) makes up the majority of the organ’s bulk and is responsible for the majority of its function. There are densely packed, unstriped smooth muscle fibres in this tissue, which gives it its firmness. There are also blood vessels, lymph vessels, and nerves to be found. More or less, the muscle is organised into three layers of fibres that run in opposite directions to one another. A longitudinal arrangement is used for the outermost fibres. Because they are distributed throughout the intermediate layer, they do not form an organised pattern; this layer is the thickest of all. The arrangement of the innermost fibres is both longitudinal and circular in nature.