Role of muscular sytem of uterus in the genesis if labor forces

During the contraction phase each of the fundamental contracting units that are parts of the functional contracting module generates the thrust force directed along the cell’s axis and consequently the modele’s axis. This thrust force evenly affects the functional fixation points thus attempting to shift them towards the imagined centre of module. The shifting becomes possible when the thrust force is larger than the elastic resistance of the structures forming a fixation point. It is well-known today that NO shifting of fixation points of the functional contracting modules into a systole of a uterine contraction does take place. The contraction happens in an isometric mode without the fundamental contracting units being shortened. The rasom for this is the thrust force is not enough for overcoming the elastic resistence on the part of both anatomic and functional points of fixation. So, the “rectraction” phenomenon introduced by Bumm at the beginning of the 20^th century as well as “pacemaker” and “reciprocity” has turned out to be a myth.

At the state of optimal stretching the three-dimensional grid structure of myometrium through a synchronized contraction of the included modules forms a system of line tension crossing in the three planes, that result in the fact that the sametangential tension is generated at every moment of time in each of the points of the muscular shell of uterus. But we have to clarify here. The muscular shell of the uterus body is separated from the muscular shell of the lower segment by the contraction ring. The ring is a morphological lump cutting off a straight mechanic connection between the different parts of one and the same contracting module if it belongs both to the body and lower segment of uterus. This does not allow the line tension appearing in one part of module transmit it to the other, as they are divided by a mechanically functional point of fixation located in the contraction ring, despite all the three parts of module located in the body, contraction ring and the lower sgment are parts of one (in anatomical sense) lump. Firstly, under the law on compliance of structure and function, it allows to divide the muscular shell of uterus during labor into three functional sections – the body of uterus, contraction ring and the lower segment in which the total line tensions result in different biomechanical effects. Secondly, it should be phenomenologically apparent in the analysis of “biomechanic behavior” of each of the functional sections of the muscular shell of uterus.

Isometric contraction of the external muscular layer of the uterus body that consist of tightly packed contracting modules and is tightly glued to the pelvic bones, forms the same tough “shirt” of the uetus body in a contraction systole, in which all (or almost all) contracting energy in converted into tension directed at the imagined sphere centre. From the other side due to the mechanism of the contracting feedback, the “shirt” of the uterus body the contracting elements from stretching and the uterus body- from cranial shift during the whole period of systole of uterine contraction. The “shirt” covers a huge vascular lump – dividing venous sinus, that allows for accommodating up to
250 ml of blood, and the vascular internal muscular layer pierced by a large number of lacunary transformed venous vessels. Consequently, the part of myometrium armouring the zone of the dividing venous sinus and being the basis for the internal muscular layer is an intrauteral structure towards the external muscular layer of uterus body.this strcture has the same hydraulic characteristics as intervillous space and the amniotic cavity. Moreover, the three-dimentional grid structure of the inner musclualr body of uterus by sinchronic contraction together with the musculature of external muscular layer totally converts the energy of tension into intramiometric and intrauteral tension. Herewith, the internal structure of internal sections of myometrium from the shell of uterus prospective creates a special “hydraulic environment” thus influencing the intrauteral volume by defining transmural tension in the venous depots of dividing venous sinus, in lacunas of the internal muscular layer, and tension variations in the intevillous space.

Thus, the main biomechanic task of myometrium of uterus body contracting into a uterine contraction is to convert the energy of isometriccontraction of functional modules into intrametric and intraamniotic tensions.

Not only does the contraction ring cut off the straight mechanic connection between the functional contracting modules of the body and lower segment of uterus but also divides the cavity of body and lower segment into two reatively isolated cavities. The difference of labor from pregnancy is in the appearance of the contaction ring as a functional lump. The beginning of its functioning is linked to increasing basal tonus of activated myometrium. The “hydraulic isolation” of the cavities of body and lower segment of uterus during labor is relative. Parts of the fetus are located in both cavities, but circular tension generated in half-loop parts of functional modules clearly divide the amniotic cavity into two parts. Today it is unquestionable. In physiological and pathological labor the contraction ring ensures hydraulic isolation of the cavities of body and lower segment of uterus during labor during the whole period of labor action – inbetween contracions and at the phases of systole and diastole of a contraction. This circumstance gives rise to some biomechanic differences in functioning of the muscles of body and lower segment of uterus which do not relate to the presence of genotypic or phenotypic differences in myocites of these parts of uterus. Though these differences do exist and are quite demonstrative. If the amniotic tension is a function of myometrium tension in a definite section of uterus, so its dynamics can be helpful in defining the differences in the functioning of different sections of muscle. It has been clearly stated that in the pause between uterine contractions the tension in the uterine cavity is higher than the one in the segment of the lower cavity. At the beginning of a contraction the tension in both cavities starts to rise, but its speed in the lower segment turns out to be higher that that in the cavity of the body of uterus.

The gradient of tension for the benefit of the cavity of the body of uterus disappears when tension in both cavities reaches 27-30 mm Hg. Afterwards, the gradient of tensions inverts for the benefit of the lower segment cavity and remains till the peak of uterus contraction. When uterus relaxes a reverse inversion of gradients takes place untill the tension in the body cavity of uterus becomes 27-30 mm Hg., afterwards the tension in the uterus cavity exceedes the tension in the lower segment cavity during the second phase of diastole, the pause between contractions and the first phase of systole. Though, we have to specify the notion of “cavity of the lower segment”. From the formal morphology perspective this cavity is the space containing part of the fetoamniotic complex. By the beginning of labor this cavity is a spherical layer, and to the end of the dilatation period – cylinder. From the “hydraulic” point of view cavity of the lower segment is a part of its cavity filled with amniotic fluid. It is restricted by the contraction ring on top, and by a zone of а dense connection of tissue of its shell at the place of its maximal deformation from bottom (zone of “destruction”, zone of stretching, etc). This zone ususally corresponds to a minimal diameter of the presenting part of fetus. By the beginning of physiological labor when cervix is totally mature this hydraulic cavity of the lower segment is clearly identified by ultrasonic examination. This cavity is preserved during the whole period of dilatation independent on presence or absence of forewaters.

It is necessary to make one more basically important specification. Unlike the musculature of the uterus body which is evenly stretched on the fetoamniotic complex, the level of initial stretching of the musculature of the lower segment is unequal. These differences in the level of stretching of terminal parts of the functional modules are not that important while defining differences in isometric stretching peak as by defining the peak of elastic resistence of stretching.

It is necessary to remind that the distal one-third of the lower segment has a different morphology in comparison to the two proximal one-third. The two latter’s muscular tissue is totally identical to the structure of the internal muscular layer and formally can be treated as its “continuation”, while the distal one-third is a vascular connective-tissue and muscular lump. Strong development of venous lacunary transformed vessels is preserved in this part of the lower segment, while the volume of muscular tissue which is presented by fixation points of functional modules of different “height” of positioning is dramatically cut down. That is why the flow point of the lower segment tissue during labor is located very close to the point of the peak resistance to deformation (coefficient of elasticity). It dramatically lowers the range of immunity to extradeformation in radical stretching of the tissue of this section of the lower segment during labor. This is the lower third of the lower segment of uterus that is «locus minoris resistencio» which the process of overdistension of its tissue during pathologic labor starts from.

What is actually the role of the musculature of the lower segment of uterus at the first stage of labor?

Unquestionably, the musculature of the lower segment does not “hand over” any efforts from the body of uterus to cervix, as it is impossible because of the absence of direct mechanic connection between these muscular systems.

There is no doubt that during latent and some active phase of dilatation a process of adaptive deformation takes place in the distal sections of the lower segment. The process is a transformation from a spherical layer into a cylinder, which is linked to the consequent stretching of its tissue in a radial direction. This process is conditional on the external influence on the tissue of distal sections of the lower segment and is not related to active relaxaion of its musculature. As it has been proved, that the process of adaptive deformation of distal sections of the lower segment is related to the increase in volume of hydraulic cavity there is every reason to state that the forces generated in the upper segment of the delivering uterus make the basis for deformation of distal sections of the lower segment.

The role of the muscular shell of the lower segment of uterus is brought down to:

А. Modulation of the actions of these external forces to their own tissue;

B. Maintenance of biomechanic effect through generation of its own line tensions in functional modules. But (which is very important!) unlike the musculature of the upper segment of uterus, the musculature of of the lower segment has a direct connection with cervix, thus directly influencing its tissue. It was the first point.

Secondly,isometric peaks of contrctions in parts of functional modules located in the body of uterus and its lower segment are equal (around 9,8 9,8 Н/mm²). Moreover, at the peak of isometric tension of the uterus musculature into the systole of a contraction line tensions are generated in the lower segment during physiological contraction. Their intensity is much higher than that generated in the muscular of the uterus body. All this contributes to the fact that the role of the lower segment of uterus is still not quite clear and has not been studied properly.

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