I. Fundamental mechanical properties of myometrium in the first stage of labor

The obstetricians are united in the opinion that contractions of myometrium during uterine contraction carry out a job that is actualizes in cervical dilatation at the first stage of labor and delivery of the presenting part of the fetus at the active phase of this stage. Basically, labor as a natural phisiological process cannot take place without uterine contractions. Obstetricians do not have an understanding though, how a uterine contraction turns the energy of tension of myometrium into effective external capacity. This understanding is quite contradictory. There is still no unity in understanding of what mode allows myometrium contraction to contract into the systole of a uterine contraction. Even the main ideas concerning these issues are presented by very simple phisiological patterns. Though it is important for us that those obstetricians tha are directly involved with the development of the issues of theory of labor process understand that a uterine contraction is, first of all, a biomechanical force macroprocess. It producesthe so-called labor forces that at the first stage of labor transform uterus from a bearing organ into an expulsing one. All the concepts describing biomechanics of this process such as “The Triple Descending Gradient”, “Contraction-Retraction-Destraction”, and “Reciprocal Relationsof Various Sections and Layers of Myometrium” actually describe the biomechanics of uterus dilatation process. We have been doing the same for the three decades. That’s why the main goal of this section is the necessity to discuss the question of myometrium activity of uterus as a phisical system during the first stage of labor. That is why the question of fundamnetal mechanic characteristics of myometrium it has in the first stage of labor are essentially important for understanding “its participation” in the genesis of labor forces.

· All the contracting elements of myometrium are found in a stretched state starting from the period of 8 weeks of pregnancy till the beginning of labor. The degree of stretching of the uterus shell and its tempo are defined by an external factor – volume dynamics of fetoamnionic complex. When it is expulted, all the contracting modules of myometrium become two times shorter. The size of stretch of each of the functional contracting modules at this moment of dilatation is definedby the size of fetoamnionic complex. If for active (contracting) or passive reasons a reduction in its intrauterine size takes place, then each of the stretched functional contracting modules of myometrium instantly and finally fixes its new shorter length. It cannot become longer again independantly on the fact in what phase state it had its smooth muscle elements at the moment of the fixation of length changing. In order to make longer an optimally stretched module that has just finished the cycle of shortening as a result of a uterine contraction, it is necessary to use the force that is times stronger than the spontaneously contracting module at the peak of its isometric upper limit. Consequently, neither any area nor layer of myometrium consisiting of the same functional contracting modules and being in the same morphofunctional state and level of stretching are able to stretch another area or layer of myometrium at the moment of contracting. Not only the muscled tissue of myometrium but also its connective tissue matrix is counteracting their stretching.

· Myometrium activated during labor possesses mechano-receptoric properties – which means that stretched myometrium instantly and appropriately responds to every impulse of extrastretching by increasing the strain resistance. Each of the smooth muscle cell of myometrium possesses the properties of stretch receptor which is to instantly respond to every impulse of extrastretching by using three variants of response that are defined by peculiarities of the phase state of cell at the moment of being affected by an impulse of extrastretching. If the latter starts affecting the activated smooth muscle cell that is found between the contractions, mechanoreceptoric stretching feedback mechanism will lead to either a rise in a basal tone of the cell or shortening of pause and activation of the contracting phase (flutter).

If this impulse starts affecting the cell in the contracting phase it will respond by an abrupt rise in the tension speed.

If the extrastretching impulse finds the cell in the relaxation phase it can provoke an out-of-turn cell contarcting (extrasystole).

The presented, unique in many aspects experimental research allows us to state that it is the mechanoreceptoric stretching feedback mechanism that allows for synchronizing the phase contracting activity of each of the functional contracting modules of myometrium. Besides, all the cells that have the same morphofunctional state also have the same level of stretching and a direct mechanic relation between each other. This very mechanism functioning on the level of the functional contracting module of myometrium, syncronizes their phasal contracting activity with the help of extrastretching impulse which affects them simultaneously, equally and evenly. Consequently, this synchronizing impulse is generated outside the tissue of the functional contracting module.

· Direct mechanic relation means that all the smooth –muscled elements of functional contracting module are mechanically related between each other by connective-tissue structures that allow not only to support the same tension level of each of the contracting elements of the system but also freely broadcast any changes in tension of each of the contracting elements through the force axis of the system. The mechanic relation is not identical to the anatomic and does not presuppose the presence of a direct intimate contact between the modules. Moreover, the presence of anatomic integrity does not necessarily guarantee the presence of a mechanic relation- its damage takes place in case of fibers twisting (for instance, in the area of the contraction ring).

· The same stretching level of each of the smooth muscle elements of any functional module of myometrium during labor at any level of activation of myocites is supported by firm counteraction on the part of the modules fixation points. All anatomic modules fixation points located in the ligamentous apparatus of uterus are tightly fixed and almost cannot be moved. Firstly, at least it means that all the functional contracting modules of the external muscular layer with their fixation points being located only in the ligamentous apparatus of uterus can only contract in an isometric mode (without shortening in smooth-muscular cells). Secondly, the possibility for a freely broadcasting tension force of myocites through the force axis modules can only be carried out when the counteraction this broadcasting in every part of the module is the same. Peculiarities of modules mioarchitectonics indicate that curvature of the force axis in some parts of the module course is so considerable that broadcasting of tension may be terminated. Thus, a functional point of fixation appears on a separate part of the contracting module that divides it into separate consistent contracting units. Functional points of fixation of this kind can be found in a laboring uterus in the areas of the transition of module from ligaments to uterus body, from uterus body to the ligament or the contraction ring or exiting it into the lower segment tissue. Our experiment has shown that the functional point of fixation of the modules part is always created by reducing the angle of modules entry less than 90⁰. Consequently, there is every reason to state that definite parts of a single (in the anatomic sense) functional contracting module of myometrium as independent contracting units can form the contracting system of uterus, contraction ring and lower segment.

· Every myocite possesses spontaneous phase activity with its force and time characteristics being fixed in the cells genome. The highest tension (isometric upper limit) is achieved through the stretching level of myometrium at 2l₀. By exceeding this stretching level the value of isometric maximum lowers progressively. The activation of the pacemaker mechanism in every myocite is carried out by extraorgan factors (humoral control) and extrastretching impulse.

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Дата добавления: 2015-10-30