sign in sign up
<<
Home , Birth, Cephalic presentation, Cervical canal, Pregnancy (mammals), Uterine contraction, Uterus

The Physiology of the in Labor

D. N. DANFORTH, M.D., R. J. GRAHAM, B.M. and A. C. IVY, M.D.l

HE purpose of this article is to (8). This ring becomes T synthesize into a rather complete fairly evident during labor at the junc­ picture the experimental observations tion of the upper and lower segments. that have been made during the past It is a more or less definite, tapering ten years in our laboratory regarding ledge, the formation of which is due the processes concerned in the evacu­ to the greater thickness or "retraction" ation of the uterus. of the muscle fibers of the upper than the lower segment. (This r ing is the The Anatomic and Physiologic " fundal or cornal sphincter" in the Divisions of the Uterus .) In obstructed labors it becomes It is important first to obtain a clear a very pronounced ring or band and concept (a) of the anatomical divisions is then called Bandl's ring. The ap­ of the uterus, which may be pearance of a Bandl's ring means --­ traced directly to those in lower forms, threatened rupture of the uterus in the and (b) of the obstetrical or physio­ lower segment ; it is a pathological re­ logical divisions. See figure 1. traction ring. This is a subject that has been greatly III. The lower uterine segment is an­ confused because our knowledge of the alogous to the isthmus uteri oj Asch­ gross and microscopic of the off in the non-pregnant uterus. In human uterus, and of the , its upper level is generally comparative anatomy, and physiology marked by the reflection of the per­ of the uterus has developed in ~ hap­ itoneum. The musculature of the isth­ hazard manner; and the facts have not mus in , like the upper seg­ been correlated until recently. A .corre­ ment or corpus, undergoes definite lation of the known facts practically hypertrophy, and a lower segment is establishes the of Aschoff's grossly and definitely evident at 3 divisions of the human uteru~ with the months (6). The normally obstetrical or physiological divisions. is located above the upper limit of The Divisions: the isthmus, i.e., in the corpus or cornua, or above the physiological re­ 1. The upper uterine segment. The traction ring. " corpus uteri" are the uterine horns in the dog, , and cow. The " The isthmic musculature shows is normally implanted in this segment. definite hypertrophy, but to a lesser II. The physiologic retraction ring. The extent than that of the corpus. In the function of this ring, or Aschoff's ana­ first months of pregnancy only the tomical internal os (A, fig. 1), on the corpus cavity forms the chamber for basis of comparative appears the . Thereafter the isthmus be­ to be to restrict implantation to the comes taken up more and more , un­ upper segment or corpus uteri or the til at the end of pregnancy there exists an egg chamber of two divi­ l From the Department of Physiology and Pharo sions: an upper, the cavum corporis, macology, Northwester n University Medical School. Received for pUblication, July 23, 1940. which has the function of contraction · This research was rendered possible by a gift in honor of Dr. I. N . Danforth. and discharge of the egg during de- 21 22 QUARTERLY BULLETIN, N. U. M. S.

NON-PRmNAN! PREGNANT AT TEI!M ~...... -... -.:..------WNOOS

GROSS UPPER ANAT. CORPUS SIi'GMEIIT CORPUS UTERI UTERI

GROSS ANAT. __ u:n:il..______CERVIX _ A.-Anatomic internal OB, physiological retraction ring. B.-Ob~tflt.ric internal os and location of cervical sphincter. Co-External os. A.-C. An ..t. . B.-C. Obstbt. cervix. Figure 1.

Fig. 1. The divisions Of the human uterus. Grossly the non-pregnant uterus appears to be divided only into two parts, the corpus and the cervix. Histo­ logically, and from the viewpoint of comparative embryology and anatomy, the uterus pregnant and nonpregnant is divided into three parts, the corpus uteri or upper segment, the isthmus uteri or lower segment, and the cervix. "A" is the junction of the upper and lower segments and "B" the junction of the lower segment with the cervix, at which site is found the cervical sphincter in the dog and monkey.

livery, and the lower already dilated tance to the exit of the . part, corresponding to the isthmus (Stieve). See figure l. (or LUS), which during delivery ex­ The early in periences some further dilation. Un­ pregnancy occupy the upper segment der normal circumstances there is of the uterus. no doubt that the entire isthmus As the products increase and the down to the limit of typical cervical uterus grows, the isthmus uteri en­ mucosa is taken up by the egg cham­ larges, the anatomic internal os disap­ ber." (1) pears, a definite lower segment is IV. The cervix, or the obstetrical or formed, and the products of conception physiological cervix uteri. also occupy the lower segment. This A. The cervical sphincter is located is grossly evident at about 3 months. at the level of the fornix or at the The obstetric and functional division junction of the obstetrical cervix with of the uterus into an upper and lower the lower segment, or at the upper segment and cervix is demonstrated by end of the histologic , a study of the comparative anatomy or at the obstetrical os internum. and physiology of the uterus and is This sphincter definitely exists in dog generally recognized by obstetricians. and monkey (3, 4) , and some ob­ The upper segment is relatively stetricians report its presence in more active and "stronger" than the woman. Its presence in woman is lower segment. Such a statement does not definitely established, nor uni­ not imply that the musculature of the versally recognized. lower segment is inactive, because it B. The external os, which becomes manifests resistance to stretch (tone), quite soft and patulous in pregnancy, and it contracts very actively under could not normally offer much resis- certain conditions. When labor is ob- DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 23 structed as in the case of a small or the length of the muscle fiber is rela­ deformed , or malposition of the tively fixed, and at which length fetus, the lower segment, if not relieved it contracts and relaxes. by the obstetrician, ruptures. In the The stomach, when one eats, in­ various stages of labor the lower seg­ creases the length of its muscle fib­ ment is only relatively passive when ers without a change in intra gastric compared to the upper segment. That tension. As the stomach empties, the is, most of the "expulsive work" of the length of the muscle fibers decreases uterus is performed by the upper seg­ without a change in intra gastric ten­ ment. sion. Intragastric tension, of course, increases and decreases during con­ The Four Major Properties of the traction and relaxation. Uterine Musculature When the muscle fibers increase or The 4 major properties of the uter­ decrease in length and at the in­ ine musculature are the same as those creased or decreased length mani­ possessed by smooth muscles in gen­ fest the same tension as before, a eral: (1) contraction, (2) relaxation, metro static adjustment has occurred. (3) adjustments in length without To denote the directional change in change in intra-uterine tension, (4) co­ length, we use the e x pre s s ion ordination. "mecystatic adjustment" to indi­ I. Contraction. When the uterus con­ cate that the muscle has relaxed or tracts, the intra-uterine pressure rises become elongated and has become equally throughout, but because of the relatively fixed at an tncreased form of the uterus, pressure is exert­ len g t h; we use the expression ed on the breech of the fetus and the "brachystatic adjustment" to in­ presenting part tends to descend or dicate that the muscle has shortened, to serve as a wedge about which the or after contraction it does not re­ cervix becomes effaced and dilated. turn to its original length. (This is Actual descent does not occur until not to be confused with "tone," be­ the cervix is considerably dilated. cause a change in tone means a Actual descent is the result of efface­ change in tension, which mayor may ment and dilation of the cervix. The not be accompanied by a change in cervix is pulled up and about the pre­ length.) senting part. B. Mecystasis is a state manifested Contraction costs food energy. Un­ by a muscle fiber after it has as­ less food and adequate are sumed a relatively fixed increase in available, the uterus will become ex­ length, at which length it resists hausted. If properly nourished, the stretch, contracts and relaxes, and uterine musculature should be diffi­ manifests the same tension as before cult to fatigue; for example, the stom­ elongating. See figure 2. ach and intestine in many animals 1. If mecystasis of the uterine fibers work day and night. did not occur in the latter stage of II. Relaxation. The pressure in the pregnancy, the fibers would either uterus returns to the previous level. have to grow in length, or a grad­ During the period of relaxation and ually increasing intra-uterine ten­ rest that follows a contraction, the sion would result. This latter phe­ properly nourished muscle returns to nomenon does not normally occur, its original chemical state. and the muscle fibers cease growing III. Adjustments in length without at about midterm. changes in intra-uterine tension (4). 2. If mecystasis of the cervical A. Metrostasis is a state in which sphincter did not occur, then after 24 QUARTERLY BULLETIN, N. U. M. s.

o M!VW!WWV\

COIiTlllC"rIONS ur • SlIICLJ: 11115CU: FIIIU

lIUCHYSTASIS JIIl:YSTASlS

Fig. 2. GraphicaL representation of metro static adjust­ ments. Brachystasis is iHustrated by the reLative shortening of a singLe muscle fiber during successive contractions with­ out a change in the average tension exerted by the fiber. Mecystatsis is iLLustrated by the reLative Len gthening of a singLe muscle fiber during successive contractions without a change in the average tension exerted by the fiber.

each ute r i n e contraction the 2. If brachystasis did not occur after sphincter would contract to its for­ descent of the head was caused by mer state. Instead, the cervix an -active contraction, the advan­ shows mecystasis normally after tage gained by the contraction each vigorous contraction of the would be lost during relaxation and uterus. If mecystasis did not occur, no progress would be attained. A a continuous continuous active contraction of the with continuous intra-uterine ten­ uterine fibers could compensate for sion would be required to hold the the lack of brachystasis, but would cervix open. If mecystasis of the be an expensive and exhausting cervix occurred too rapidly, a pre­ mechanism. cipitate labor would occur. 3. If brachystasis did not occur after C. Brachystasis is a state manifested separation of the placenta, bleed­ by a muscle fiber after it has as­ ing from the placental site would sumed a relatively fixed decrease in be excessive and probably fatal. length, at which length it resists stretch, contracts and relaxes, and Metrostatic Adjustments in the manifests the same tension as be­ Uterus Applied to the Processes of Pregnancy and Labor fore shortening. See figure 2. 1. If brachystasis of either the upper 1. During pregnancy when the or lower segment, or of both did growth of the uterine fibers has not occur, the cervix could not be ceased and the fetus continues to dilated because as the cervix di­ grow, the fibers relax or elongate lates the slack created in the uter­ to accommodate the fetus without ine fibers must be taken up. Slack a change in intra-uterine tension, could be taken up by sustained ac­ like the stomach or urinary blad­ tive contraction, but this would be der that is being filled. This is a uneconomical, provide no rest peri­ "mecystatic adjustment." od, and lead to early exhaustion of The Braxton-Hicks' contractions the uterus. of the uterus that appear early in DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 25

At start of 1st stage At end of 1st stage At end of lot st"8e Brachystasis of L.U.S. only Brachyste,sis of U.U.S. only, e.cc:ording to curr8J\t vleW9.

Brachystasi s of both segment s Uonkey _ as in IV but more dra.wing up of eel""fix

Fi re 3.

Fig. 3. " I " is the condition at the start of the first stage. AB represents the lower uterine segment. " II" represents what would have occurred during dila­ tions of the cervix if only the lower uterine segment retracted or if only its fibers manifested brachystasis. The shortening would involve only A LB'. "III" represents what is said to be the usual type of retraction in the human, i.e. , r etraction or brachystasis occurs only in the upper segment, the lower segment being represented by A2B'. " IV" represents what would happen if retraction or brachystasis occurred in both segments, the lower segment being represented py A 3B '. In the monkey, retraction occurs in both segments and more pulling' up of the cervix occurs than is generally thought to occur in woman. pregnancy, or about the second " pains" are experienced, signifying month, are non-propulsive in type, a "call to labor." like those rhythmic contractions In "false labor," painful contrac­ that occur in the tions occur but are not accompa­ for some time before a "call to nied by brachystasis of the upper urinate" is experienced. When segment and dilation of the cervix. these intermittent B-H contractions " False labor" may occur and the become more forceful, labor contractions disappear, or it may 26 QUARTERLY BULLETIN, N. U . M. S.

F" 4 A and B Showing the uterus at the start and end of the first stage in w?ma~. F" ~~ . S'h . th~ uterus at the end of the first stage in the ~on~ey. P .R .R ., physwlogt- caltg .retractwn . .owmg rmg or C .R ., the contraction ring. O.I. , obstetncal mternal os or C .S., the cervi cal sphincter. O.E., the external os.

be followed by "true labor." It is portion of the second stage of la­ difficult to define exactly the "on­ bor. This gradual dilation and tak­ set of true labor." Arbitrarily, true ing-up thins out the sphincter and labor starts when the uterine mo­ obstetrical cervix until it is com­ tility is of such a character as to pletely dilated and practically ef­ lead to dilation and effacement of faced. the cervix. It should be pointed out here that the two halves of the uterus must 2. During Labor. First Stage. Stage contract together, or the various of Dilation and Effacement of the portions of the uterus must con­ Cervix. The upper segment con­ tract in a coordinated manner, in tracts then the lower, and then order to effect normal effacement the c:rvical sphincter. The polarity and dilation of the cervix (9). If for of the wave of contraction is il­ one reason or other the uterus is lustrated in figure 5. The head (or not in proper alignment with the presenting part) is pushed against bony canal, or the uterine attach­ the cervix and sphincter. The ments are under an uneven tension, sphincter on relaxing along with or the forces exerted by the uterine the remainder of the uterus as­ contractions are unequally applied sumes a more dilated than to the breech, the "axis of descent" before the uterine contraction, or of the fetus will not be in proper a mecystatic adjustment has oc­ relation to the cervix or to the curred. Obviously dilation of the canal, and the cervix will not be cervix tends to create some slack in properly effaced and dilated; and the upper and lower segments. This in the second stage descent will not slack must be taken up in order to occur normally. Thus, the lack of keep the head in contact with the brachystasis, or the lack of coordi­ cervix. The slack is taken up by nation in the first stage results in brachystatic shortening (not by ac­ no dilation or very slow and un­ tive and sustained contraction) of equal dilation. This is why the cer­ the upper and possibly also the low­ vix is sometimes referred to as the er segment. In the monkey (fig. " barometer of the uterus" during 3) longitudinal brachystasis occurs the first stage. in both segments, only chiefly in the upper segment during the latter Second Stage. This stage starts DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 27

when the cervix is completely di­ ing that the sphincter has undergone lated. It is chiefly the stage of de­ during the passage of the head. See scent. The head (or presenting part) figure 3. descends with each uterine con­ During the second stage "bearing traction. During uterine relaxation down" or a "defecatory type" of move­ the head rises a little, but does not ment of the occurs. This is return normally to its original po­ both a voluntary and an involuntary sition or level because brachystatic reflex act. It occurs in women or ani­ shortening of the upper and lower mals with a high or a thoracic transec­ segment, chiefly the upper, accord­ tion of the . ing to current views, occurs. (In Third Stage. After the birth of the the monkey the cervix retracts infant, the upper segment contracts over the head to the level of the and manifests much brachystatic short­ of the fetus and the brim of ening. The lower segment is flaccid. the pubis of the mother soon after Several contractions and much brachy­ the cervix has been completely di­ stasis, or a "tetanic contraction" may lated and before the head has de­ be required to separate the placenta scended more than one or two cen­ and to close the maternal blood sin­ timeters.) In woman, the station uses. That the latter result is not ef­ or level of the vertex when the fected rapidly is shown by the occur­ cervix surrounds the neck is not rence of retroplacental hematoma. Ob­ known, but should be answered by viously the placenta separates by con­ work which Drs. Caldwell and Still­ traction of the uterine fibers, and the of Sloane Hospital, New York, hematoma forms in the cavity created have in progress (5) . See figure 4.* due to incomplete closure of the ma­ After the birth of the. head a tempo­ ternal sinuses. A hematoma cannot rary arrest in descent is frequently ob­ form until some placental separation served. This means that several uterine occurs. The hematoma is the result contractions must occur, or some time and not the cause of placental separa­ must elapse to permit sufficient tion. During this period the lower seg­ brachystatic shortening to occur so as ment remains flaccid. After a varying to bring the uterine musculature in period the placenta is expelled, the firm contact with the hind parts, and uterus as a whole contracts firmly, be­ to again pull taut the uterovaginal at­ coming almost completely a pelvic or­ tachments. The extent to which the gan. cervix retracts cephalward will depend Then after a period the uterus re­ upon the extent of fixation of the vagi­ laxes, becoming an abdominal , nal wall and cervix to adjacent soft but manifests much brachystatic short­ and hard parts of the pelvis. ening relative to the ante partum state, The cervical sphincter may contract in which condition it manifests rhyth­ to some extent, about the neck, and mic postpartum contractions for sev­ dilate again about the shoulders as the eral days. evident contractions start. On the oth­ In all of our frozen sections in the er , this may not occur because of monkey, the lower segment retracts the marked thinning and stretch- considerably, or manifests much longi­ 'By placing metal clips on the posterior and tudinal brachystasis, when the fetus is anterior cervical and then making X·ray passing through the vaginal canal. plates at different stages. Drs. Caldwell and Still· man have found that, at least in somElll women, (figs. 6, 7, and 8). But, direct observa­ the cervical lips may retract almost as high as in the monkey. The extent to which the cervix is tions on the uterus of the monkey in retracted cephalward will vary with the degree of fixation of the vaginal walls and cervix. labor show that as soon as the fetus is 28 QUARTERLY BULLETIN, N . U. M. S.

• T.

______U.U.5.

JUNCTIOll ______OJ' U.U.5. Ii L.U.S. .,; ______L. U. S.

------O.E.

Fig. 5. Showi ng the wave of contraction in the pregnant uterus of the monkey in labor. P .C., pace maker. 1. The concentric rings passing from P .C . about the insertion of the tubes ( F .T .) toward the mid-line. 2. The longitudinal shortening. 3. The circular waves of contraction. See text.

born, the lower segment relaxes longi­ ply to the fetus. (The placental site tudinally. and remains dilated circular­ is relatively inert or passive, how­ ly and is relatively flaccid, compared ever, during contractions.) to the upper segment. This condition 3. Metrostasis permits the mainten­ of the lower segment persists until aft­ ance of volumetric and postural ad­ er the delivery of the placenta, after justments at a very low energy cost which it contracts and manifests longi­ and without the continuous mainten­ tudinal and circular brachystasis, ance of an intra-uterine hyper­ which markedly reduces the of tension. the lower segment. After delivery of IV. Coordination (3, 4, 9) . Anyone who the placenta, the non-pregnant form of knows physiology and has studied the the uterus returns sooner in the mon­ mechanism of evacuation of the uter­ key than in woman. us in the , dog, and monkey soon becomes convinced that a coor­ Role Played by the Various dinating mechanism for uterine mo­ Activities of the Uterine Muscle. tility exists_ The nature of this coor­ 1. Contraction provides the force which dinating mechanism is unknown, al­ is necessary for evacuation. though it has been shown that the 2. Relaxation and the rest period per­ uterovaginal ganglion is involved mits recovery of the metabolic or (dog) and that an independent mecha­ chemical state of the muscle and nism exists in the uterus. Purkinje tis­ probably of the maternal blood sup- sue has been described in the uterus, DANFORTH ET AL-PHYSIOLOGY OF THE UTERUS 29

but the evidence is equivocal. The evi­ physiology of the uterus cannot be dis­ dence is against a net mecha­ cussed here ( 4, 7). nism. Some evidence favors a mus­ cle fiber to muscle fiber mechanism Polarity. The uterus of the dog (4) of conduction. and the monkey (1) manifests a defin­ ite polarity. This has been maintained Dog. In the postpartum dog, many for years by some obstetricians. The of the waves of contraction start at the contraction moves caudalward. In the insertion of the Fallopian tubes, but dog we have occasionally seen it re­ they still occur after excision of the verse; the fetus retreated from the tubes and the tissue about their area fused portion of the uterus (lower seg­ of insertion; and normal labor results ment) into the ampulla in the horn (up­ after excision of the tubes and the apex per segment). We have seen contrac­ of the pregnant horns (2). tions occur in the uterine horns after Monkey. (1). Wave of contraction. In distension of the cervix and upper va­ the monkey's uterus the wave of con­ gina. This may be the basis of induc­ traction may be divided into three tion of labor by placing a bag in the phases. First, a wave of contraction cervix and lower segment. starts on both sides about the insertion of the tubes (pacemakers) and spreads Extrinsic and Labor medially in enlarging concentric waves; The utero-vaginal ganglion receives second, by the time they meet in the nerves from the hypogastric ganglion mid-line longitudinal shortening is evi­ (sympathetics from the lumbar chain) dent and continues; third, shortly after and from the nervi erigentes or pelvic the longitudinal shortening is evident, nerves (sacral division of the auto­ circular a contraction, somewhat peri­ nomic). In the rabbit the extrinsic staltoid in nature appears to spread nerves unquestionably affect uterine from above downward. Then the uterus and vaginal . In the dog real is very firm throughout, includin~ the movements of the uterus have not been lower segment, and the cervical obtained by direct stimulation of the sphincter can be felt and observed to extrinsic nerves. The same is true of contract and to dilate. The placental the monkey. The vagina will con­ site manifests some contraction, but is tract (9). relatively inert; it manifests some brachystasis. See figure 5. Labor will start and proceed norm­ ally in the absence of extrinsic nerves If a wave of contraction starts only from one "pacemaker," then an obliq­ and the uterovaginal ganglion in the uity of the uterus occurs. Or, it is pos­ dog, except that the last fetus is not evacuated from the paralyzed vagina sible for a contraction to start on one (9). side and then later on the other. Or, it is possible for the contraction to be "Reflex" contraction. Contractions of stronger on one side than on the other. the postpartum uterus of the monkey Or, brachystasis may occur unequally. can be elicited by stimulation of the Such disturbances in the coordinating nipple of the (3). mechanism or brachystasis would ren­ One of us (A. C. Ivy) has seen the der the "axis of descent" abnormal; ef­ same in the goat. The exact mechan­ facement and dilation of the cervix ism by which these contractions are would be interfered with and a type elicited is unknown. of "cervical dystocia" would result. These and other possible abnormalities Pregnancy and labor have been resulting from a disturbance of the known to occur in women with paraly- 30 QUARTERLY BULLETIN, N. U. M. S.

GROSS CORPUS UTERI ANATOOCAL Upper ltter1ne CORPUS segment. histologically UTERI confirmed.

GROSS

.ANATO}(ICAL ''''no,~o'il:l cally CERVIX nDed.

Figure 6.

Figure 7. Figure 8.

Fig. 6. Median sagittal section of the uterus of a mature non-pregnant Macacus rhesus monkey (magnification 2lh times). Observe contour of cervical canal. Fig. 7. Median sagittal section of a Macacus rhesus monkey near term. (lh natural si ze.) Breech presentation. Observe mucous plug in cervical canal. Fig. 8. Median sagittal section of a Macacus rhesus monkey in labor. (lh natural si:::e). . Head on . Observe height of cervical lips. DANFORTH ET AL---PHYSIOLOGY OF THE UTERUS 31

.sis (transverse myelitis) of the lower Portion of Uterine Horns Upon Labor, Proc. Soc. Exper. BioI. & Med., 1936-37, portion of the spinal cord. 35;482. It is possible that nervous factors 3. Ivy, A. C., Hartman, C. G., and Koff, A.: may retard dilation of the cervix and The Contractions of the Monkey Uterus disturb the pelvic mechanism con­ at Term, Am. J. Obst. & Gynec., 1931, 22;388. cerned in birth. It is reported that in 4. Ivy, A. C. and Rudolph, L .: The Physi­ . woman a sacral anesthesia favors di­ ology of the Uterine Musculature, Surg., la tion of the cervix. Since in the mon­ Gynec. & Obst., 1938,67;188. key "reflex" contractions of the uterus 5. Personal communication. can be elicited, it is rational to assume 6. Rudolph, L. The Phenomenon of Lighten­ ing in Pregnancy and the Lower Uterine that "reflex inhibition" may occur on Segment, Surg. Gynec. & Obst., 1937, the basis that motor and inhibitory 64;906. nerves or effects occur together. 7. Rudolph, L .: Constriction Ring Dystocia, J. Obst. & Gynaec. Brit. Emp., 1935, 42;992. REFERENCES 8. Rudolph, L. and Ivy, A. C.:: The Physi­ 1. Aschoff, L.: Die Dreiteilung des Uterus, ology of the Uterus in Labor. An Experi­ das untere Uterinsegment (lsthmusseg­ mental Study in the Rabbit and Dog, Am. ment) und die , Klin. J . Obst. & Gynec., 1930, 19;317. Wchnschr., 1907,44;979. 9. Rudolph, L. and Ivy, A. C.: Coordination 2. Danforth, D. N., Greene, R. R ., and Ivy, of the Uterus in Labor, Am. J. Obst. & A. C.: Effect of Amputation of Apical Gynec., 1931, 21;65.