Uterine Contractility
Dr. Nelson Soucasaux, Brazilian gynecologist
In my article "The Physiology of Menstruation" [here]
I have analyzed the hormonal actions upon the endometrium - the mucosa that
lines the interior of the cavity of the uterine corpus -, as well as the
main endometrial histological structures and, finally, menstruation. Now,
let us proceed to a brief analysis of some topics on the physiology of the
myometrium - the thick and potent uterine muscular layer.
Since the uterus is endowed with this potent muscular structure (whose
development and trophicity depends on the estrogens), this organ is capable
not only of modifying its basal tonus, but also of contracting and relaxing
through systoles and diastoles that vary greatly in intensity and duration.
The variations in the uterine contractile intensity are so wide that they
include: 1) weak and moderate contractions that occur in the periods in
which the organ is predominantly relaxed, as during the greatest part of
the menstrual cycle and pregnancy; 2) stronger contractions that characterize
menstruation and the orgasmic response, as well as those resulting from
pathological conditions; 3) the extremely powerful contractions of parturition.
The degree of contractility/relaxation of the uterus depends on the
excitability level of the smooth muscle cells of the myometrium.
The increase in the excitability of the myometrial cells stimulates
the uterine contractility, while its reduction induces to a relaxation of
the organ. These phenomena depend on an interaction of hormonal, biochemical
and neurovegetative factors, the most important of which are the first two
ones.
By reducing the excitability of the myometrial cells, progesterone is
the hormonal factor that promotes uterine relaxation, keeping the uterus
quiescent (in "repose"). Conversely, the estrogens have the opposite
effect - by increasing the myometrial excitability, they promote a moderate
stimulation of the uterine contractility. Due to the estrogenic action,
the uterus also becomes highly sensitive to the action of oxytocin - the
most important hormone in the causation of uterine contractions. Oxytocin
has a powerful contractile action upon the myometrium, being the hormonal
factor responsible for the very strong uterine contractions of parturition.
Oxytocin production takes place in the hypothalamus and its release is triggered
by a reflex neurogenic pathway that starts in nerve endings especially sensitive
to pressure located inside the cervical canal and in the nipples. During
parturition, it is the strong mechanical stimulation of the endocervix that
causes the hypothalamic liberation of a great quantity of oxytocin, giving
rise to increasingly potent uterine contractions.
As non-hormonal factors that also cause strong myometrial contractions,
we must mention several prostaglandins. The formation of these prostaglandins
in the endometrium during the menstrual necrosis of this tissue generates
the increase in the uterine contractility typical of this phase of the cycle,
giving rise to the menstrual cramps. Together with several other factors,
these substances also play a role in the triggering of parturition labour.
In spite of the fact that the uterine contractility is predominantly
commanded by the hormonal and biochemical factors mentioned here, there
are indications that the sympathetic and parasympathetic innervation of
the uterus may also have a considerable influence upon it. Independently
of the majority of the uterine contractions being endocrinally and biochemically
triggered, the myometrial contractile activity exhibits a very peculiar
characteristic that seems to demonstrate the existence of a precise and
subtle nervous coordination: it is the "triple descending gradient."
This gradient gives the uterine contractility its typical expulsive pattern.
According to Caldeyro-Barcia, the uterine contractile waves originate
in "pacemakers" situated around the uterine insertion of the Fallopian
tubes, one on the right and the other on the left. This author describes
the "triple descending gradient" with the following characteristics:
1) the propagation of the contractile wave along the uterus has a descending
direction. That happens because, after starting in one of the "pacemakers,"
the contractile wave spreads throughout the uterine fundus and propagates
downwards; 2) the systolic phase of the contraction lasts more at the uterine
fundus and less at the inferior parts of the organ; 3) the contractions
are stronger in the upper parts of the uterus than in the lower ones (Caldeyro-Barcia,
R.- "Fenômenos ativos do parto: Contratilidade uterina"
- in: Rezende, J.- "Obstetrícia, Vol 1" - Guanabara Koogan,
Rio de Janeiro, 1962).
In my opinion, it seems somewhat difficult to explain this so precise
and symmetric coordination of the myometrium contractile waves exclusively
by means of the hormonal mechanisms that trigger them. Even the double spiral
arrangement of most myometrial fibers throughout the uterus does not seem
to be, only by itself, capable of entirely explaining the "triple descending
gradient" - regardless of being an essential condition for its occurrence.
Several facts suggest that the neurovegetative system may have some coordinating
activity upon the uterine contractions. For details on this subject, see
my article "Fundamentos para o Estudo das Influências Neurovegetativas
em Ginecologia" ("Basis for the Study of the Neurovegetative Influences
in Gynecology"), published in Jornal Brasileiro de Medicina, Vol. 57,
Nº 4, October 1989, Rio de Janeiro.
Specifically about the orgasmic contractions of the uterus, everything
indicates that they are fundamentally triggered and commanded by the vegetative
innervation of this organ. These uterine contractions are the acme of the
neurogenic myotonic reaction that occur in the female genitals during sexual
excitement, and happen simultaneously with the widely known contractions
of the muscles that surround the vaginal entrance. Still regarding the neurovegetative
influences upon the myometrial contractile activity, it is pertinent to
remind that neurogenic uterine contractions are well-known in gynecologic
practice.
This article is an excerpt from my book "Os Órgãos
Sexuais Femininos: Forma, Função, Símbolo e Arquétipo"
("The Female Sexual Organs: Shape, Function, Symbol and Archetype"),
published by Imago Editora, Rio de Janeiro, 1993. For information on the
book, see page http://www.nelsonginecologia.med.br/orgaos.htm
, from my Website www.nelsonginecologia.med.br
Copyright Nelson Soucasaux 1993, 2001
__________________________________________________________
Nelson Soucasaux is a gynecologist especially dedicated to clinical,
preventive and psychosomatic gynecology. Graduated in 1974 by Faculdade
de Medicina da Universidade Federal do Rio de Janeiro, Brazil, he is the
author of several articles published in medical journals and of the books
"Novas Perspectivas em Ginecologia" ("New Perspectives in
Gynecology") and "Os Órgãos Sexuais Femininos: Forma,
Função, Símbolo e Arquétipo" ("The
Female Sexual Organs: Shape, Function, Symbol and Archetype"), published
by Imago Editora, Rio de Janeiro, 1990, 1993.
©2001 Harry Finley. It is illegal
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