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Uterine physiology
Physiology
Uterine physiology
rogesterone deprives the endometrium of its hormonal support p and menstruation occurs. During menstruation uterine contractions are stimulated by prostaglandins.
Gillian Campling
Myometrial contractility Figure 1 shows the nerve supply to the uterus. The body of the uterus is supplied by sympathetic fibres from T10–L1 via the superior hypogastric plexus, whilst the cervix is supplied by parasympathetic fibres from the sacral region (S2–S4). The uterine nerves originate from the uterovaginal plexus of the inferior hypogastric plexus. Sympathetic activation produces uterine contraction, whilst parasympathetic stimulation inhibits contraction and produces vasodilatation. Control of uterine contraction is largely governed by hormones. Muscle contraction is initiated by changes in both intra- and extracellular calcium. Pacemakers in the myometrium are capable of spontaneous depolarization. These pacemaker regions cause action potentials that are conducted through the myometrium, which acts as a syncytium. When these reach a threshold level, contraction occurs. Prostaglandins enhance the release of intracellular calcium, and oxytocin lowers the thresh old needed for contraction to occur.
Abstract The uterus provides an environment to nurture the fetus until parturition. It must also be capable of contracting to help deliver the infant. The endometrium undergoes hormonally controlled changes during each menstrual cycle and during pregnancy. The contractility of the myometrium is governed by hormones and plays a key role in parturition. This article provides a brief overview of the functions of the uterus and how these functions are achieved.
Keywords hormones; oestrogen; parturition; progesterone; prostaglandin
Parturition The precise mechanism for triggering parturition in humans is still not completely understood. Parturition is a complex, integ rated process, which involves both neural and endocrine input from mother and fetus. Research has suggested that the maturation of the fetal adrenal cortex contributes to the process. During most of pregnancy placental progesterone secretion is greater than that of oestrogen. It is thought that secretions from the fetal
The non-pregnant uterus is approximately 7.5 cm long, pearshaped and suspended in the peritoneal cavity. The detailed anatomy of the uterus is considered on page 107. The role of the uterus is to allow the embryo to implant and the placenta to form, providing an environment where the fetus can grow until parturition. The uterus needs to be able to contract forcefully to expel the fetus at parturition, whilst remaining quiescent during gestation to allow successful fetal development. The uterus consists of three functional layers: the serosa (an outer covering), the myometrium (a middle layer of smooth muscle) and the endometrium (the innermost layer composed mostly of epithelial cells, blood and lymph vessels and nerves).
Nerve supply to the uterus Uterus Uterine artery
Endocrine influences During each menstrual cycle the endometrium and myometrium undergo changes in appearance and function in response to ovarian steroids (to prepare for fertilization, implantation, placentation and fetal development). During the follicular phase of the ovarian cycle oestrogens have a uterotrophic effect, stimulating the proliferation of endometrial cells and increasing the number of glandular structures. This is the proliferative phase of the uterine cycle. (In general, oestrogen stimulates uterine activity, enhancing its sensitivity to oxytocin.) Rising progesterone levels in the luteal cycle cause further stromal proliferation and spiral arteries to develop fully. Endometrial glands become fully active, and this is known as the secretory phase of the uterine cycle. Progesterone inhibits myometrial contraction by opposing the action of oestrogens and locally generated prostaglandins. If fertilization does not occur the fall in plasma oestrogen and
T10 T11
Ovarian plexus
T12 Superior hypogastric plexus
L1
S2 S3 S4 Uterovaginal plexus Inferior hypogastric plexus Right and left hypogastric nerves
Gillian Campling, DSc, is Biomedical Sciences Course Leader in the School of Biological and Biomedical Sciences at the University of Durham, Durham, UK.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:3
Figure 1
122
© 2008 Elsevier Ltd. All rights reserved.
Physiology
adrenal gland may induce a change from progesterone to oestrogen dominance in the last few days of pregnancy. This increase in the ratio of oestrogen to progesterone stimulates the production of prostaglandins and increases myometrial excitability and sensitivity to other agents that can affect its contractility. Progesterone levels do not decrease but it is thought that placental oestrogen secretion is stimulated via androgen precursors from a specialized region of the fetal adrenal cortex. The number of oxytocin receptors in the myometrium increases, and oxytocin secretion is stimulated by stretching of the reproductive tract.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:3
The increase in uterine contractions causes a positive feedback loop resulting in more oxytocin secretion and, ultimately, the delivery of the baby. ◆
Further reading Jabbour HN, Kelly RW, Fraser HM, Critchley HO. Endocrine regulation of endometrial function. Endocr Rev 2006; 27: 17–46. Johnson MH. Essential reproduction. 6th edn. Oxford: Blackwell, 2007.
123
© 2008 Elsevier Ltd. All rights reserved.
Uterine physiology
rogesterone deprives the endometrium of its hormonal support p and menstruation occurs. During menstruation uterine contractions are stimulated by prostaglandins.
Gillian Campling
Myometrial contractility Figure 1 shows the nerve supply to the uterus. The body of the uterus is supplied by sympathetic fibres from T10–L1 via the superior hypogastric plexus, whilst the cervix is supplied by parasympathetic fibres from the sacral region (S2–S4). The uterine nerves originate from the uterovaginal plexus of the inferior hypogastric plexus. Sympathetic activation produces uterine contraction, whilst parasympathetic stimulation inhibits contraction and produces vasodilatation. Control of uterine contraction is largely governed by hormones. Muscle contraction is initiated by changes in both intra- and extracellular calcium. Pacemakers in the myometrium are capable of spontaneous depolarization. These pacemaker regions cause action potentials that are conducted through the myometrium, which acts as a syncytium. When these reach a threshold level, contraction occurs. Prostaglandins enhance the release of intracellular calcium, and oxytocin lowers the thresh old needed for contraction to occur.
Abstract The uterus provides an environment to nurture the fetus until parturition. It must also be capable of contracting to help deliver the infant. The endometrium undergoes hormonally controlled changes during each menstrual cycle and during pregnancy. The contractility of the myometrium is governed by hormones and plays a key role in parturition. This article provides a brief overview of the functions of the uterus and how these functions are achieved.
Keywords hormones; oestrogen; parturition; progesterone; prostaglandin
Parturition The precise mechanism for triggering parturition in humans is still not completely understood. Parturition is a complex, integ rated process, which involves both neural and endocrine input from mother and fetus. Research has suggested that the maturation of the fetal adrenal cortex contributes to the process. During most of pregnancy placental progesterone secretion is greater than that of oestrogen. It is thought that secretions from the fetal
The non-pregnant uterus is approximately 7.5 cm long, pearshaped and suspended in the peritoneal cavity. The detailed anatomy of the uterus is considered on page 107. The role of the uterus is to allow the embryo to implant and the placenta to form, providing an environment where the fetus can grow until parturition. The uterus needs to be able to contract forcefully to expel the fetus at parturition, whilst remaining quiescent during gestation to allow successful fetal development. The uterus consists of three functional layers: the serosa (an outer covering), the myometrium (a middle layer of smooth muscle) and the endometrium (the innermost layer composed mostly of epithelial cells, blood and lymph vessels and nerves).
Nerve supply to the uterus Uterus Uterine artery
Endocrine influences During each menstrual cycle the endometrium and myometrium undergo changes in appearance and function in response to ovarian steroids (to prepare for fertilization, implantation, placentation and fetal development). During the follicular phase of the ovarian cycle oestrogens have a uterotrophic effect, stimulating the proliferation of endometrial cells and increasing the number of glandular structures. This is the proliferative phase of the uterine cycle. (In general, oestrogen stimulates uterine activity, enhancing its sensitivity to oxytocin.) Rising progesterone levels in the luteal cycle cause further stromal proliferation and spiral arteries to develop fully. Endometrial glands become fully active, and this is known as the secretory phase of the uterine cycle. Progesterone inhibits myometrial contraction by opposing the action of oestrogens and locally generated prostaglandins. If fertilization does not occur the fall in plasma oestrogen and
T10 T11
Ovarian plexus
T12 Superior hypogastric plexus
L1
S2 S3 S4 Uterovaginal plexus Inferior hypogastric plexus Right and left hypogastric nerves
Gillian Campling, DSc, is Biomedical Sciences Course Leader in the School of Biological and Biomedical Sciences at the University of Durham, Durham, UK.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:3
Figure 1
122
© 2008 Elsevier Ltd. All rights reserved.
Physiology
adrenal gland may induce a change from progesterone to oestrogen dominance in the last few days of pregnancy. This increase in the ratio of oestrogen to progesterone stimulates the production of prostaglandins and increases myometrial excitability and sensitivity to other agents that can affect its contractility. Progesterone levels do not decrease but it is thought that placental oestrogen secretion is stimulated via androgen precursors from a specialized region of the fetal adrenal cortex. The number of oxytocin receptors in the myometrium increases, and oxytocin secretion is stimulated by stretching of the reproductive tract.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 9:3
The increase in uterine contractions causes a positive feedback loop resulting in more oxytocin secretion and, ultimately, the delivery of the baby. ◆
Further reading Jabbour HN, Kelly RW, Fraser HM, Critchley HO. Endocrine regulation of endometrial function. Endocr Rev 2006; 27: 17–46. Johnson MH. Essential reproduction. 6th edn. Oxford: Blackwell, 2007.
123
© 2008 Elsevier Ltd. All rights reserved.