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Uterine physiology
PHYSIOLOGY
Nerve supply to the uterus
Uterine physiology
Uterus
Iain Campbell
Uterine artery T10 T11
Ovarian plexus
T12
S2 S3 S4 Uterovaginal plexus Inferior hypogastric plexus Right and left hypogastric nerves 1
Fallopian tubes and potentially aiding conception. Progesterone, secreted by the ovary in the second half of the cycle, inhibits muscle contraction and so encourages implantation. Uterine contractions during menstruation are stimulated by prostaglandin. In pregnancy, the uterus increases from 30–60 g to 750–1000 g. by both hyperplasia and hypertrophy. Glycogen is laid down and there is an increase in ATP. Muscle contraction is induced by calcium, both intra- and extracellular, and there are spontaneous depolarizing pacemakers in the myometrium. If these reach a threshold, contraction occurs. Prostaglandins enhance the liberation of intracellular calcium and oxytocin lowers the excitation threshold.
Nerve supply The nerve supply to the uterus passes via autonomic pathways (Figure 1), the body via the sympathetic fibres from T10–L1 and the cervix via the parasympathetic from the sacral outflow S2–4. The sympathetic nerves pass via the superior hypogastric plexus; this lies in front of the abdominal aorta and gives off branches to the ovarian plexus and to the inferior hypogastric plexus (via the right and left hypogastric nerves), which lies on the front of the sacrum. The uterine nerves come from the uterovaginal plexus of the inferior hypogastric plexus. They are distributed with the blood vessels. They communicate with the ovarian plexus and some pass to the cervix. Motor function – sympathetic activation produces uterine contraction; parasympathetic activation inhibits uterine contraction and produces vasodilatation. In pregnancy these actions are complicated by the hormonal control of uterine function. Sensory function – the body of the uterus is innervated from T10 to L1 via the sympathetic nerves, and the cervix via the parasympathetic at S2–4.
Parturition During pregnancy, synthesis of prostaglandins (which stimulate uterine contraction) is inhibited but, at parturition, a rise in the oestrogen to progesterone ratio results in this inhibition being lifted and prostaglandins are synthesized and released, principally by the endometrium. Oestrogen also increases the number of myometrial oxytocin receptors and oxytocin stimulates the release of prostaglandin directly. Release of oxytocin from the posterior pituitary is produced by tactile stimuli from the reproductive tract. This increases uterine contractions further and a positive feedback loop is established. The precise mechanisms surrounding the induction of parturition in the human are not certain. Maternal progesterone does not generally fall; the fetal adrenal gland stimulates placental oestrogen synthesis via androgen precursors from a specialized area of the fetal adrenal cortex, thus raising the oestrogen to progesterone ratio. This specialized area of the fetal adrenal cortex regresses soon after birth. u
Myometrial contractility Uterine growth and activity are largely under hormonal control. In broad terms, oestrogen stimulates myometrial activity (and renders it more sensitive to oxytocin) and progesterone inhibits it (and makes it less sensitive to oxytocin). During the menstrual cycle, oestrogen levels are highest before ovulation, increasing the motility of the
Iain Campbell is Consultant Anaesthetist at the University Hospitals of South Manchester and Visiting Professor of Human Physiology at Liverpool John Moores University, UK. He qualified from Guy’s Hospital Medical School, London, and trained in anaesthesia in Zimbabwe, Southend, Montreal and Leeds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:3
Superior hypogastric plexus
L1
The role of the uterus is to nurture the fetus until parturition. The anatomy of the uterus is described on page 74. Functionally it consists of a lower cervix and an upper body in which the fetus develops. The uterus is supported at its lower end by a number of ligaments connected to the bladder, the rectum and the walls of the pelvis. Some of these are peritoneal folds, others are unstriped muscle and fibrous tissue. The uterus has three layers: serosa (peritoneum), myometrium (muscle), and endometrium (blood and lymph vessels and nerves). The myometrium is arranged in three layers. In late pregnancy it forms a functional syncytium with electrical coupling of the branching bundles of cells at gap junctions. The endometrium is lined by columnar epithelium and contains many tube-like glands that open into the uterine cavity and secrete mucus. The cervix is a more collagenous structure with a muscle content of 10–15%.
FURTHER READING Johnson M H, Everitt B J. Essential reproduction. Oxford: Blackwell Science, 2000.
76
© 2005 The Medicine Publishing Company Ltd
Nerve supply to the uterus
Uterine physiology
Uterus
Iain Campbell
Uterine artery T10 T11
Ovarian plexus
T12
S2 S3 S4 Uterovaginal plexus Inferior hypogastric plexus Right and left hypogastric nerves 1
Fallopian tubes and potentially aiding conception. Progesterone, secreted by the ovary in the second half of the cycle, inhibits muscle contraction and so encourages implantation. Uterine contractions during menstruation are stimulated by prostaglandin. In pregnancy, the uterus increases from 30–60 g to 750–1000 g. by both hyperplasia and hypertrophy. Glycogen is laid down and there is an increase in ATP. Muscle contraction is induced by calcium, both intra- and extracellular, and there are spontaneous depolarizing pacemakers in the myometrium. If these reach a threshold, contraction occurs. Prostaglandins enhance the liberation of intracellular calcium and oxytocin lowers the excitation threshold.
Nerve supply The nerve supply to the uterus passes via autonomic pathways (Figure 1), the body via the sympathetic fibres from T10–L1 and the cervix via the parasympathetic from the sacral outflow S2–4. The sympathetic nerves pass via the superior hypogastric plexus; this lies in front of the abdominal aorta and gives off branches to the ovarian plexus and to the inferior hypogastric plexus (via the right and left hypogastric nerves), which lies on the front of the sacrum. The uterine nerves come from the uterovaginal plexus of the inferior hypogastric plexus. They are distributed with the blood vessels. They communicate with the ovarian plexus and some pass to the cervix. Motor function – sympathetic activation produces uterine contraction; parasympathetic activation inhibits uterine contraction and produces vasodilatation. In pregnancy these actions are complicated by the hormonal control of uterine function. Sensory function – the body of the uterus is innervated from T10 to L1 via the sympathetic nerves, and the cervix via the parasympathetic at S2–4.
Parturition During pregnancy, synthesis of prostaglandins (which stimulate uterine contraction) is inhibited but, at parturition, a rise in the oestrogen to progesterone ratio results in this inhibition being lifted and prostaglandins are synthesized and released, principally by the endometrium. Oestrogen also increases the number of myometrial oxytocin receptors and oxytocin stimulates the release of prostaglandin directly. Release of oxytocin from the posterior pituitary is produced by tactile stimuli from the reproductive tract. This increases uterine contractions further and a positive feedback loop is established. The precise mechanisms surrounding the induction of parturition in the human are not certain. Maternal progesterone does not generally fall; the fetal adrenal gland stimulates placental oestrogen synthesis via androgen precursors from a specialized area of the fetal adrenal cortex, thus raising the oestrogen to progesterone ratio. This specialized area of the fetal adrenal cortex regresses soon after birth. u
Myometrial contractility Uterine growth and activity are largely under hormonal control. In broad terms, oestrogen stimulates myometrial activity (and renders it more sensitive to oxytocin) and progesterone inhibits it (and makes it less sensitive to oxytocin). During the menstrual cycle, oestrogen levels are highest before ovulation, increasing the motility of the
Iain Campbell is Consultant Anaesthetist at the University Hospitals of South Manchester and Visiting Professor of Human Physiology at Liverpool John Moores University, UK. He qualified from Guy’s Hospital Medical School, London, and trained in anaesthesia in Zimbabwe, Southend, Montreal and Leeds.
ANAESTHESIA AND INTENSIVE CARE MEDICINE 6:3
Superior hypogastric plexus
L1
The role of the uterus is to nurture the fetus until parturition. The anatomy of the uterus is described on page 74. Functionally it consists of a lower cervix and an upper body in which the fetus develops. The uterus is supported at its lower end by a number of ligaments connected to the bladder, the rectum and the walls of the pelvis. Some of these are peritoneal folds, others are unstriped muscle and fibrous tissue. The uterus has three layers: serosa (peritoneum), myometrium (muscle), and endometrium (blood and lymph vessels and nerves). The myometrium is arranged in three layers. In late pregnancy it forms a functional syncytium with electrical coupling of the branching bundles of cells at gap junctions. The endometrium is lined by columnar epithelium and contains many tube-like glands that open into the uterine cavity and secrete mucus. The cervix is a more collagenous structure with a muscle content of 10–15%.
FURTHER READING Johnson M H, Everitt B J. Essential reproduction. Oxford: Blackwell Science, 2000.
76
© 2005 The Medicine Publishing Company Ltd