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Uterine Contraction - Anatomy & Physiology (view source)
Revision as of 11:17, 2 July 2008
, 11:17, 2 July 2008New page: '''Myometrium''' consists of: * Non-striated smooth muscle fibres * Nerves, Blood and Lymph * During pregnancy, '''Oestrogens''' stimulate muscle cell hypertrophy. * Myometrial cells be...
'''Myometrium''' consists of:
* Non-striated smooth muscle fibres
* Nerves, Blood and Lymph
* During pregnancy, '''Oestrogens''' stimulate muscle cell hypertrophy.
* Myometrial cells behave as a functional syncytium, being electrically coupled via specialised regions of contact known as '''gap junctions/nexi'''.
* The combination of the above factors allows '''strong, coordinated myometrial contraction'''.
* Excitation of smooth muscle depends on alteration in '''membrane potential'''. Myometrial cells must be '''depolarised''' (-50mV) to achive contraction.
* '''Spontaneous depolarizing pacemaker potentials''' occur. If the magnitude of these exceeds the '''critical threshold''', a burst of '''action potentials''' will fire.
* This causes '''calcium influx''' from endoplasmic reticular stores and extracellular fluid.
* Intracellular calcium binds to regulatory sites on '''actin''' and '''myosin''', allowing expression of ATPase, thus causing contraction.
* Uterine contractions increase in both frequency and amplitude during labour. This causes an increase in '''uterine pressure''' from 10mmHg to 50-100mmHg.
'''''Brachystasis'''''
* Myometrial cells contract and shorten, but do not regain their original length upon relaxation.
* Reduces the uterine volume and causes retraction of the lower uterine segment and the cervix to create a '''birth canal'''.
'''''Endocrine Control'''''
Two hormones directly regulate myometrial contractions:
* '''Oxytocin''': from the posterior pituitary gland (not the corpus luteum) lowers the excitation threshold of muscle cells to increase the likelihood of action potential firing. Oxytocin also stimulates prostaglandin release.
** The uterine Oxytocin receptor is a G protein coupled receptor with a Gαq subinit. Thus the signalling pathway is via a calcium second messenger. Hence prostaglandin and oxytocin are complementory in action.
* '''Prostaglandins''': Stimulate liberation of calcium from intracellular stores to cause the rise in intracellular calcium concentration required for myometrial contraction.
'''''Retraction Ring'''''
*The Uterus is divided into two segments:
** Upper segment: contractile
** Lower segment: passive
*The junction between these segments is the retraction ring.
* Palpation of the retraction ring is used as an indicator of progression through labour.
* Non-striated smooth muscle fibres
* Nerves, Blood and Lymph
* During pregnancy, '''Oestrogens''' stimulate muscle cell hypertrophy.
* Myometrial cells behave as a functional syncytium, being electrically coupled via specialised regions of contact known as '''gap junctions/nexi'''.
* The combination of the above factors allows '''strong, coordinated myometrial contraction'''.
* Excitation of smooth muscle depends on alteration in '''membrane potential'''. Myometrial cells must be '''depolarised''' (-50mV) to achive contraction.
* '''Spontaneous depolarizing pacemaker potentials''' occur. If the magnitude of these exceeds the '''critical threshold''', a burst of '''action potentials''' will fire.
* This causes '''calcium influx''' from endoplasmic reticular stores and extracellular fluid.
* Intracellular calcium binds to regulatory sites on '''actin''' and '''myosin''', allowing expression of ATPase, thus causing contraction.
* Uterine contractions increase in both frequency and amplitude during labour. This causes an increase in '''uterine pressure''' from 10mmHg to 50-100mmHg.
'''''Brachystasis'''''
* Myometrial cells contract and shorten, but do not regain their original length upon relaxation.
* Reduces the uterine volume and causes retraction of the lower uterine segment and the cervix to create a '''birth canal'''.
'''''Endocrine Control'''''
Two hormones directly regulate myometrial contractions:
* '''Oxytocin''': from the posterior pituitary gland (not the corpus luteum) lowers the excitation threshold of muscle cells to increase the likelihood of action potential firing. Oxytocin also stimulates prostaglandin release.
** The uterine Oxytocin receptor is a G protein coupled receptor with a Gαq subinit. Thus the signalling pathway is via a calcium second messenger. Hence prostaglandin and oxytocin are complementory in action.
* '''Prostaglandins''': Stimulate liberation of calcium from intracellular stores to cause the rise in intracellular calcium concentration required for myometrial contraction.
'''''Retraction Ring'''''
*The Uterus is divided into two segments:
** Upper segment: contractile
** Lower segment: passive
*The junction between these segments is the retraction ring.
* Palpation of the retraction ring is used as an indicator of progression through labour.