Difference between revisions of "Uterine Contraction - Anatomy & Physiology"

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== Myometrial Contraction==
 
== Myometrial Contraction==
  
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* Myometrial cells contract and shorten, but do not regain their original length upon relaxation.
 
* Myometrial cells contract and shorten, but do not regain their original length upon relaxation.
* Reduces the uterine volume and causes retraction of the [[Uterus - Anatomy & Physiology|lower uterine segment]] and the [[Cervix - Anatomy & Physiology|cervix]] to create a '''birth canal'''.
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* Reduces the uterine volume and causes retraction of the [[Female_Reproductive_Tract_-The_Uterus_-_Anatomy_%26_Physiology|lower uterine segment]] and the [[Female_Reproductive_Tract_-The_Cervix_-_Anatomy_%26_Physiology|cervix]] to create a '''birth canal'''.
  
  
 
'''''Retraction Ring'''''
 
'''''Retraction Ring'''''
  
*The [[Uterus - Anatomy & Physiology|Uterus]] is divided into two segments:
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*The [[Female_Reproductive_Tract_-The_Uterus_-_Anatomy_%26_Physiology|Uterus]] is divided into two segments:
 
** Upper segment: contractile
 
** Upper segment: contractile
 
** Lower segment: passive
 
** Lower segment: passive
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*Two hormones directly regulate myometrial contractions:  
 
*Two hormones directly regulate myometrial contractions:  
** '''[[Pituitary_Gland_- Anatomy & Physiology#Oxytocin|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.
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** '''[[Pituitary_Gland_-_Anatomy_%26_Physiology#Oxytocin|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.
 
*** 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.
 
** '''Prostaglandins''': Stimulate liberation of calcium from intracellular stores to cause the rise in intracellular calcium concentration required for myometrial contraction.
 
 
[[Category:Parturition]]
 
[[Category:Bullet Points]]
 

Revision as of 09:01, 17 September 2008



Anatomy and PhysiologyWikiAnt and Phys Banner.png
()Map REPRODUCTIVE SYSTEM (Map)
NORMAL PARTURITION



Myometrial Contraction

  • 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.


The Birth Canal

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.


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.



Endocrine Control

Schematic Diagram to show the Endocrine Influence on Myometrial Contraction,Copyright RVC 2008
  • 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.