Changes

* 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 [[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'''.

* Reduces the uterine volume and causes retraction of the [[Female_Reproductive_Tract_-The_Uterus_- Anatomy & Physiology|lower uterine segment]] and the [[Female_Reproductive_Tract_-The_Cervix_- Anatomy & Physiology|cervix]] to create a '''birth canal'''.

'''''Retraction Ring'''''

'''''Retraction Ring'''''

*The [[Female_Reproductive_Tract_-The_Uterus_-_Anatomy_%26_Physiology|Uterus]] is divided into two segments:

*The [[Female_Reproductive_Tract_-The_Uterus_- Anatomy & Physiology|Uterus]] is divided into two segments:

** Upper segment: contractile

** Upper segment: contractile

** Lower segment: passive

** Lower segment: passive

*Two hormones directly regulate myometrial contractions:  

*Two hormones directly regulate myometrial contractions:  

** '''[[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.

** '''[[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.

*** 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.