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

Revision as of 11:17, 2 July 2008

Myometrium consists of:

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.

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 '''''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.