Parturition

Parturition is the act of giving birth to a baby. It consists of many different stages which interact to provide delivery of child and the placenta.

Organs Involved

The primary organ involved is the uterus. This contains a layer known as the myometrium which contains bundles of non-striated (smooth) muscle as well as connective tissue (collagen), nerves and blood vessels. The muscles of the inner wall of the myometrium are circular, whereas the muscles on the outer wall are longitudinal. During pregnancy this layer increases in mass approximately 20 fold, with hypertrophy increasing from 50um to 500um, hyperplasia increasing and glycogen deposits increasing.

The muscle cells act as a syncytium, in which spontaneous depolarising pacemaker potentials occur. If these are above a certain threshold (known as the action potential) then a wave of calcium is released and a contraction occurs.

Cervical Softening

The cervix consists largely of fibrous collagen in a proteoglycan matrix. It is normally hard, and needs to be softened to allow passage of the baby. The presence of keratan sulfate decreases the amount of collagen, whereas dermatan sulfate increases the amount of collagen.

There are several chemicals which allow the softening of the cervix, including:

• Metalloproteinases
• Proinflammatories such as cytokines which induce an inflammatory response
• Nitrous oxide (although not directly responsible it is the product following the catabolism of inhibiting NOS)
• Prostaglandins

Contractions

The contractions are responsible first for dilating the cervix, moving the foetus through the pelvis and eventually expelling the baby. The primary hormone involved is oxytocin, a protein (9 amino acids long) which is released following a nervous response which results from the weight of the baby on the cervix, among other things.

Oxytocin stimulates the decidual layer to release prostaglandins (which in turn enhance the effect of oxytocins). These act on the myometrium to produce a contraction by initiating the release of contraction associated proteins (CAP's) which:

• increasing gap junctions between cells to increase the effectiveness of the synctium and increase the likelihood that an action potential will occur
• increasing ion channels for calcium to increase the effect of an action potential
• increasing oxytocin and prostaglandin receptors to enhance the above two effects

Calcium activates myosin light chain kinase, which in turn induce contractions.

A high estrogen:progesterone ratio is required for contractions, as this results in an increase in the release of prostaglandins (by stimulating the release of phosphorlipase from lysosomes) and increasing the number of oxytocin receptors.