Non-Pregnant Uterine Contractions

A successful conception is the result of a set of coordinated transport phenomena within the male and female reproductive tracts. The detached ovum is gathered by the fimbria of the fallopian tube (oviduct), then transferred to the ampula by contractile actions of the fallopian tube. Spermatozoa are transported first through the vas deferens and urethra in the male tract towards the vaginal fornix and stored for up to 48 hours in the cervical mucous. This storage site provides a steady supply of living sperm for transport into the uterine cavity. Sperm are transported through the uterine fluid towards the ampulla of the fallopian tube where fertilization occurs. The zygote forms over 2-4 days, when it is then transported to the uterine cavity, again by actions of the fallopian tube. It is then conveyed during another 4 days to its implantation site, usually in the fundal area (the upper part) of the uterus. The embryo does not have a self-propelling mechanism (e.g., cilia or flagella), thus it is a passive passenger. These essential events depend on concomitant intrauterine fluid motion induced by uterine wall motility. Uterine wall motility is the result of myometrial contractions that are normally directed from the cervix towards the fundus.

The myometrium is made of myocytes (smooth muscle cells), which are the fundamental contractile elements that also pass the electrical signals required to coordinate the contractions of the uterine wall. Regions of the myometrium are interconnected via gap junctions to form a functional syncytium, which link microscopic activity (e.g. cellular) to macroscopic function (e.g. tissue level). The greater the connectedness, the larger the syncytium. When a single myometrial cell, or a group of cells contract, the deformation results in a local geometric change according to viscoelastic properties of the tissue. As a result, the neighboring regions are also forced to undergo some deformation. Summation of all the deformations from all the cells provides the instantaneous geometry of the intra-uterine fluid-tissue interface. The dynamics of the intra-uterine fluid-tissue interface (variations with time) induces the forces that control intra-uterine pressures as well as intra-uterine fluid motions in the non-pregnant uterus. This combined action moves the embryo towards the fundus.