Myocytes, myometrium, and uterine contractions.

The pregnant uterus is unique because of the dramatic functional changes that occur in the peripartum period. To promote the concept that we have a relatively poor understanding of the physiology of parturition, we will posit 10 facts that are so obvious and so clearly accepted as facts that they probably are not even facts at all. (1) The laboring uterus undergoes peristalsis to dilate the cervix, deliver the fetus, and expel the placenta. (2) The human uterus is composed of longitudinal and circular layers of smooth muscle. (3) The functional cells of the uterus are the myocytes, which are a homogeneous cell type responsible for the generation of contraction forces, passage of action potentials, and control of contractility. (4) The phasic contractions of the uterus are typical for visceral smooth muscle. (5) The primary, and perhaps only, role of gap junctions is to allow passage of action potentials through the tissue. (6) Action potential propagation as the mechanism for global communication (over many centimeters throughout the uterus) is sufficient to recruit all regions and all myocytes of the uterus. (7) Slow waves pace the contractions of human myometrium. (8) Calcium-activated potassium channels are responsible for repolarization of the membrane potential that terminates each contraction. (9) Chloride channels are not important in uterine electrophysiology. (10) With enough computing power, it would be straightforward to build a closed model of human labor, given our current understanding of the components of myometrium. This manuscript discusses each point to stimulate questions for future investigation.