Abnormal uterine action

Abstract

The arrangement of the three layers of muscle fibres of the uterus in late pregnancy and labour has been described in Chapter 7. The layers are not as clearly defined as shown diagrammatically in Figure 51, but the fibres in general thin out as they approach the uterine isthmus and cervix. The waves of contraction and retraction which commence in the area of the cornua become less intense as they pass downwards to the lower segment. As the latter has a higher percentage of circular muscle fibres than the fundus it has been aptly compared to a sphincter which may go into spasm through fear stimulating the sympathetic nervous system. Yet the lower segment needs to remain passive and relaxed in order to efface and dilate, and any effort to do so if in spasm would cause pain. Pain increases fear and thus a vicious circle is set up which must be broken if normal labour is to occur. These views, much debated years ago, may be oversimplistic, but even today the innervation of the uterus in labour is neither fully established nor understood. The pathways by which fibres of the autonomic nervous system reach the uterus may be agreed, but the physiological effects of stimuli remain unestablished.