Changes in uterine innervation during pregnancy and labour

A Special Authors Introduction

By Berith Tingåker

Changes in uterine innervation during pregnancy and labour clearly indicate that cervix uteri plays a crucial role in labour pain and parturition as well as in pregnancy maintenance and labour initiations.

A number of studies have demonstrated an almost complete denervation of the corpus uteri in term pregnancy and labour in both humans and rodents compared to the non-pregnant state. In contrast, the cervix uteri remains densely innervated throughout pregnancy and labour. Additionally, pregnancy increases excitability of mechanosensitive afferents innervating the uterine cervix.

Moreover, gonadal hormones are involved in this intricate interplay. Recent studies on rodents have revealed reduced cervical nerve density secondary to treatment with progesterone. However, estrogen treatment was associated with increased cervical nerve density.

The Transient Potential Vanilloid Receptor (TRPV1), a key molecule in nociception, has been detected in the human uterus and its presence is affected by pregnancy. In term pregnancy and during labour, TRPV1 positive nerve fibres "disappear" from the corpus in contrast to the non-pregnant state. However, TPRV1 positive nerve fibres are clearly demonstrated in cervical tissue throughout pregnancy and labour. TRPV1 is expressed primarily in nociceptors but also by nonneuronal tissues, such as smooth muscle, the bladder urothelium and inflammatory cells like polymorphonuclear granulocytes and macrophges. Cervical ripening is a prerequisite for a normal obstetric outcome. It is considered to be a neuroimmune-mediated inflammatory process, and a number of inflammatory and algogenic biomolecules are released in the cervical tissue. Moreover, several of these chemical mediators also generate nociceptive signals by activation or modulation of TRPV1.

In conclusion, corpus uteri is denervated in term pregnancy and during parturition. In contrast, the cervical innervation remains preserved. These results clearly indicate that cervix uteri can be regarded as the main site from where labour pain mainly emanates. This finding and further studies may open up for new approaches and strategies concerning labour pain relief.

This article was published in Current Opinion in Anesthesiology
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