Components of the tract and their innervation

A diagram of the human female genital tract is given in Fig. 10.17.

Ovaries

The human ovaries are covered by a layer of germinal epithelium surrounding a thick cortex that contains the follicles and corpora lutea, and a vascular medulla. The ground substance or stroma of the cortex and medulla is interstitial connective tissue containing several cell types, some of which are probably smooth muscle cells.

Infundibuium of uterine tube

Mesovarium and Reproductive Tract Systems

Infundibuium of uterine tube

Mesovarium

Fig. 10.17 Diagram of the human female reproductive tract. On the left the tract is shown in sagittal section.

The ovaries are innervated both by sensory nerves that project to the central nervous system and by adrenergic and cholinergic autonomic nerve fibres. Sympathetic fibres arise from the pre-aortic plexus and enter the ovaries with the ovarian arteries. Some of the parasympathetic fibres may enter the ovary along the branches of the uterine artery, distributed to the ovary by way of the broad ligament and mesovarium (the peritoneal fold surrounding the ovaries). Many of the autonomic nerves innervate the blood vessels, but there is some evidence for association with non-vascular elements in the uterus. Some 25% of the adrenergic nerves seem to be associated with the follicles. It has been demonstrated in some mammals that the ovaries undergo contractile activity, which is increased by a-adrenoceptor activation, and this probably involves activity of the smooth muscle cells in the stromal fibromuscular tissue. It is possible that the contraction of these cells may assist in the rupture of the follicles. Early work on transplanted ovaries has, however, shown them to function normally although they show no evidence of re-innervation, suggesting that intact innervation is unnecessary.

Fallopian tubes

The ovaries are linked to the uterus via the oviducts, or Fallopian tubes, which have muscular walls and complex mucosal linings. The nearest part of each tube to the ovary is called the ampulla, and opens via a flower-like expansion, the infundibulum, into the abdominal cavity. The mucosal chapter 10

lining of the ampulla extends into the fringe-like fimbriae lining and Renal, Urinary extending from the infundibulum, one of which, the ovarian fimbria, is and Reproductive attached to the inner surface of the ovary. The ampulla joins to the isthmus Tract Systems at the ampullary-isthmic junction, and the isthmus reaches the uterus at the uterotubal junction, with its last part, the intramural portion, running inside the wall of the uterus, entering the cavity of the uterus at the small uterine ostium. The oviducts and ovaries are suspended in peritoneal folds

(the broad ligaments of the uterus) and joined to the ovaries by membranes containing smooth muscle cells, the mesotubarium and mesosalpinx.

The oviducts function to capture the released ova, and transport them to the uterus. The ova are, however, retained in the oviducts for three to four days, to increase the chances of them meeting a sperm and becoming fertilised. In situ observation of the human oviduct shows a great deal of contractile activity, which varies depending on the state of the menstrual cycle. The contractile activity of the smooth muscle cells in the mesotubarium and mesosalpinx alters the anatomical relationship between the oviducts and their fimbriae with the ovaries, and this may assist capture of released ova. The activity in the ampulla and isthmus of the oviducts is not synchronous, and there is no synchrony between the two oviducts. The contractions in the oviduct can run as waves along the tubes. In the ampulla, the waves most often run towards the ovaries, whereas in the isthmus the waves normally run towards the uterus. There is no structural sphincter at the ampullary-isthmic junction, although it is at this point that the egg is delayed in its passage to the uterus. It seems likely that activity in the smooth muscles provides a physiological sphincter.

Although, as has been mentioned earlier, animal experiments show that transport and fertilisation of the ovum can occur normally in denervated organs, there is nevertheless a dense innervation of the oviducts, particularly at the ampullary-isthmic junction, and it seems likely that nervous activity plays a role in controlling the smooth muscle activity. An excitatory sympathetic input reaches the tubes from the hypogastric plexus along the ovarian and uterine arteries, and forms its densest innervation at the isthmus, close to the ampullary-isthmic junction. The parasympathetic input is both from vagal fibres (which are thought to supply the lateral part of the tubes) and pelvic splanchnic nerves to the more medial parts. This is probably also excitatory to the non-vascular smooth muscle. It is not certain where the ganglia in the parasympathetic pathway are situated, but they are possibly in the paracervical ganglia. More recently, peptide-containing nerves have been demonstrated in the walls of the oviducts, apparently innervating the smooth muscle cells. The dominant peptide is VIP, and the VIPergic nerves also densely innervate the isthmus and inhibit smooth muscle activity. The concentration of VIP in the human oviduct is affected by the hormonal status. Sensory nerves run with the adrenergic input.

chapter 10

Renal, Urinary and Reproductive Tract Systems

Uterus

The uterus in the human differs from that of many commonly used experimental animals in that it is a single triangular-shaped dorsoventrally flattened hollow organ, rather than a bicornate one. The uterus is linked to the vagina via the cervix, at the narrow end of the uterus. Moving through the cervix towards the uterus proper, the cervical cavity narrows at the internal os, and then expands into the triangular cavity of the uterus, with the Fallopian tubes entering at the two corners made by the uterine walls with the fundus of the uterus. The thick uterine walls have a thin outer serous layer, a thick muscular layer composed of smooth muscle and an internal mucous layer. The muscular layer is composed of an outer longitudinally arranged layer, a middle layer in which the muscle bundles run in all directions, and an inner layer that has both longitudinal and circularly arranged bundles. The smooth muscle shows myogenic activity, regular slow depolarisations of the cells occurring with superimposed spike activity that initiates contraction (Fig. 10.18). The contractile activity is under hormonal control, which varies during the menstrual cycle and in pregnancy. The mucosal layer, or endometrium, is highly vascular and contains uterine glands. Its thickness and structure change during the menstrual cycle, and it is of the utmost importance in the implantation and development of the placenta. The body of the uterus is innervated by cholinergic, adrenergic and peptidergic neurones, which innervate both the smooth muscle and the epithelial layers. Parasympathetic input is from the second, third and fourth sacral segments, via the pelvic plexus, and sympathetic input from the twelfth thoracic and first lumbar segments, via the hypogastric plexus, entering the uterus along the uterine arteries. The paracervical ganglia contain the cell bodies of many VIP-containing neurones, which innervate the whole of the uterus. Stimulation of the intrinsic nerves in strips of myometrial smooth muscle from non-pregnant non-parous women demonstrates excitation through muscarinic receptors (blocked by atropine) and a-adrenoceptors

Fig. 10.18 Electrical and mechanical activity recorded from strips of smooth muscle from the guinea-pig uterus. The upper trace shows microelectrode recordings of spontaneous myogenic slow waves with superimposed spikes, and the bottom trace shows the contractile response. Trace kindly supplied by H. Parkington.

Fig. 10.18 Electrical and mechanical activity recorded from strips of smooth muscle from the guinea-pig uterus. The upper trace shows microelectrode recordings of spontaneous myogenic slow waves with superimposed spikes, and the bottom trace shows the contractile response. Trace kindly supplied by H. Parkington.

(blocked by phentolamine). There is no evidence of a nerve-mediated inhi- chapter 10 bition of activity Exogenously applied VIP inhibits smooth muscle activity Renal, Urinary in the non-pregnant uterus, and causes vasodilation. This may have a role in and Reproductive menstrual bleeding. Afferent nerves leave the uterus through the hypogas- Tract Systems trie plexus.

During pregnancy, there is an almost total loss of the innervation to the body of the uterus, although the smooth muscle continues to show myogenic activity. Waves of contraction pass along the uterus at regular intervals, but these do not elevate the intrauterine pressure. The size and frequency of these waves change during pregnancy as the hormonal status changes. Towards the end of pregnancy, the smooth muscle becomes increasingly sensitive to oxytocin, which can powerfully enhance this activity, and just before delivery, there is a large increase in the number of gap junctions between the smooth muscle cells, which allows the electrical activity of the cells to be synchronised, thus initiating co-ordinated contractions of the smooth muscle that are necessary to expel the fetus.

Cervix

The innervation of the cervix is similar to that of the body of the uterus, but more dense. In contrast to the uterus, however, its innervation is preserved during pregnancy. There is a particularly high density of VIP-containing fibres, and it has been suggested that they may play a role in the relaxation of the cervix during parturition. These neurones, along with some of the noradrenergic neurones, arise in the paracervical ganglia. The cervix also contains a large number of NPY-containing nerves, some of which are probably adrenergic. The cervix has interesting functions - it facilitates the entry of sperm into the uterus around the time of ovulation, allows the escape of menstrual debris, permits passage of the fetus at delivery, but prevents entry of infectious organisms. Many of its functions depend on the quantity and characteristics of the mucus secreted from the cervical epithelium. This is probably largely controlled by the hormonal state of the body, but the dense innervation and the observation that the transmitter contents also change with the hormonal state suggest that nervous control may also play an important role.

Vagina

This extends from the cervix to the vulva. The outer wall of the vagina is covered in a connective tissue coat, which is continuous with the pelvic fascia. This coat contains striated muscle fibres near the opening of the vagina. The main vaginal wall is muscular, with a thin outer longitudinal smooth muscle layer, and an ill-defined inner circular layer, both interspersed with elastic fibres. The mucosa is thick and non-glandular. It is lined by a stratified but non-keratinised squamous epithelium, which has a high chapter îo

Renal, Urinary and Reproductive Tract Systems

External genitalia

The only part of the external genitalia in which the autonomic innervation plays an important role is the clitoris. Since the structure and mechanisms involved in the erection of the clitoris are very similar to those of the penis, they will not be dealt with here. The importance of the sensory innervation of the clitoris is, however, worth mentioning, since its activation plays a significant role in the pleasure of sex, and appears largely responsible for the initiation of female orgasm. In some societies, female 'circumcision' is practised - a misnomer, since what occurs is excision of the clitoris. This presumably helps to emphasise the subjugation of women in societies that practise it. Orgasm in the female has similarities with the ejaculatory reflex in the male, and also involves reflex activation of muscles and glands, including rhythmic activation of many of the smooth muscles, both of the reproductive tract and the urethra, as well as the striated muscles of the pelvic floor.

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