Nervous and Endocrine Regulation of the Sexual Function
The nervous and endocrine systems take part in regulation of the sexual function together. Sex hormones, produced by the sex glands and the adrenal cortex, get distributed in the entire organism with the bloodstream and form an overall informational background for the regulation of various parts of the reproductive system, including various structures of the nervous system. The so-called “target organs” for each type of hormones contain special cells — “hormone receptors”; molecules of hormones form compounds with the molecular structures of these cells. This mechanism is what hormones use in order to initiate various coincident processes in the nervous, glandular, and other tissues of the body.
In its turn, the production of sex hormones is regulated by the corresponding structures of the central nervous system, namely the hypothalamo-hypophysial complex. In this complex, the hypothalamic structures regulate the activity of the “primary” endocrine gland of the organism — the hypophysis, which in its turn regulates, among other processes, the activity of the sex glands and the adrenal cortex by means of its own hormones.
There are three major groups of sex hormones produced by the sex glands and by the adrenal cortex: androgens (male hormones), estrogens and progesterone (female hormones). The synthesis of sex hormones starts with transformation of cholesterol into progesterone, after that androgens are created from progesterone, and then estrogens are produced from androgens. This succession of hormones’ transformation takes place in organisms of both sexes; and all three groups of hormones are present in the body tissues of representatives of each sex. But depending on sex, i.e. due to sex-specific biochemical and histological differences in the glands structure, hormones that are peculiar to the sex of the organism get primarily accumulated and secreted into the bloodstream.
Numerous electrophysiological experiments on animals demonstrated that virtually all major brain structures take part in providing the complex of sexual behavioral reactions. One can easily understand this if one imagines what a great amount of information from both outside and inside the organism gets into the central nervous system, where it is processed into the output in the form of commands to numerous structures of the body.
Communication between the central nervous system and the genitals is carried out through nervous pathways and by means of the endocrine system.
A certain role in the regulation of the level of sexuality in males is played by the so-called auxiliary sex glands — by the seminal vesicles, in particular. We are going to dwell on this issue a little more.
The seminal vesicles are paired glands of the male reproductive system, which are aligned along the walls of the urinary bladder and connected through their canals with the deferent duct. The secretion of these glands takes part in the production of sperm. Its main component is apparently fructose which is utilized as nourishment for spermatozoids. The walls of the seminal vesicles have a muscle fiber layer which indicates their ability to contract.
Back in the end of the 19th century, experiments on frogs  demonstrated that artificial filling of seminal vesicles with liquid leads to dramatic increase of sex drive. There is indirect evidence that these glands participate in the regulation of sexuality of human beings in a similar manner . However, it was not confirmed in experiments on either humans, or mammals.
In 1978 we attempted to clarify this issue in the course of experiments on chinchilla rabbits by implanting hard objects into their seminal vesicles. According to an accepted working hypothesis, these objects were supposed to press against presumable baroreceptors that send impulses to the brain centers that regulate the sex drive of the rabbits, and this would increase their sex drive.
Our experiments involved measurement of the background sex drive in 8 male rabbits for several days; as its measure we used the number of copulation attempts (male’s sexual mounts on the female) per 30 minutes (we used female rabbits which were not in heat — in order to avoid copulations, as well as to exclude the stimulating influence of sex pheromones and the factor of sexual activity of female rabbits).
Later on we implanted pieces of a PVC rod with the diameter of 2 millimeters and length of 10 millimeters into the both seminal vesicles of the male rabbits. This was performed using thiopental (5 males) and ether-type (3 males) anesthesia.
The tests were resumed within 2 days of the surgeries. The results were evaluated by comparing the average number of sexual mounts during the last three testing sessions before the surgeries — with the average number of such mounts during the first three post-surgery tests.
In order to determine possible influence of a) 2 days interval between the experiments and b) narcotization — on the results of the tests, we conducted the following check tests: five males which were not operated were given a 2-days testing interval, while we injected similar doses of thiopental sodium (40 mg per 1 kilogram of the body weight) into 3 other males that were also not operated. Besides that, we excised seminal vesicles in 5 other male rabbits.
As a result of implantation of alien objects into the seminal vesicles, all male rabbits that underwent the surgery (except for one case when the rod implantation perforated the wall of the seminal vesicle of one of the animals, which resulted in that the average number of mounts remained the same) demonstrated an increase of the average number of mounts by 10.6, 10.3, 5.1, 1.8, 1.6 and 1.1 times respectively (by 4.7 times in average). Despite the fresh surgical sutures on the abdominal wall, 6 out of 8 rabbits demonstrated an increase of the average number of mounts during the first postoperative test compared to three tests prior to the surgery. Four of them demonstrated more than a two-fold increase. All 8 males made the maximum number of mounts during one of the postoperative days.
The results of the check experiments were the following.
After a 2-days interval the level of sex drive in all 5 rabbits decreased slightly.
Narcotization of test animals did not increase the number of mounts as well.
Therefore, the above listed results cannot be explained by the influence of these side factors.
Ablation of the seminal vesicles in 5 rabbits resulted in an insignificant decrease of the sex drive in the case of two of them (1.9 and 1.2 times), and the other three demonstrated a slight increase (2.4; 1.5 and 1.2 times).
Thus the conducted research proved that irritation of the baroreceptors located in the seminal vesicles increases the sex drive in male rabbits, which is expressed in the increase of the number of mounts. Ordinarily, such an impact on the baroreceptors is made by secretion that is produced and accumulated by the seminal vesicles and which afterwards gets excreted during the ejaculation.
At first glance, this conclusion contradicts the results of the experiments on ablation of the seminal vesicles, since the significant decrease of the sex drive that was expected did not occur. Similar results were obtained earlier in experiments on rats [28,64], which led those authors to the conclusion that the regularity observed in frogs is inapplicable to mammals. However, this seeming contradiction disappears if we remember that the seminal vesicles represent only one of several mechanisms of regulation of sexuality. These mechanisms can be divided into those which: a) create a background level of sexuality and b) perform its prompt regulation.
Among the first ones are the impact of sex hormones discussed above, an activating influence of the seminal vesicles that get filled with secretion, a possible inhibitory action of the prostate secret that gets into the bloodstream when there have been no ejaculations for a long time , as well as a stimulating or suppressing influence from the side of the parasympathetic and sympathetic nervous systems.
Among the second ones are congenital or acquired reflexes.
This, of course, is not a complete list of factors that determine sexual behavior of a mature human being. Ethic and moral factors, as well as many others, play a huge role in this case.
The multifactor nature of sexual behavior regulation ensures a high flexibility of the entire reproductive system, including its ability to continue functioning even when some parts of the regulation mechanisms fail. Continuation, in some cases, of sexual activity for a long time after castration serves as the best illustration of the above said.
This also allows a therapist to take various “bypass routes” while treating sexual disorders. Most benefits can be reaped in this case if one uses the knowledge and practical methods that will be discussed in the chapter Bioenergetic Aspects of Sex.