Nervous and Endocrine Organization of Sexual Function
Nervous and endocrine systems take part in regulation of the reproductive function together. Sex hormones, produced by sex glands and the adrenal cortex, get distributed within 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. So called “target organs” for each type of hormones contain special cells — “hormone receptors”, molecular structures of which form compounds with molecules of hormones. This mechanism is what hormones use in order to initiate various coincident processes in the neurous, glandular, and other tissues of the body.
In its turn, production of sex hormones is regulated by corresponding structures of the central nervous system, namely, the hypothalamo-hypophysial complex. In this complex 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 sex glands and the adrenal cortex by means of its own hormones.
There are three major groups of sex hormones that sexual glands and the adrenal cortex produce: androgens (male hormones), estrogens, and progesterone (female hormones). Synthesis of sex hormones starts with transformation of cholesterol into progesterone, after that androgens are created from progesterone, then estrogens are produced from androgens. This succession of hormones’ transformation takes place in the organisms of both sexes, all three groups being 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, those 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 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 central nervous system and genitals is carried out through nervous pathways and by means of the endocrine system.
A certain role in the regulation of a sexual attraction in males is played by auxiliary sex glands — seminal vesicles, in particular. We are going to dwell on this issue a little more.
Seminal vesicles are paired glands of the male reproductive system that are aligned along the walls of the urinary bladder and connected through their canal with the deferent duct. The secretion of these glands takes part in the production of sperm. Its main component is apparently fructose that gets utilized by spermatozoids. The walls of seminal vesicles have a muscle fiber layer which indicates their ability to contract.
Back in the end of the 19th century experiments on frogs [68] demonstrated that artificial filling of seminal vesicles with liquid leads to dramatic increase of sexual drive. There is indirect evidence that these glands participate in the regulation of sexual drive of human beings in the similar manner [45]. However, it had been confirmed in experiments on neither humans, nor mammals.
In 1978 we attempted to clarify this issue in the course of experiments on chinchilla rabbits by implanting hard foreign objects into their seminal vesicles. According to our working assumption these objects were supposed to press against presumable baroreceptors that send impulses to the brain centers that regulate sexual drive of the rabbits, which in its turn would lead to increase of the latter.
The experiments involved measurement of the background sexual drive in 8 male rabbits for several days, as an indication of which we used the number of mounts per 30 minutes (we used female rabbits out of heat in order to exclude copulations, as well as to eliminate the stimulating influence of sex hormones 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 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 mounts during the last three testing sessions before the surgeries with the average number of mounts during the first three post-surgery tests.
In order to identify possible influence of a) 2 day 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-day testing interval, while we injected similar doses of thiopental sodium (40 mg per 1 kilogram of the body weight) into 3 other males who 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 of seminal vesicle wall perforation caused by the rod implantation in one of the animals — in which case the average number of mounts remained the same) demonstrated an increase of the average number of mounts 10.6, 10.3, 5.1, 1.8, 1.6 and 1.1 times respectively (4.7 times on 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-day interval the level of sexual drive in all 5 rabbits slightly decreased. Narcotization of test animals also did not lead to an increase of the number of mounts. Thus, 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 sexual drive in case of two of them (1.9 and 1.2 times), and the other three demonstrated a slight increase (4; 1.5 and 1.2 times).
The conducted research showed that irritation of baroreceptors located in the seminal vesicles leads to an increase of sexual drive in male rabbits, which is expressed in the increase of the number of mounts. Ordinarily such an impact on 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 sexual drive that was expected did not occur. Similar results were obtained earlier in the experiments on rats [75,93], which led us to conclude that the regularity observed in frogs is inapplicable to mammals. However, this seeming contradiction disappears if we remember that seminal vesicles represent only one of the several mechanisms of regulation of sexual attraction. These mechanisms can be divided into those, which: a) create a background sexual drive and b) perform its operational regulation.
Among the first ones are the impact of sex hormones discussed above, an activating influence of 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 [35], as well as a stimulating or suppressing influence from the side of the parasympathetic and sympathetic nervous systems.
Among other mechanisms of operational regulation of sexual drive 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. (It should be noted that all these factors are profoundly correlated).
The multifactor nature of sexual behavior regulation ensures a high flexibility of regulation of the entire reproductive system, including its ability to continue functioning even when some parts of the regulation mechanisms fail. Continuation of sexual activity for a long time after castration in some cases serves as the best illustration of the above said.
This also allows a therapist to take various “bypass routes” while treating sexual disorders. The most benefits can be derived from this approach if one uses the knowledge and practical methods, which will be discussed in the “Bioenergetic Aspects of Sex” chapter.
In the next chapters we will touch on several other aspects of regulation of the reproductive system.
