There are sexually differentiated cognitive behaviors and processes in many species (including the human being): parental behavior, aggressiveness and territoriality, regulation of intake and body weight, social behaviors, learning and memory, etc.
These physiological, behavioral and cognitive differences between males and females result, at least in part, from the sexual differentiation of the central nervous system made by sex steroids.
- 1 sex hormones
- 2 Oxytocin and vasopressin
- 3 Sexual differentiation of the nervous system
- 4 Sexually dimorphic nervous system structures
- 5 Structural sexual differences in the human brain
- 6 Pheromones and vomeronasal organ
- 7 How do pheromones affect humans?
Sex hormones, which include progestogens, androgens and estrogens, are steroids derived from cholesterol.
In mammals, steroid hormones usually come from the gonads and adrenal glands.
Cholesterol, in the adrenal gland and gonads, in response to different peptide hormones of the adenohypophysis, is transformed into the pregnenolone steroid hormone. This will be the precursor of progesterone (steroid hormone that will be the precursor of the rest of steroids).
Sex hormones are steroids secreted by the gonads, and also by the cortex of the adrenal gland.
Cholesterol is the common precursor of both adrenal and gonadal steroids.
Androgens are the sex hormones of masculinizing action secreted by the adrenal cortex, the testicles, and, with a small amount, by the ovaries.
Testosterone, androstenedione, 5- a-dihydrotestosterone and 5- b-dihydrotestosterone are androgens of great importance within the sexual development of male mammals. These hormones are produced by Leydig testicular cells.
Androgens are male sex steroid hormones.
All estrogens come from androgens: different ovarian enzymes convert testosterone and androstenedione into estrogens, through a process called aromatization.
Thus, 17 b-estradiol, estrone and estriol are the three natural estrogens. For therapeutic use (contraception, menopause disorders, lactation inhibition, female hypogonadism, osteoporosis, palliative treatment of breast cancer and prostate) there are semisynthetic estrogens (ethinyl estradiol, mestranol), and also non-steroidal synthetic estrogens, such as diethylstilbestrol.
Estrogens are secreted, especially in the ovary, testicles and adrenal cortex. Androgens are produced in the ovaries and are immediately converted into estrogens. However, some androgens can pass into the bloodstream without having aromatized.
The blood flow of the ovary moves the androgens produced through the interstitial granular cells. In these cells, androgens are converted into estrogens.
Estrogens, in addition to their influence on sexual and reproductive behavior and on the development of secondary sexual characteristics in females, are able to influence the metabolism of water (promote fluid retention) and calcium (regeneration and bone growth).
For example, estrogens intervene in the regulation of the conditioning menstrual cycle of the endometrium to receive the fertilized egg.
Androgens are the precursors of all estrogens.
Sex steroids of the adrenal gland
The adrenal cortex produces sex steroids. These hormones are very similar, structurally, to those of other adrenal steroids, such as glucocorticoids and mineralocorticoids.
For example, Androstenedione is a sex hormone, secreted by the adrenal cortex, which is involved in the development of body hair.
Some sex steroids are secreted by the cortex of the adrenal gland.
Oxytocin and vasopressin
It has been found that the neurohypophytic hormone oxytocin has a regulatory role on the sexual and parental behavior of mammals.
It has been described that oxytocin facilitates the formation of affective bonds in promoting tactile contacts between subjects.
Vasopressin is involved in sexual behaviors related to the establishment of hierarchies of social dominance. It has also been proven that tactile stimulation serves as a trigger for the release of this hormone.
Vasopressin and oxytocin are two hormones secreted by neurohypophysis, involved in sexual and parental behavior.
Sexual differentiation of the nervous system
Different brain structures have a primary role in the control of reproductive behavior, of gonadal function and even ovulation. Sexual differences, dependent on gonadal hormones, have been described in some of these structures.
Thus, for example, a classic experiment, done in 1936 by Carroll Pfeiffer, revealed the relationship between hormones and sexual differentiation of the nervous system. This researcher implanted female baby testicles of rats. Pfeiffer saw that the ovulation of these animals was permanently blocked. Therefore, the secreted products of the implanted male gonads were able to inhibit normal sexual behavior in females such as ovulation because the testicular hormones had changed the differentiation of the brain.
In 1959, Phoenix and colleagues based on studies on early exposure to testosterone during sexual development and their subsequent influence on sexual behavior, proposed that sexual steroids could have two different effects:
- Organizing Effects: These hormones would act during early developmental periods by organizing the neural structures and pathways involved in sexual and reproductive behavior.
- Activating effects: when the subject is an adult, sex steroids would fulfill an activating role of previously organized behaviors.
In 1980, Goy and McEwen distinguished three types of dimorphism:
- Type I: differentiation where hormones organize, during early developmental periods, different tissues, and generate an activating effect during periods of adulthood.
- Type II: dimorphism where only the activating effect of hormones occurs.
- Type III: Dimorphism related to behaviors that need the organizing effects of hormones but not of activators to be carried out.
There are differences in the structure of the nervous system between males and females. Normally, sexually dimorphic nerve structures agglutinate in the hypothalamus anterior, around the third ventricle.
Certain sexually dimorphic behaviors require the organizing action of gonadal hormones during development and their activating action during adulthood.
The effects of gonadal steroids on the nervous system and behavior are carried out during critical periods, where there is maximum susceptibility for the action of these hormones on different cell types involved in the control of sexually dimorphic behaviors (different in males and females). ).
The perinatal period is critical for sexual differentiation, both in males and females.
Mechanism of action of gonadal steroids on the nervous system
In neurons, testosterone can be converted into estradiol and affect the gene processes that would alter the neural circuits during development, both in males and females.
Gonadal steroids affect the brain, acting in two ways:
- Non-genomic action: gonadal steroids act directly on the membrane of the presynaptic or postsynaptic neuron, altering the synthesis, release or collection of a specific neurotransmitter. It would be related to the activating effects of sex hormones.
- Genomic action: modifies the expression of genes, acting through receptors inside the neuron. It would be related to the organizing effects of sex hormones.
Estrogens are the main factors that mediate the differentiating effects of gonadal hormones on the brain.
Exposure to testicular steroids during development
Mammalian perinatal exposure to testosterone promotes the differentiation of patterns of male sexual behavior typical of the species.
The masculinization of sexual behavior and preference for the couple require a long period of exposure to testicular steroids; Thus, testosterone is needed during fetal life and during the period after birth.
During early development, in the presence of androgens, the brain will be organized so that at adult age male sexual behaviors develop. In the absence of these male steroids, the nervous system will be structured to give rise to female sexual behavior.
Some sexual behaviors do not respond to early androgen exposure, but exposure to these hormones at adulthood will generate dimorphic changes. Other sexual behaviors need the action of testicular steroids during development (organizing effects) and during adulthood (activating effects).
For example, the behavior of mating to male rats, for example, follows a pattern of type I dimorphism. Testicular steroids organize the nervous system of males to enable subsequent mating behavior. But if a male's testicles are removed at adulthood, his copulatory behavior will be inhibited unless he is given testosterone.
Exposure to testicular steroids during development generates sexual differences in the nervous system: Estradiol from the aromatization of testosterone seems to be primarily responsible for the male differentiation of the nervous system.
Estrogen effects on sexually dimorphic brain characteristics
As we have seen so far, the metabolism of testosterone into estradiol is a necessary condition for masculinization of the brain. It also seems that, in the absence of these sex steroids, the differentiation of the brain is female.
During pregnancy, the gonads and the placenta release a large amount of estrogen in the blood. Also, just after birth, plasma estrogen levels are quite high.
There is a liver protein in the blood and in the cerebrospinal fluid, a -fetoprotein, which is able to bind to estrogen, avoiding its masculinizing effect on the brains of females. However, in males, testosterone (which does not bind to a -fetoprotein) can reach the Central Nervous System, penetrating neurons and being metabolized into estradiol to exert its masculinitzadors effects on the brain.
Estradiol also has effects on neuronal morphology:
During development, this sex steroid increases neuritic growth and branching of dendrites.
Estrogens have a masculinizing action to the brain.
Sexually dimorphic nervous system structures
There are different aspects of sexual dimorphism in the nervous system: differences in the number or size of neurons in specific areas, neuronal shape, synaptic density, neurotransmitters used, etc.
Preoptic area of the hypothalamus
In 1978, Gorski et al. Found that in the preoptic area of the rat hypothalamus there was a nucleus that was larger in males than in females. This nucleus was called the sexual nucleus of the preoptic area (NSD).
Several experiments have shown that androgens secreted just after birth are responsible for this structural difference in the NSD, between males and females.
It seems that aromatized testosterone is responsible for the masculinization of the sexual-dimeric nucleus of the preoptic area of the hypothalamus.
Anteroventral Periventricular Nucleus
The anteroventral periventricular nucleus is one of the few nuclei that is larger in female rats than in males. However, size differences only become plausible from puberty.
Both the administration of testosterone to females and the castration of baby males, eliminates this difference in the number of neurons in this nucleus.
The anteroventral periventricular nucleus is greater in female rats than in males.
In the lumbar segment of spinal cord Of the rodents there is a nucleus, the spinal nucleus of the bulbocavernosum (NEB), which is a small set of motor neurons. This spinal nucleus is almost absent in the brains of females.
In both sexes, NEB neurons are present at birth, but after the first week of life they disappear into the female brain.
In humans, the region corresponding to the NEB is called the Onuf nucleus and is located in the sacral spinal cord. This nucleus is divided into two cell groups: the ventrolateral and the dorsomedial. Men have a greater number of neurons in the ventromedial cell group than women.
Testosterone acts on the pelvic musculature, promoting the survival of bulbocavernosal spinal nucleus neurons in the spinal cord of male rats.
Structural sexual differences in the human brain
Structural differences according to sex have been found in the human brain. However, both these differences and their physiological importance are smaller than in the case of rodents.
Thus, for example, the sexdimorphic nucleus of the hypothalamic preoptic area or the interstitial nucleus of the anterior hypothalamus are larger and with a greater number of neurons in men than in women. On the contrary, it has also been observed that the back of the hard body (splenium) and the anterior commissure are selectively longer in women than in men.
There are fewer sexual-dimorphic differences in human brain structure than in other mammals.
Pheromones and vomeronasal organ
The pheromone concept was created in 1959 by M. Luscher and P. Karlson, to designate the chemical messages that generally affect development, reproduction and behavior.
Pheromones are nonvolatile molecules secreted by specialized epithelial glands, which provide signals between males and females of numerous species. These substances carry out various social functions such as, for example, communication between mothers and young, the demarcation of the territory or the attraction between individuals, among others.
Pheromones are detected by sensory receptors located in the vomeronasal organ, the axons of which they make the first synapse in the olfactory bulb accessory. The accessory olfactory bulb is projected towards the cortical and medial nuclei of the tonsil. From this last nucleus it is projected towards the nucleus of the bed of the terminal stria, the preoptic area, the
How do pheromones affect humans?
At first it was thought that humans did not have a vomeronasal organ, but currently it has been found that it is present in the human olfactory system. However, there is no clear evidence about the role and exact functionality of this organ in sexual behavior.
In 1989, an experiment was carried out that had as its setting the waiting room for a dental consultation. The experience consisted of applying a male hormonal substance, androstenol (which is normally diluted in men's axillary sweat), on a chair in the same waiting room. In order to establish a control of the experimental situation the position of the chair was systematically changed and thus avoid preferences for the position of the chair occupied in the specific space of the consultation. It was observed that most women chose to sit the chair that contained androstenol.
Some studies have shown that women who spend more time together are more likely to menstruate once.
In 1971, McClintock described that women who slept in the same room in a university residence had their menstrual cycles synchronized. In 1998, Stern and McClintock saw that the application of sweat from other women with a strip at the base of the nose of volunteers altered the menstrual cycle of the latter, to synchronize with the former.
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Kandel, E.R .; Shwartz, J.H. and Jessell, T.M. (eds) (1997) Neuroscience and Behavior. Madrid: Prentice Hall.
Nolte, J. (1994) The human brain: introduction to functional anatomy. Madrid: Mosby-Doyma.Related tests
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