Chapter 10 - Sexual Behaviour Flashcards
SEX CHROMOSOMES VARIATIONS
Sex is determined by the 23rd pair of chromosomes, called the sex chromosomes. Sex chromosomal abnormalities occur rather frequently, yet most of them produce relatively mild differences. Syndromes due to sex chromosome variations include:
1) TURNER SYNDROME (X0);
2) KLINEFELTER SYNDROME (XXY);
3) 47,XYY SYNDROME.
TURNER’S SYNDROME (X0)
TURNER’S SYNDROME is a chromosomal disorder characterised by a 45,X genotype that occurs 1 every 2000 births. This abnormality is caused by NONDISJUNCTION, a mistake during the process of meiosis that produces monosomic cells. Individuals with this syndrome develop typical FEMALE external genitalia, but their ovaries can develop abnormally, leading to infertility. Phenotypical traits of this syndrome include short height and neck deformities. Intelligence is normal, but specific deficits in spatial cognition can occur.
KLINEFELTER SYNDROME (XXY)
KLINEFELTER SYNDROME is a chromosomal disorder characterised by a 47,XXY genotype that occurs 1 every 1000 male births. Individuals with this syndrome are phenotypically MALE, but experience reduced fertility. Phenotypical traits include taller than average height, poor development of secondary sex male characteristic and larger than average breasts. Intelligence is not impaired, although they can show reduced verbal skills.
47,XYY SYNDROME
47,XYY SYNDROME is a chromosomal disorder that occurs 1 every 1000 male births. Individuals with this syndrome develop as phenotypical MALES, even though they tend to be taller, thinner and prone to acne. They do not experience reduced infertility, but below average QI, verbal deficiencies and autism spectrum disorder are common among XYY individuals. Correlation between this syndrome and antisocial behaviour has been the subject of considerable debate.
3 STAGES of PRENATAL SEXUAL DEVELOPMENT
Development of male and female reproductive structures involves 3 distinct processes:
1) development of GONADS;
2) development of INTERNAL ORGANS;
3) development of EXTERNAL GENITALIA.
INTERSEX individuals show elements of both male and female development.
1) development of GONADS
Up until the 6th week of gestation, both male and female embryos have identical gonads that have the capacity to develop into either OVARIES or TESTES. About 6 weeks after conception, the SRY gene ( SEX-DETERMINING REGION of the Y CHROMOSOMES), located on the SHORT ARM of the Y chromosome, is expressed in male embryos. SRY encodes for proteins called TESTIS-DETERMINING FACTORS.
2) development of INTERNAL ORGANS
Up until the third gestational month, both male and females embryos possess a MALE WOLFFIAN SYSTEM and a FEMALE MULLERIAN SYSTEM, which will develop into the internal organs of the reproductive system. During the 3RD gestational month, the TESTES will begin to release of two hormones:
1) TESTOSTERONE, an androgen, which promotes the development of the Wolffian system;
2) ANTI-MULLERIAN HORMONE, which initiates the degeneration of the Mullerian system.
Unlike the testes, the OVARIES are not active during fetal development - in other words, a female development will take place unless interrupted by male hormones.
ANDROGEN INSENSITIVITY SYNDROME (AIS)
AIS is an intersex condition that disrupts the second stage of prenatal sexual development - specifically the normal development of the Wolffian system in genetic MALES . In individuals with AIS a defective gene produces ABNORMAL ANDROGEN RECEPTORS, making the fetus’s tissues blind to the presence of androgens. As a result - since the testes still produce ANTI-MULLERIAN HORMONE - both the WOLFFIAN and the MULLERIAN system will fail to develop.
Individuals with AIS are (1) INFERTILE, (2) GENETIC MALES, and (3) PHENOTYPICAL FEMALES.
This condition is believed to be more common among female athletes and fashion models than in the general population.
3) development of EXTERNAL GENITALIA
The male external genitalia include the penis and scrotum. The female external genitalia include the labia, clitoris, and outer part of the vagina.
No hormonal activity is required to develop female external genitalia.
On the other hand, a particular androgen, 5-ALPHA-DIHYDROTESTOSTERONE - produced by a reaction between TESTOSTERONE and the enzyme 5-ALPHA-REDUCTASE - is essential for the development of male external genitalia.
CONGENITAL ADRENAL HYPERPLASIA (CAH)
CAH is a heritable condition in which the fetus receives abnormally elevated amounts of ANDROGENS during the period of sexual differentiation. While males with CAH show very few observable effects, CAH in female fetuses results in AMBIGUOUS EXTERNAL GENITALIA.
DEVELOPMENT at PUBERTY
At the onset of puberty, males and females release these same hormones, but with different effects. GONADOTROPIN-RELEASING HORMONE (GnRH) is released by the HYPOTHALAMUS, which in turn initiate the release of two hormones by the ANTERIOR PITUITARY GLAND, namely FOLLICLE-STIMULATING HORMONE (FSH) and LUTEINIZING HORMONE (LH).
These two hormones act on the gonads:
1) in MALES, the testes produce additional TESTOSTERONE, which stimulate muscular development, growth of facial hair, and enlargement of the larynx. In conjunction with LH and FSH, testosterone regulates the production of SPERM.
2) in FEMALES, the ovaries produce ESTRADIOL, which produces breast growth, maturity of the uterus. LH and FSH regulate the menstrual cycle.
The testes also produce small amounts of estrogens, including estradiol, and the ovaries produce small amounts of androgens, including testosterone.
SEXUAL DIMORPHISM
Exposure to prenatal androgens masculinIzes the brain as well as the internal and external reproductive systems. Therefore, certain areas of the brain are SEXUALLY DIMORPHIC, meaning that they display structural differences between the sexes.
1) In rats, the SEXUALLY DIMORPHIC NUCLEUS of the PREOPTIC AREA (SDN-POA) , located in the hypothalamus, is much larger in male rats than in female rats. This difference emerges after birth - hormone exposure masculinizes the brain.
The human equivalent of SDN-POA is a set of 4 nuclei known as the INTERSTITIAL NUCLEI of the ANTERIOR HYPOTHALAMUS (INAH). INAH2 and INAH3 are about twice as large in males than in females.
2) In rats, the SPINAL NUCLEUS of the BULBOCAVERNOSUS (SNB) of the spinal cord is richer of neurons in males. This is because in male rats additional neurons in the SNB innervate the penis.
3) Additional structural differences between male and female brains have been discovered in the hypothalamus, thalamus, and in the white matter of the cerebral hemispheres. These differences could be related to sexual behaviour and performance in COGNITIVE TASK: males have been shown to have a slight advantage in VISUOSPATIAL tasks, whereas females have a slight advantage in VERBAL tasks.
SEXUAL ORIENTATION
SEXUAL ORIENTATION refers to a stable pattern of attraction to members of a particular sex. Sexual orientation has been the subject of correlational studies between genes, hormones and brain structures.
1) HORMONES exposure influences adult sexual behavior in animals and humans by producing SEXUAL DIMORPHISM.
If male rats do not receive exposure to testosterone during this critical period, their SDN-POA remains small, and their adult sexual behavior differs from rats receiving typical exposure to testosterone.
In humans, this phenomenon could explain why men who have older brothers are slightly more likely to be homosexual than men who have no siblings, younger siblings only, or older sisters. According to the BIRTH ORDER HYPOTHESIS, The mother’s immune response to pregnancy with a male fetus could become stronger with each pregnancy, thus affecting masculinization of the fetus’ brain.
2) Differences in BRAIN STRUCTURE are correlated to sexual orientation. A landmark study showed that INAH-3 was two to three times larger in heterosexual men than in homosexual men. This results have been criticised because of it methodological limits: sexual behaviour histories of participants were poorly assessed, for the researchers studied autopsied brains.
3) GENETICS appear to influence sexual orientation. The fact that the chances of a homosexual male twin having a homosexual brother are 25% for fraternal twins and 50% for identical twins points to a potential genetic origin of homosexuality.
LIGHT and SEX HORMONES
The HYPOTHALAMUS regulates the release of sex hormones by secreting GONADOTROPIN-REALEASING HORMONE (GnRH), which reach the ANTERIOR PITUITARY GLAND and stimulate the release of LH and FSH.
GnRH release is inhibited from MELATONIN, and melatonin is inhibited by light. Light, therefore, promotes GnRH release and thus sexual behaviour.
In non human species this mechanism provides means for producing offspring at the right time of the year. Some evidence for seasonality of birth rates in humans is provided by the phenomenon of a spring BABY BOOMLET - in countries which experience strong contrasts in seasons in terms of length of days, fertility is higher during the summer.
MOOD, MENSTRUATION and POSTPARTUM DEPRESSION
Some women experience PREMENSTRUAL SYNDROME in response to hormone shifts that regulate the menstrual cycle - symptoms include bloating, breast enlargement, depression and irritability. Severe cases of these symptoms are diagnosed as PREMENSTRUAL DYSPHORIC DISORDER (PMDD).
Within one year of childbirth, approximately 10% of women experience POSTPARTUM DEPRESSION as hormones shift from the pregnant state back to normal monthly cycles.
