BIO Lecture 8 - Sex

• The lecture covers sex and the brain, referencing Ellison Chapter 10 and Kolb & Whishaw Chapter 12

Chromosomes, Genes, and Sex

• Genotype refers to the complete set of genes an organism possesses
• Genes are organized into chromosomes
• Somatic cells contain 23 pairs of chromosomes, 23 from each parent, totaling 46
• The 23rd pair determines sex and influences physical and behavioral sexual characteristics
• Sex chromosomes are either X or Y
• Females have two X chromosomes (XX), while males have one X and one Y chromosome (XY)

Factors Determining Sex

• Genotype (XX or XY)
• Prenatal action of sex hormones
• Postnatal proportion of sex hormones

MAMAWAWA vs WEARBOH

• Men are men and women are women
• Men are women
• Everyone is influenced by hormones

Sex Hormones

• Androgens (testosterone) and Estrogens (estradiol), along with Progesterone, are gonadal hormones
• These hormones are released in differing proportions by both male and female reproductive organs
• Testosterone can be converted into 5-a-dihydrotestosterone by 5-a-reductase
• Aromatase converts testosterone into 17-B-estradiol

Brain Differences

• The preoptic nucleus in rats is initially the same size in both sexes
• In males, estradiol acts on the nucleus, causing it to grow larger than in females
• This is called the sexual dimorphic nucleus, correlates with testosterone levels
• In both sexes, alpha fetoprotein binds estrogen, rendering it inactive
• Estradiol, converted from testosterone by aromatase in the brain, has a masculinizing effect
• INAH3 is larger in males than in females
• The suprachiasmatic nucleus (SCN) contains more neurons in the male brain
• Gonadal hormones influence the brain throughout life, changes in hormone levels affect behavior

Genetic Mutations and Sexual Identity

• Genotypic and phenotypic sex are usually aligned
• Genetic mutations can lead to ambiguous sexual phenotypes
• Turner's syndrome (XO) results in short stature and lack of sexual development in women
• Congenital adrenal hyperplasia (CAH) causes high androgen levels
• This results in masculinization of XX individuals
• Androgen insensitivity syndrome (AIS) makes individuals unresponsive to androgen, leading to feminization of XY individuals

Gender Differences in Sexual Behavior

• Many animal behaviors vary between sexes
• This is referred to as sexually dimorphic behavior
• Female rats display proceptive behaviors (hopping, darting, ear wiggling) and adopt a lordosis posture when ready to mate
• These behaviors are mediated by the ventromedial hypothalamus, which is rich in estrogen receptors
• The female behaviors prompt the male to mount and mate, mediated by the preoptic area of the hypothalamus

Songbirds and Sexual Behavior

• Sexually dimorphic behaviors are designed to attract mates or warn off rivals
• Male songbirds produce complex songs, while females do not
• Male song comprises characteristic repeating elements
• Humans also show differences in attitudes towards sex, with it being a bigger evolutionary investment for females

Amygdala and Sexual Motivation

• Lesions in the hypothalamus stop mating behavior, but interest in receptive females persists
• Monkeys with hypothalamic lesions do not mate but masturbate while watching receptive females
• Rats with hypothalamic lesions press a bar to access females but do not copulate
• Rats with amygdala lesions show no interest in accessing females but copulate if placed in the same cage
• Hypothalamus mediates sexual behavior
• Amygdala mediates motivation

Frontal Lobe and Sexual Behavior

• Sexual behavior involves thinking, dreaming and other cerebral cortex regions
• Visual imagery about sex activates the visual cortex
• Frontal lobe damage can cause loss of sexual inhibition or libido
• May also lead to erotomania
• Lesions to the left dorsolateral prefrontal cortex (DLPFC) may lead to aggressive sexual behavior

Sexual Reward

• Completing sexual behavior results in reward
• Orgasm causes release of dopamine and relaxation due to oxytocin
• Oxytocin promotes pair bonding
• Oxytocin is structurally linked to endorphins
• Agitation felt from separation may be due to lower oxytocin levels

Cognitive Abilities Between the Sexes

• Testing reveals women excel in verbal fluency
• Men do better in spatial reasoning tests.
• One theory suggests men ranged over larger territories
• Greater spatial abilities might have been advantageous
• Also argued that females developed tools for social communication and were left behind
• The neural basis for these differences may include frontal lobe variations and a larger corpus callosum in women
• This suggests women may be more symmetrical

Other Factors

• Pheromones influence behavior
• Pheromones are chemicals that are not consciously perceived as smells
• These activate the amygdala and hypothalamus

Sexual Orientation

• Studies show gay women have asymmetric brains like straight men in 2008
• Gay men have symmetric brains like straight women
• The amygdala of straight women and gay men connects to brain areas involved in mood, fear, and anxiety
• The amygdala of gay women and straight men connects more to the sensorimotor system and basal ganglia
• Sexual orientation and identity may stem from hypothalamic structures
• The suprachiasmatic nucleus (SCN) contains more neurons in male brains
• SCN is larger in gay men than in heterosexual men
• INAH3 is larger in males but smaller in heterosexual male brains
• Sexual orientation is linked to the X chromosome (Xq28)
• Disturbed interactions between brain development and hormones cause orientation
• Social factors can modulate the expression of the genotype