Chapter 17: Sex and the Brain

1. Neuroscience: Exploring the Brain, 4e Chapter 17: Sex and the Brain

2. Introduction • Neural machinery underlying reproduction • Life-maintaining functions • Regulated by subcortical structures • Conscious control by cerebral cortex • Influence sex hormones on brain • Sexual and reproductive behaviors • Male and female brains

3. Sex and Gender • Concepts of biological sex and gender • Biological characteristics and qualities • Gender-specific behaviors • Introspection • Upbringing • Societal expectations • Genetics, hormones • Gender identity • Perception of gender

4. The Genetics of Sex • Genotype: male = XY, female = XX • X chromosome larger than Y • X contains 800 genes, and Y contains 50 genes. • X-linked diseases • Occur more often in men than women • SRY: gene on Y chromosome

5. The Genetics of Sex—(cont.) • Discovery of SRY • Location of SRY on Y chromosome • Encodes testis-determining factor (TDF) • Causes development of testes and testicular hormones • Makes fetus develop as male • Without SRY, female development

6. Sex Chromosome Abnormalities • Rarely, too few, or too many sex chromosomes • Minimal to lethal health consequences • Turner syndrome • Partial or complete absence of one X chromosome in a female (XO genotype) • Klinefelter syndrome • Extra X chromosome in a male (XXY genotype)

7. Differentiation of Fetus and Development of Gonads

8. Hormones regulate physiological processes. Sex hormones: steroids Endocrine glands release sex hormones. Pituitary gland regulates endocrine glands. Hormone structure affects function. Male hormone: testosterone Female hormone: estradiol The Hormonal Control of Sex

9. The Principal Male and Female Hormones • Men: high concentration of androgens • Women: high concentration of estrogens • Testosterone (androgen) + aromatase  estradiol (estrogen) • Protein hormonesdo not pass through cell membranes, bind surface receptors. • Steroid hormonespass through cell membrane, bind to cytoplasmic receptors. • Receptor concentrations vary in different brain regions.

10. Concentration of Estradiol Receptors in Sagittal Section of Rat Brain

11. Principal Male and Female Hormones—(cont.) • Males: testes release androgen • Testosterone increase at puberty leads to development of secondary sex characteristics. • Females: ovaries secrete estradiol (estrogen) and progesterone (progestin) • Estrogen increase at puberty leads to maturation of female reproductive system and development of breasts. • Blood concentrations of sex hormones vary. • Males—levels fluctuate rapidly each day • Females—levels fluctuate in 28-day cycle

12. Pituitary and Hypothalamus Control of Sex Hormones • Gonadotropins: LH and FSH • Males—LH stimulates testosterone production; FSH aids sperm maturation. • Females—LH, FSH cause estrogen secretion. • Menstrual cycle: follicular phase, luteal phase • Estrous cycle in non-primates

13. Reproductive Organs and Their Control • Sexual response cycle • Arousal, plateau, orgasm, resolution • Cerebral cortex • Neural control of sexual response • Spinal cord • Mediates sexual response of genitals • External genitals • Males—penis • Females—labia and clitoris

14. Neural Control of Human Sex Organs

15. Reproductive Organs • Role of parasympathetic division of the ANS • Neurotransmitters relax smooth muscle in penis and clitoris. • Acetylcholine, vasoactive intestinal polypeptide (VIP), and nitric oxide (NO) • Activity from the sympathetic division of the ANS • Male orgasm: muscular contractions • Ejaculation • Female orgasm: muscular contractions

16. Mammalian Mating Strategies • Polygyny • Male mates with many females. • Polyandry • Female mates with many males. • Monogamy • Male and female mate exclusively. • Different mating strategies in humans in various cultures and eras—mostly monogamy

17. The Neurochemistry of Reproductive Behavior • Pair bonding • Prairie voles: social and monogamous • Montane voles: asocial and promiscuous • Role of oxytocin and vasopressin receptors in reproductive behaviors and bonding • Prairie voles: more oxytocin (female) and vasopressin (male) • Montane voles: fewer receptors for oxytocin and vasopressin

18. Pair Bonding in Prairie and Montane Voles

19. Oxytocin and Vasopressin Receptors in Voles

20. Love, Bonding, and the Human Brain • Human plasma oxytocin levels increase • During breastfeeding in mothers • During sexual intercourse in men and women • Brain activation demonstrates strong reinforcing nature of partner and parental relationships. • fMRI studies: Oxytocin and vasopressin play roles in human bonding.

21. Imaging Maternal and Romantic Love in Human Brain

22. Why and How Male and Female Brains Differ • Sexual dimorphisms of the CNS in humans • Onuf’s nucleus—motor neuron pool • Sexually dimorphic nucleus (SDN) • INAH

23. Sexual Dimorphisms of Cognition • Evolutionary explanations for cognitive dimorphisms • Possible role of hormonal differences • Innate versus differences in experience • Larger differences between individuals than between sexes

24. Sex Hormones, the Brain, and Behavior • Steroids • Alter membrane excitability, sensitivity to neurotransmitters, neurotransmitter release • Modulate functions of various enzymes, channels, and transmitter receptors • Diffuse across outer cell membrane • Bind to specific steroid receptors in cytoplasm and nucleus

25. Sex Hormones, the Brain, and Behavior—(cont.) • Organizational effects of hormones • Development, anatomy • Tend to be irreversible • Activational effects of hormones • Behaviors • Tend to be temporary

26. Masculinization of the Fetal Brain • Role of elevated testosterone levels • Response of fetal brain to maternal circulating hormones • α-Fetoprotein in fetal blood binds estrogens and protects female fetus from masculinization. • Role of prostaglandins downstream from sex hormones • Rat experiments with COX inhibitors • Reduced copulatory behavior in male rats • Male-like copulatory behavior in female rats

27. Mismatches between Genetic Sex and Hormone Action • Normally, genetic sex determines hormonal function. • Treatment with testosterone in early development of mammals  reduced female reproductive behaviors • Androgen-insensitive genetic males: defective androgen gene on X chromosome • Congenital adrenal hyperplasia in genetic females: abnormally large adrenals overproduce androgens • External genitals intermediate between normal clitoris and penis.

28. Direct Genetic Effects on Sexual Differentiation of the Brain • Gynandromorph: having both male and female tissues • Gynandromorphic zebra finch (naturally occurring) • Brain: female left side, male right side • Differential gene expression • Role of genes in complex sex behaviors • Fruit flies • Sexual behaviors encoded in genes • The fru and dsx genes

29. The Activational Effects of Sex Hormones • Temporary modifications in brain organization, structural changes in neurites • Men: anticipation of sex  rise in testosterone, fall in testosterone associated with decreased sexual interest • Women: rise in estrogen  increased sexual interest

30. Brain Changes Associated with Maternal and Paternal Behavior • Changes in pregnancy • Rise in leptin levels • Somatosensory map plasticity in rats during lactation and nursing • In males, interaction with offspring may alter brain structure. • Density of dendritic spines • Increased vasopressin receptors

31. Effect of Lactation on SensoryRepresentation in Rat Cortex

32. Activational Effects of Steroid Hormones • Estradiol treatment has dramatic effects on hippocampal neuronal dendrites. • Increase in hippocampal spine numbers coincides with female rat’s peak fertility. • May play a role in changing reproductive needs • Estradiol’s protective effect on neurons (in culture) • In humans, estrogen: • May protect against stroke • May benefit women with multiple sclerosis

33. Activational Effects of Steroid Hormones—(cont.)

34. Sexual Orientation • Unrelated to activational effects of hormones in adults • INAH-3 size differences • Results difficult to interpret

35. Concluding Remarks • Male and female nervous systems not obviously different • Apparent sex differences in cognition not known to have neurobiological basis • Most human behaviors not distinctly masculine or feminine • Reproduction demands sex-specific behaviors. • Sex hormones have crucial role in sexual development and behavior.