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Exploring Sex and Gender: A Neuroscientific Perspective

This chapter delves into the neural machinery behind reproduction, life-maintaining functions, and how subcortical structures regulate sexual behaviors. It explores the influence of sex hormones on the brain, the differentiation of male and female brains, and the complexities of biological sex versus gender identity. Genetic factors, hormonal control, and brain structures affecting sexual response and mating strategies are also discussed. Neurochemical processes underlying reproductive behaviors, pair bonding, and love are examined, along with the differences in male and female brain structures.

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Exploring Sex and Gender: A Neuroscientific Perspective

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  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.

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