1. Chapter 17: Sex and the Brain
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. The Hormonal Control of Sex
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

9. The Principal Male and Female Hormones
Men: high concentration of androgens
Women: high concentration of estrogens
Testosterone (androgen) + aromatase  estradiol (estrogen)
Protein hormones do not pass through cell membranes, bind surface receptors.
Steroid hormones pass 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 Sensory Representation 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.