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

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

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

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

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

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)

Differentiation of Fetus and Development of Gonads

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

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.

Concentration of Estradiol Receptors in Sagittal Section of Rat Brain

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

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

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

Neural Control of Human Sex Organs

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

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

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

Pair Bonding in Prairie and Montane Voles

Oxytocin and Vasopressin Receptors in Voles

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.

Imaging Maternal and Romantic Love in Human Brain

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

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

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

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

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

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.

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

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

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

Effect of Lactation on Sensory Representation in Rat Cortex

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

Activational Effects of Steroid Hormones —(cont.)

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

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.