Chapter 17 Sex and the Brain

Introduction

Influence of brain on sex hormones

• Sexual and reproductive behaviors

• Male and female brains

Sex and Gender

• Concept of Gender

– Biological characteristics and qualities

– Gender-specific behaviors

• Self-assessment– Societal expectations– Genetics– Hormones

• Gender-identity– Perception of gender

Sex and Gender

The Genetics of Sex

– Encodes testis-determining factor– Location of SRY on Y

chromosome– Causes development of testes and

testicular hormones– Makes fetus develop as male– Default pathway, female

Sex and GenderDifferentiation of fetus and development of gonads

The Hormonal Control of SexHormones regulate physiological processes

– Sex hormones: Steroids

– Endocrine glands: Release sex hormones

– Pituitary gland: Regulates endocrine glands

– Structure affects function of hormone

– Primary male hormones: testosterones

– Primary female hormones: estradiols

The Hormonal Control of SexThe Hormonal Control of SexThe Principle Male and Female Hormones

– Men: High concentration of androgens

– Women: High concentration of estrogens

Aromatization Process for Steroid Hormones:

Testosterone (androgen) + aromatase estradiol (estrogen)

The Hormonal Control of SexConcentration of estradiol receptors in sagittal section of rat brain

The Hormonal Control of Sex

– Males: Testes- release androgen

• Testosterones – increase at puberty leads to development of secondary sex characteristics

– Females: Ovaries- secrete estradiol (estrogens) and progesterone (progestins)

• Blood concentrations of sex hormones vary• Males- levels fluctuate mildly during a 24 hour cycle• Females- levels fluctuate more extensively but on a 28-day cycle

The Hormonal Control of SexThe Hormonal Control of SexPituitary and Hypothalamus Control of

Sex Hormones

– Gonadotropins: LH and FSH

– Males- LH produces testosterone; FSH aids sperm maturation

– Females- LH, FSH cause estrogen secretion

The Neural Basis of Sexual Behaviors

Mammalian Mating Strategies:

– Polygyny• Male mates with many females

– Polyandry • Female mates with many males

– Monogamy • One mate

The Neural Basis of Sexual Behaviors

The Neural Basis of Sexual Behaviors

• The Neurochemistry of Reproductive Behavior

– Prairie voles: Solidly monogamous

– Meadow voles: Asocial and promiscuous

– Affected by oxytocin and vasopressin… Prairie voles exhibit higher levels of vasopressin and oxytocin…. Suggesting a role in the brain

The Neural Basis of Sexual Behaviors

Role of oxytocin and vasopressin receptors in reproductive behavior

• Prairie voles display more oxytocin receptors in females and more vasopressin receptors in males

• Meadow voles, fewer receptors in both sexes

Why and How Male and Female Brains Differ

Sexual Dimorphisms of the CNS

– Sexually dimorphic nucleus (SDN)

– INAH in humans analogous to rat SDN

Why and How Male and Female Brains Differ

Why and How Male and Female Brains Differ

– Organizational effect of hormones• Tend to be irreversible

– Activational effect of hormones• Tend to be temporary

Why and How Male and Female Brains DifferWhy and How Male and Female Brains Differ

An example of the activational effects of hormones… in the somatosensory cortex, the plasticity of the ventral surface for the nipples increases by ~100% to encourage lacation .

This increase in sensation is associated also with increases in prolactin, oxytocin, and a variety of birthing, parental care, and reproductive behaviors.

Why and How Male and Female Brains Differ

Sexual Orientation

– INAH-3 - twice as large in heterosexual males compared to heterosexual females: Sexually dimorphic

– INAH-3 in homosexual males: Similar in size to that in heterosexual females

– INAH – 3 in homosexual females does not appear to differ from heterosexual females

– See Levay, 1991.