1. Topic 18 Sex and the Brain Lange
Biology Neurobiology
Topic 18
Sex and the Brain
Lange

2. Introduction
Influence of brain on sex hormones…. and the influence of the sex hormones back on the brain.
Sexual and reproductive behaviors
Male and female brains

3. Sex and Gender
Concepts of Gender are very much inter-realated, but will involve specifics of:
Biological characteristics of gender associated with architecture:
Morphology
Organization of the brain
Gender features associated with psychology and perception:
Self-assessment
Societal expectations
Gender-identity
Perception of gender
Gender features associated with biochemistry:
Genetics
Hormones

4. Some chromosomal systems of sex determination
44 + XY XX
22 + X Y
Parents
Ova
Sperm
Zygotes
(offspring)
(a) The X-Y system
(b) The X-0 system
(c) The Z-W system
76 + ZW ZZ 32
(Diploid) 16
(Haploid)
(d) The haplo-diploid system
Female = default sex
Male = default sex

5. Sex and Gender The Genetics of Sexual Determination
Y Chromosome encodes testis-determining factor (Tdf)
Tdf is produced from the SRY gene on Y chromosome
Guides development of testes and production of testicular hormones
Promotes fetal development as male

6. Sex and Gender
Differentiation of fetus and development of gonads

7. The Hormonal Control of Sex
Hormones regulate physiological processes
Sex hormones: Steroidal hormones
Endocrine glands: Gonads release sex hormones
Pituitary gland: Regulates endocrine glands (LH & FSH)
Small structural differences affects function of hormone
Primary “male” hormones: androgens (testosterones)
Primary “female” hormones: estradiols and progestins

8. The Hormonal Control of Sex
The Principle Male and Female Hormones in adulthood
Men: High concentration of androgens
Women: High concentration of estrogens & progestins
Aromatization Process for Steroid Hormones:
Testosterone (androgen) + aromatase  estradiol (estrogen)

9. The Hormonal Control of Sex
Concentration of estradiol receptors in sagittal section of rat brain

10. The Hormonal Control of Sex
Males: Testes- release androgens
Androgens (testosterones) – increase at puberty leads to development of secondary sex characteristics
Females: Ovaries- secrete estradiol (estrogens) and progesterone (progestins)
For both sexes, 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

11. The Hormonal Control of Sex
Pituitary and Hypothalamus Control of Sex Hormones
Gonadotropins: LH and FSH
Males- LH stimulates testosterone; FSH aids sperm maturation
Females- LH, FSH cause estrogen secretion

12. The Neural Basis of Sexual Behaviors
Mammalian Mating Strategies:
Polygyny
Male mates with many females
Polyandry
Female mates with many males
Monogamy
One mate

13. 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

14. 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

15. Why and How Male and Female Brains Differ
Sexual Dimorphisms of the CNS
Sexually dimorphic nucleus (SDN)
INAH in humans analogous to rat SDN

16. Why and How Male and Female Brains Differ
Organizational effect of hormones
Tend to be irreversible
Activational effect of hormones
Tend to be temporary

17. Why 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.

18. 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.

19. END.