Neuroscience: Exploring the
Brain, 3e
Chapter 17: Sex and the Brain
Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
Introduction
• Neural machinery underlying reproduction
• Reproduction and eating
– Regulated by subcortical structures
– Conscious control by cerebral cortex
• Influence of brain on sex hormones
• Sexual and reproductive behaviors
– Male and female brains
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Sex and Gender
• Concept of Gender
– Biological characteristics and qualities
– Gender-specific behaviors
• Self-assessment
• Societal expectations
• Genetics (XX: female, XY: male)
• Hormones
• Gender-identity
• Perception of gender
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Sex and Gender
• The Genetics of Sex (Cont’d)
–
Discovery of SRY
–
Location of SRY on Y chromosome
–
Encodes testis-determining factor
–
Causes development of testes and
testicular hormones
–
Makes fetus develop as male
–
Default pathway, female
–
Exposure to testosterone
masculinizes the brain
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Sex and Gender
• Differentiation of fetus and
development of gonads
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The Hormonal Control of Sex
• Hormones regulate physiological
processes
– Sex hormones: Steroids
– Endocrine glands: Release sex
hormones
– Pituitary gland: Regulates
endocrine glands
– Structure affects function of
hormone
– Male hormone: testosterone
– Female hormone: estradiol
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The Hormonal Control of Sex
• The Principle 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
cytoplasmic receptors
– Receptor concentrations vary in different brain
regions
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The Hormonal Control of Sex
• Concentration of estradiol receptors in sagittal section of
rat brain
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The Hormonal Control of Sex
• The Principle Male and Female Hormones (Cont’d)
– Males: Testes- release androgen
• Testosterone – increase at puberty leads to
development of secondary sex characteristics
– Females: Ovaries- secrete estradiol (estrogen) and
progesterone (progestin)
• Blood concentrations of sex hormones vary
• Males- levels fluctuate daily
• Females- levels fluctuate, 28-day cycle
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The Hormonal Control of Sex
• Pituitary and Hypothalamus Control
of Sex Hormones
– Gonadotropins: LH and FSH
– Males- LH produces
testosterone; FSH aids sperm
maturation
– Females- LH, FSH cause
estrogen secretion
• Menstrual cycle- Follicular
phase, Luteal phase
• Estrous cycle in
nonprimates
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The Neural Basis of Sexual Behaviors
• Reproductive Organs and Their Control
– Cerebral cortex
• Neural control of sexual response
– Spinal cord
• Mediates sexual response of genitals
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The Neural Basis of Sexual Behaviors
• Neural control of human
sex organs
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The Neural Basis of Sexual Behaviors
• Mating Strategies: Ecology
– Polygyny
• Male mates with many females
– Polyandry
• Female mates with many males
– Monogamy
• One mate
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The Neural Basis of Sexual Behaviors
• The Neurochemistry of
Reproductive Behavior
– Prairie voles: Solid ‘family
values’; monogamous
– Meadow voles: Asocial and
promiscuous
– Affected by oxytocin and
vasopressin
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The Neural Basis of Sexual Behaviors
• Role of oxytocin and vasopressin receptors in reproductive
behavior
• Prairie voles more oxytocin (female) and vasopressin
male receptors
• Meadow voles, fewer receptors
• Experiments: Vasopressin antagonists prevent pairbond formation; increasing vasopressin receptors in
meadow voles promotes pair-bond formation.
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Why and How Male and Female Brains Differ
• Sexual Dimorphisms of the CNS
– Onuf’s nucleus
• Controls motor neuron
pool
– Sexually dimorphic nucleus
(SDN)- in rats, much larger
in males
– INAH in humans analogous
to rat SDN
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Why and How Male and Female Brains
Differ
• Sexual Dimorphisms of Cognition
– Evolutionary explanations for
cognitive dimorphisms
– Role of distinctive hormonal
environment triggers differences
in performance
– Low estrogen levels
• Better performance by women
in spatial reasoning
– Relation between cognition and
hormones
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Why and How Male and Female Brains Differ
• 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
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Why and How Male and Female Brains
Differ
• Sex Hormones, The Brain, and Behavior (Cont’d)
– Prostaglandins - downstream from sex hormones
• Synthesized from arachadonic acid via COX enzyme
• Involved in tissue damage, pain/fever
– Fetal and neonatal rats exposed to COX inhibitors
• Reduced copulatory behavior in male rats
• Male-like copulatory behavior in female rats
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Why and How Male and Female Brains
Differ
• Sex Hormones, the Brain, and Behavior (Cont'd)
– Organizational effect of hormones
• Tend to be irreversible
– Activational effect of hormones
• Tend to be temporary
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Why and How Male and Female Brains
Differ
• Sex Hormones, The Brain, and Behavior (Cont’d)
– Mismatches Between Genetic Sex and Hormone
Action: Treatment with testosterone in mammals,
reduced features of female reproductive behavior
• Androgen-insensitive genetic males: Defective
androgen gene, X chromosome
• Congenital adrenal hyperplasia genetic females:
Abnormally large adrenals overproduce androgens
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Why and How Male and Female Brains Differ
• Sex Hormones, The Brain, and
Behavior (Cont’d)
– Direct Genetic Effects on Sexual
Differentiation of the Brain
– Gynandromorphic zebra finch
(naturally occurring)
• Brain: Female left side, male
right
• Differential gene expression
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Why and How Male and Female Brains
Differ
• The Activational Effects of Sex Hormones
– Temporary modifications in brain organization,
structural changes in neurites
– Men: Rise in testosterone, anticipation of sex, fall in
testosterone, decreased sexual interest
– Women: Rise in estrogen, increased sexual interest
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Why and How Male and Female Brains
Differ
• The Activational Effects of Sex Hormones
– Brain Plasticity and Maternal Behavior
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Why and How Male and Female Brains Differ
• The Activational Effects of Sex Hormones
(Cont’d)
– Estrogens, Neurite Growth, and Disease
– Toran-Allerand – estradiol treatment of
hypothalamic tissue promotes neurite
outgrowth
– Songbirds: Testosterone mediates
seasonal fluctuation in size of sexually
dimorphic vocal-motor regions.
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Why and How Male and Female Brains
Differ
• The Activational Effects
of Sex Hormones
– Gould, Woolley, and
McEwen – estradiol
treatment has
dramatic effects on
hippocampal
neuronal dendrites
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Why and How Male and Female Brains
Differ
• The Activational Effects of Sex Hormones
– Estrogens, Neurite growth, and Disease (Cont’d)
• Increase in hippocampal spine numbers coincides with
female rat’s peak fertility
• May play a role in changing reproductive needs
• Estradiol
• Protective effect on neurons (in culture)
• Hypoxia, oxidative stress, toxins, etc.
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Concluding Remarks
• Most human behaviors
– Not distinctly masculine or feminine
• Reproduction demands
– Sex-specific behaviors
• Sex hormones
– Crucial role in sexual development and behavior
Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
Sex and Gender
• The Genetics of Sex
– Genotype- Male: XY, Female: XX
• X chromosome larger than Y
• X contains 1500 and Y contains 50
genes
– X-linked diseases
• Occur more often in men than women
• SRY: Gene on Y chromosome, encodes
TDF (testis determing factor)
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Why and How Male and Female Brains
Differ
• Sexual Orientation
– INAH-3
• Twice as large in
men: Sexually
dimorphic
– INAH-3 in gay men:
Similar to that in
women
• Significance: Results
difficult to interpret
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End of Presentation
Copyright © 2007 Wolters Kluwer Health | Lippincott Williams & Wilkins
The Neural Basis of Sexual Behaviors
• Reproductive Organs and Their Control (Cont’d)
– Neurotransmitters: Relax smooth muscle
• Acetylcholine, vasoactive intestinal polypeptide
(VIP), and nitric oxide (NO)
– Male orgasm: Muscular contractions
• Ejaculation
– Female orgasm: Muscular contractions
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