BIO 132

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BIO 132

• Neurophysiology

Lecture 37 Sex and the Brain
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Sexual versus Asexual Reproduction

• Asexual reproduction consists of a cell dividing
  into two genetically identical offspring (clones)
• Bacteria reproduce this way
• Sexual reproduction requires the genetic material
  from two individuals combine to make a new
  genetically unique offspring.
• Advantage
• Increased diversity in the population offers
  protection against disasters
• Disadvantage
• Slower proliferation and is costlier

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Sex and Gender

• The Y-chromosome (only found in men) will change
  the gender from the default female to male.
• The SRY-gene (sex-determining region of the
  Y-chromosome) turns on other sex-determining
  genes.
• Genitalia of men are derived from the same tissue
  as women. The presence of high levels of
  testosterone trigger the transformation of the
  tissue.
• Clitoris becomes the head of the penis
• Labia fuse to form scrotum
• Ovaries descend and become testes

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Sexual Behavior

• Sexual behavior in humans is a complex thing that
  most certainly has powerful genetic and social
  components.
• Not well understood.
• Sexual Behavior in non-human animals have less
  social pressures and is easier to study.

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Monogamy and Promiscuity

• Monogamy (one spouse) a male and female form
  a tightly bound relationship that includes
  exclusive mating with each other.
• Only 3 of mammals practice monogamy
• 12 of primates practice monogamy
• 90 of bird species practice monogamy
• Promiscuity mating with many partners
• Polygyny (many women) and Polyandry (many
  men).
• Polygyny is far more common than is polyandry

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Studying Sexual Behavior

• Choosing an animal model
• Voles (very similar to mice) were chosen to be
  studied because there are two species that are
  very similar (physically and genetically) in most
  respects except that one species uses promiscuous
  and the other uses monogamous sexual behavior
• Prairie Vole Monogamous
• Male forms a tight pair-bound with female right
  after mating
• Both parents live in same nest and raise
  offspring
• Montane Vole Promiscuous
• Male forms no pair-bound and tries to mate with
  many females
• Male lives in an isolated nest and takes no part
  in raising offspring
• Females care for offspring only briefly

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Brain Differences in Voles

• Studies looking at the size of different brain
  areas were not significantly different between
  the two species.
• Studies looking at differences in receptors found
  little difference between the two species except
  for the receptor locations of oxytocin and
  anti-diuretic hormone (ADH).

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Oxytocin/ADH Receptor Mapping
Montane Voles
Prairie Voles
Oxytocin receptors
ADH receptors
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Different Neural Pathways Activated

• Different neural pathways are activated by
  oxytocin and ADH in the two vole species.
• Receptor mapping of female Montane voles shows
  plasticity, temporarily looking more like that of
  the Prairie voles (monogamous) during the brief
  period they were raising young.
• Pharmacological studies were needed to further
  test the hypothesis that oxytocin and ADH are key
  components of sexual behavior in voles.

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Pharmacological Evidence

• It was found that in Prairie voles levels of ADH
  rise sharply in males and levels of oxytocin rise
  sharply in females during copulation (sex).
• Further evidence
• Injection of male Prairie voles with an ADH
  antagonist before copulation abolished the
  pair-bonding and paternal care.
• Oxytocin injections had no effect.
• Injection of male Prairie vole without copulation
  but in the presence of a female resulted in the
  forming of a tight pair-bond.
• In Prairie voles, levels of ADH in males and
  oxytocin in females was proportional to the time
  they spent parenting.

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Pharmacological Evidence in Montanes

• Injection of Montane vole males and females with
  either ADH or oxytocin, or injection of the
  antagonists of the hormones had no significant
  effect on sexual behavior.
• The reason is probably that the Montane voles do
  not have the receptors in the right brain areas
  to elicit the behavior seen in the Prairie voles.
• Imagine the implications if injecting a male
  human with a single hormone caused him to form a
  tight life-long pair-bond with the first female
  he saw.

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Gender Differences in Cognition

• Many studies have looked for physical differences
  in the brains of human men and women but nothing
  concrete or conclusive has been found.
• In humans are women better than men at some
  mental tasks and vice versa?
• Evidence seems to suggest that on average
• women are better at certain verbal and language
  skills
• e.g. naming objects of the same color, listing
  words starting with the same letter, verbal
  memory, etc.
• Men are better at spatial and mathematical
  skills
• 3D puzzles, reading maps, learning mazes, etc.

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Cautionary Notes

• Differences in performance between individuals of
  the same gender are far larger than difference
  across the genders.
• Pick any man and woman and there is almost a
  50/50 chance that one will be better at verbal or
  spatial tasks.
• Not all studies reveal the same results.
• It is unclear how much social influence and
  upbringing plays a role.

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Sex Hormones and Behavior

• There are real and significant differences in the
  brains of men and women.
• How do we know this?
• All behavior comes from the brain and men and
  women behave differently. Therefore, there must
  be difference in the brains.
• If genitalia development is very different in men
  and women and strongly effected by testosterone,
  then perhaps brains are as well.

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Masculinization of the Brain

• Evidence from other animals supports the idea
  that testosterone levels during development
  masculinize the brain, leading to male-like
  behavior.
• If a female rat is injected with high levels of
  testosterone during her pregnancy, female rat
  offspring show male-like behavior.
• Mount other females
• Overly aggressive
• Show no maternal instincts
• Mother not affected her brain was already
  developed
• Male offspring not affected they already had
  high testosterone levels

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Masculinization of the Brain (cont)

• Pregnant cows can have twin calves that are male
  and female.
• If some of the testosterone from the male twin
  reaches the female twin during development, she
  will have non-functioning ovaries and show
  bull-like behaviors as an adult.
• Farmers have long recognized this and call the
  female a Freemartin.

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Pathway of Masculinization

• Women have two X sex chromosomes (XX)
• Men have one X and one Y sex chromosome (XY)
• The SRY gene (found only on the Y-chromosome)
  turns on genes and leads to the expression of
  testis-determining factor which turns ovaries
  into testes.
• Testes produce male sex hormones (androgens) like
  testosterone.
• Androgens affect changes in nearly all tissues
  (including the brain) during development.

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Masculinization Errors in Humans

• Androgen insensitivity Males (XY) that have
  testes and produce androgens but lack androgen
  receptors because of a genetic defect on the X
  chromosome. (1 in 13,000 births)
• Individuals appear female
• Have vagina, clitoris, labia (but no
  menstruation infertile)
• Develop breasts and a female body shape
• Individuals behave like a female
• Consider themselves women, dress and act like
  women
• Choose men as sex partners

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Androgen Insensitivity
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Masculinization Errors in Humans

• Congenital adrenal hyperplasia (CAH) Females
  (XX) that have overdeveloped adrenal glands at
  birth that produce an excess of testosterone. (1
  in 13,000 births)
• Extent of the syndrome dependent on the levels of
  testosterone during development.
• Individuals have ovaries but have
  intermediate-sized external genitalia (between
  size of penis and clitoris).
• Surgery and medication are the usual treatments
• Individuals more likely to have male traits
• Tomboyish, aggressiveness, etc.
• Most are heterosexual but a greater proportion
  than average are homosexual.