Sex determination in humans

Report
Sexual arousal and intercourse
• humans
The physiology of the sexual response
Two basic physiological responses:
• Vasocongestion (penile and clitoral erection, breasts)
• Myotonia (flexion and contraction of muscles)
four-phase model
William H. Masters
and Virginia E.
Johnson (1915 - 2001,
and 1925 - )
•
•
•
•
Excitement
Plateau
Orgasm
Resolution
excitement
in women
– Vaginal lubrication:
– Inner 2/3 of vagina expands
– Labia majora flatten and move
apart
– Labia minora and clitoris enlarge
– Contraction of small muscle
fibers in nipples
in men
– Penis become engorged
– Erection of penis (variable)
– Scrotal skin tightens
Plateau
•women
–Prominent vasocongestion
in outer 2/3 of vagina
cause tissue to swell
–Clitoris pulled back
against pubic bone
• men
–Full erection of penis
–Muscular tension
–Cowper’s glands secrete
the pre-ejaculatory fluid
–Testes are pulled up
closely against the body
Orgasm
• women
– Contractions at 0.8
second intervals
– Contractions of vagina,
uterus and anal
sphincter
–
• men
– Rhythmic contractions
of genital ducts,
muscles at the base of
the penis, and penis
– May be followed by
ejaculation of semen
resolution
– Return to pre-orgasmic state
Sexual Response Cycle
Problems with Masters & Johnson
Theoretical Model
• Model does not allow for individual variation, and suggests
that most people proceed smoothly through discrete stages.
• Model focuses on orgasm as the climax of a sexual encounter;
may deemphasize other forms of sexual pleasure, and
pressure couples to meet this sexual “goal”.
• Model is physiological, and does not include cognitive or
emotional aspects of sexuality.
– Implication may be that sexual function (and dysfunctions)
are mainly physiological, rather than psychological or
relational
Human sexual response models
Other models
• Kaplan’s 3-stage model
• Desire
• Vasocongestion
• Muscle contractions
(orgasm)
Human sexual response models
Walen and Roth’s cognitive model
Emphasizes the thoughts and feelings that must
occur for physiological arousal to happen.
1. Perception of a stimulus: “That’s sexy!”
2. Positive evaluation of perception: “I like
that!”
3. Physiological Arousal
4. Perception of Arousal: “I’m turned on!”
5. Positive evaluation of arousal: “in the mood!”
6. Sexual Behaviors
7. Perception of Behaviors: “We’re getting it
on.”
8. Evaluation of Behaviors: “That was fun!”
Frequency of Female Orgasm
(Western populations)
35
30
25
26
25
5
0
10
Rarely
10
Sometimes
15
Frequently
20
Always
% Frequency of Orgasm
35
5
Never
How do women achieve orgasm?
• 95% women said could
orgasm easily with
masturbation
• Women masturbate through
manual stimulation of clitoris
• 1.5% through vaginal insertion
alone
• Little cross-cultural
information
Multiple Orgasms?
• Return to the plateau phase,
rather than resolution after
orgasm
• 14% of women have frequent
multiple orgasms
• 8-15% of younger adult males
and 3% of adult men
• Ejaculation seems to prevent
return to plateau stage
Spare slides
Sex Determination in humans
Chromosomes
Chromosomal sex
Regulator Genes
Genetic sex
Prenatal Hormones
Genital sex
External &
internal
genitalia
Phenotypic sex
Gonads
Brain
structures
Gonadal sex
Brain sex
Gonads and internal genitalia
SRYgene
Development of testes
Mullerian Inhibiting
Factor & others
Testosterone
Müllerian Duct degenerates
Wollfian duct develops
Epididymis
Vas Deferens
Seminal Vesicles
Prostate Gland
Male Differentiation
Testosterone (from testes)
Wolffian Duct develops
Epididymis
Vas Deferens
Seminal Vesicles
Prostate Gland
Male Differentiation
SRY gene
Mullerian Inhibiting Factor & other factors
Müllerian Duct degenerates
Wnt-4 Gene
• Essential for female development
• Prevents production of testosterone
• Initiates development of Mullerian
duct
• Necessary for proper oocyte
development
• Suppresses Wolffian duct
Female Differentiation
No Y, No
SRY, No MIF
Wnt-4 Gene
Activates Müllerian Duct
Fallopian Tubes
Uterus
Cervix
Inner vagina
Development
of ovaries

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