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HSM 330 (12)
Chapter 17

Chapter 17 notes

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Health Services Management
HSM 330
Daolun Chen

Chapter 17: Sex and the Brain Sex and Gender  Gender-specific behaviours result from complex interactions among self-assessment, societal expectations, genetics and hormones  These behaviours are related to gender identity—our perception of our own gender The Genetics of Sex  Females have two X chromosomes, one from each parent  Males have an X chromosome from the mother and Y chromosome from the father o Gender is determined by the X or Y contribution from the father  The X chromosome is significantly larger than the Y chromosome o The X chromosome contains about 1500 genes whereas the Y chromosome contains less than 50  If a female has a defective X gene, there may be no negative consequence if her other X gene is normal. However, any defect in the single X chromosome of the male can lead to developmental defects o These are called X-linked diseases  The smaller Y chromosome has fewer genes and less diverse functions o It contains the gene called the sex-determining region of the Y chromosome (SRY) which encodes for a protein called testis-determining factor (TDF) o A human with a Y chromosome and the SRY gene develops as a male, and without it, the individual develops as a female Sexual Development and Differentiation  How does genotype of the child lead to the male of female development of the gonads? o During the first 6 weeks of pregnancy, the gonads are in an indifferent stage that can develop into either ovaries or testes o The uncommitted gonad possesses two key structures, the Mullerian duct and the Wolffian duct.  If the fetus has a Y chromosome and an SRY gene, testosterone is produced and the Wolffian duct develops into the male internal reproductive system. The Mullerian duct is prevented from developing by a hormone called Mullerian- inhibiting factor  If there is no Y chromosome and no upsurge of testosterone, the Mullerian duct develops into the female internal reproductive system and the Wolffian duct degenerates The Hormonal Control of Sex  Ovaries and testes release hormones and the pituitary gland regulates this release  Sex hormones are crucial to the development and function of the reproductive system and sexual behaviour. These hormones are called steroids. o Steroids are molecules synthesized from cholesterol and have four carbon rings The Principal Male and Female Hormones  Men have higher concentrations of androgens and women have more estrogens  Steroids act differently from other hormones. Some like vasopressin and oxytocin are proteins (cannot cross the lipid bilayer) which act at receptors with extracellular binding sites o Steroids, however, can easily pass through cell membranes and bind to receptors within the cytoplasm, giving them direct access to the nucleus and gene expression  The testes are responsible for the release of androgens (small amounts are also secreted from the adrenal glands) o Testosterone is by far the most abundant androgen o Prenatally elevated testosterone levels are essential for the development of the male reproductive system. Increases in this hormone during puberty regulates the development of secondary sex characteristics o Female concentrations of testosterone is roughly 10% of those found in males  The principal female hormones are estradiol and progesterone, which are secreted by the ovaries o Estradiol is an estrogen; progesterone is a progestin o Estrogen levels increase dramatically at puberty and control the maturation of the female reproductive system and the development of breasts o In women, hormonal fluctuations follow a regular 28-day cycle, whereas for men these fluctuations occur rapidly each day The Control of Sex Hormone by the Pituitary and Hypothalamus  The anterior pituitary gland secretes two hormones that are important for normal sexual development and function: o Luteinizng hormone (LH) and follicle-stimulating hormone (FSH). These are gonadotropins. o LH and FSH represent about 10% of the total cell population  Gonadotropin-releasing hormone (GnRH) causes the release of LH and FSH from the pituitary o Numerous psychological and environmental factors indirectly affect the secretion of gonadotropins from the anterior pituitary  See figure 17.6  In males, LH stimulates the testes to produce testosterone. FSH is involved in the maturation of sperm cells within the testes; this activity also requires testosterone o Because there is no cortical input to the hypothalamus, it is possible for psychological factors to decrease male fertility by inhibiting gonadotropin secretion and sperm production  In females, LH and FSH cause the secretion of estrogens from the ovaries. o In the absence of gonadotropins, the ovaries are inactive (e.g. childhood) o Cyclic variations in LH and FSH levels in adult females cause periodic changes in the ovaries, and the timing and duration of LH and FSH secretion determine the nature of the reproductive cycle (menstrual cycle).  In the follicular phase, hormones (esp. FSH) have the effect of increasing the growth of a small number of follicles, the cavities in the ovaries that enclose and maintain the ova (egg cells).  In the luteal phase after egg expulsion, the small cells that surround the egg undergo chemical changes in a process called luteinisation (depends on LH release) o In the estrous cycle of nonprimates, the luteal phase is much shorter The Neural Basis of Sexual Behaviours Reproductive Organs and their Control  Sexual arousal of adult men and women can result from both erotic psychological and sensory stimuli and from tactile stimulation of the external sex organs  A full sexual response cycle consists of arousal followed by plateau, orgasm, and resolution phases  Neural control of the sexual response comes in part from the cerebral cortex, where erotic thoughts occurs, but the spinal cord coordinates this brain activity with sensory information from the genitals and generates the critical outputs that mediate the sexual responses of the genital structures  Sexual arousal causes certain parts of the external genitals of both women and men to become engorged with blood, and thus to swell o In women it includes the labia and the clitoris and in men it is the penis  The external genitals are densely innervated by mechanoreceptors o Most men who have suffered a complete transaction of the spinal cord at the thoracic or lumbar level cannot generate an erection when their penis is mechanically stimulated o Axons from mechanoreceptors in the penis and clitoris collect in the dorsal roots of the sacral spinal cord and then send branches into the dorsal horns of the cord and into the dorsal columns through which they project toward the brain  Engorgement & erection are controlled primarily by axons of the parasympathetic division of the ANS o Engorgement of the clitoris and penis depend on dramatic changes in blood flow o Parasympathetic nerves ending are thought to release a potent combination of ACh, vasoaactive intestinal peptide (VIP) and nitric oxide (NO) directly into the erectile tissues  They cause the relaxation of smooth muscles cells in the arteries and the spongy substance of the clitoris and penis. Thus the usually flaccid arteries then become filled with blood  As the penis becomes longer and thicker, the spongy internal tissues swell against two thick, elastic outer coverings of connective tissue that give the erect penis its stiffness.  In order to keep the organs sliding easily during copulation throughout the plateau phase, parasympathetic activity also stimulates the secretion of lubricating fluids from the woman’s vaginal wall and from the man’s bulbourethral gland  Completing the sexual response cycle requires activity from the sympathetic division of the ANS o As the sensory axons from the penis or clitoris become highly active, they, together with the activity descending from the brain, excite sympathetic neurons in the thoracic and lumbar segments of the spinal cord. o In men, the sympathetic efferent axons then trigger the process of emission  During ejaculation, a series of coordinated muscular contractions expel the seme from the urethra o In women, stimulate adequate to trigger orgasm probably also activates the sympathetic system  Sympathetic outflow causes the outer vaginal wall to thicken, and during orgasm itself, triggers a series of strong muscular contractions o Following an orgasm, some time must pass before another orgasm can be triggered in men. This experience in women tends to be considerably more variable in frequency and intensity o The resolution phase (ends the sexual response cycle) includes a draining of blood from the external genitals through veins, and a loss of erection and other signs Mammalian Mating Strategies  Very common among mammals is polygyny in which the male mates with many femals but the female usually mates with only one male o It has a ‘one-night stand’ quality to it and the male never looks back to check ont he outcome of his many liaisons  Polyandry is where one females mates with many males but the males mate with only that female o This is rare among mammals and vertebrates in general  In monogamy, a male and female form a tightly bound relationship that includes exclusive mating with each other. o Only about 3% of mammalian species are monogamous Why and How Male and Female Brains Differ  Because behaviour depends on the structure and function of the nervous system, we can make the strong prediction that male and female brains are also somehow different – they have sexual dimorphisms  Another good reason to expect that male and female brains different is simply that male and female bodies differ o The body parts taht are unique to each sex require neural systems that have evolved specifically to control them  Body size and general shape also vary with gender and thus somatosensory and motor maps must adjust to fit them Sexual Dimorphisms of the CNS  Few dimorphic neural structures are related to their sexual functions in an obvious way  One structure that is related is the collection of spinal motor neurons that innervates the bulbocavernosus (BC) muscles surrounding the base of the penis o They have a role in penile erection and help to eject uring o Both men and women have a BC muscle o
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