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Chapter 13

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Western University
Psychology 2220A/B
Scott Mac Dougall- Shackleton

Chapter 13: Hormones and Sex • Men-Are-Men-and-Women-Are-Women Assumption •The men-are-men-and-women-are-women assumption is the tendency to think about femaleness and maleness as discrete, mutually exclusive, opposite categories • Developmental and Activational Effects of Sex Hormones •Hormones influence in two fundamentally different ways 1. By influencing the development from conception to sexual maturity of the anatomical, physiological, and behavioural characteristics that distinguish on as female or male 2. By activating the reproduction-related behaviour of sexually mature adults NEUROENDOCRINE SYSTEM • Only the organs whose primary function appears to be the release of hormones are referred to as endocrine glands • Glands •There are two types of glands: • Exocrine glands release their chemicals into ducts, which carry them to their targets, mostly on the surface of the body • Endocrine glands release their chemicals, which are called hormones, directly into the circulatory system • Gonads •Gonads = males testes and the females ovaries •The primary function of the testes and ovaries is the production of sperm cells and ova •After copulation, a single sperm cell may fertilize an ovum to form one cell called a zygote, which contains all of the information necessary for the normal growth of a complete adult organism in its natural environment •Sex chromosomes contain the genetic programs that direct sexual developments •Females have two large sex chromosomes, called X chromosomes, in males one sex chromosome is an X chromosome and the other is called a Y chromosome •Once a chromosome has duplicated, the two products remain jointed at one point, producing an X shape •Because the Y chromosomes is much smaller than the X chromosomes, early investigators failed to discern one small arm and thus saw a Y • Classes of Hormones •Vertebrate hormones fall into one of three classes 1. Amino acid derivatives - hormones that are synthesized in a few simple steps from an amino acid molecule 2. Peptides and proteins - chains of amino acids - peptide hormones are short chains, and protein hormones are long chains 3. Steroids - hormones that are synthesized from cholesterol, a type of fat molecule •The hormones that influence sexual development and activation of adult sexual behaviour are all steroid hormones •Once inside a cell, the steroid molecules can bind to receptors in the cytoplasm or nucleus and, by so doing, directly influence gene expression Chapter 13: Hormones and Sex • Sex Steroids • The gonads also produce and release steroid hormones • The two main classes of gonadal hormones are androgens and estrogens; testosterone is the most common androgen and estradiol is the most common estrogen • A third class of steroid hormones called progestins • The most common progestin is progesterone, which in women prepares the uterus and the breasts of pregnancy • The primary function of the adrenal cortex is the regulation of glucose and salt levels in the blood, it is not generally thought of as a sex gland • However, it does release small amounts of all of the sex steroids that are released by the gonads • Hormones of The Pituitary • Pituitary gland most of its hormones are tropic hormones • Tropic hormones are hormones whose primary function is to influence the release of hormones from other glands • Gonadotropin is a pituitary tropic hormone that travels through the circulatory system to the gonads, where it stimulates the release of gonadal hormones • The pituitary gland is really two glands, the posterior pituitary and the anterior pituitary, which fuse during the course of embryological development • The posterior pituitary develops from a small outgrowth of hypothalamic tissue that eventually comes to dangle form the hypothalamus on the end of the pituitary stalk • The anterior pituitary begins as part of the same embryonic tissue that eventually develops into the roof of the mouth • It is the anterior pituitary that releases tropic hormones • Female Gonadal Hormone Levels Are Cyclic; Male Gonadal Hormone Levels Are Steady • The major difference between the endocrine function of women and men is that in women the levels of gonadal and gonadotropic hormones of through a cycle that repeats itself every 28 days or so • It is these more-or-less regular hormone fluctuations that control the female menstrual cycle • Males’ level of gonadal and gonadatropic hormones change little form day to day • Anterior pituitaries are not inherently female or male • Neural Control of the Pituitary • The nervous system was implicated in the control of the anterior pituitary by behavioural research • Visual input to the nervous system was controlling the release of tropic hormones from the anterior pituitary • Hypothalamic stimulation and lesion experiments quickly established that the hypothalamus is the regulator of the anterior pituitary • The anterior pituitary, unlike the posterior pituitary, receives no neural input whatsoever from the hypothalamus, or form any other neural structure • Control of the Anterior and Posterior Pituitary by the Hypothalamus • There are two different mechanisms by which the hypothalamus controls the pituitary: one for the posterior pituitary and one for the anterior pituitary Chapter 13: Hormones and Sex • The two major hormones of the posterior pituitary, vasopressin and oxytocin are peptide hormones that are synthesized in the cell bodies of neurons in the paraventricular nuclei and supraoptic nuclei on each side of the hypothalamus • Harris suggested that the release of hormones form the anterior pituitary was itself regulated by hormones released form the hypothalamus • Two findings provided early support for this hypothesis • The first was the discovery of a vascular network, the hypothalamopituitary portal system, that seemed well suited to the task of carrying hormones from the hypothalamus to the anterior pituitary • The second finding was the discovery that cutting the portal veins of the pituitary stalk disrupts the release of anterior pituitary hormones until the damages veins regenerate • Discovery of Hypothalamic Releasing Hormones • Hypothesized that the release of each anterior pituitary hormone is controlled y a different hypothalamic hormone • The hypothalamic hormones that were thought to stimulate the release of an anterior pituitary hormone were referred to as releasing hormones; those thought to inhibit the release of an anterior pituitary hormone were referred to as release- inhibiting factors • Thyrotropin-releasing hormone triggers the release of thyrotropin from the anterior pituitary, which in turn stimulates the release of hormones form the thyroid gland • Isolation of gonadotropin-releasing hormone stimulates the release of both of the anterior pituitaries gonadotropins: follicle-stimulating hormone (FSH) and luteinizing hormone (LH) • Regulation of Hormone Levels • Regulation by Neural Signals •All endocrine glands, with the exception of the anterior pituitary, are directly regulated by signals from the nervous system •The effects of experience on hormone release are usually mediated by signals from the nervous system •Hormone release can be regulated by experience • Regulation by Hormonal Signals •The hormones themselves also influence hormone release •Circulating hormones often provide feedback to the very structure that influence their release: the pituitary gland, the hypothalamus, and other sites in the brain •The function of most hormonal feedback is the maintenance of stable blood levels of the hormones • Regulation by Nonhormonal Chemicals •Glucose, calcium, and sodium levels in the blood all influence the release of particular hormones • Pulsatile Hormone Release • Hormones tend to be released in pulses; they discharged several times per day in large surges, which typically last no more than a few minutes • Hormone levels in the blood are regulated by changes in the frequency and duration of the hormone pulses Chapter 13: Hormones and Sex • One consequence of pulsatile hormone release is that there are often large minute-to-minute fluctuations in the levels of circulating hormones • Summary Model f Gonadal Endocrine Regulation • The brain controls the release of gonadotropin-releasing hormone from the hypothalamus into the hypothalamo-pituitary portal system, which carries it to the anterior pituitary • In the anterior pituitary, the gonadotropin-releasing hormone stimulates the release of gonadotropin, which is carried by the circulatory system to the gonads • In response to the gonadotropin, the gonads release androgens, estrogens, and progestins which feed back into the pituitary and hypothalamus to regulate subsequent hormone release HORMONES AND SEXUAL DEVELOPMENT OF TEH BODY • Humans are dimorphic • It is the genetic information on the sex chromosomes hat normally determines whether development will occur along female or male lines • We are all genetically programmed to develop female bodies; genetic males develop male bodies only because their fundamentally female program of development is overrules • Fetal Hormones and Development of Reproductive Organs • Gonads • 6 weeks after fertilization each fetus, regardless of its genetic sex, has the same pair of gonadal structures called primordial gonads • Each primordial gonad has an outer covering, or cortex, which has the potential to develop into an ovary; and each has an internal core, or medulla, which has the potential to develop into a testis • Six weeks after conception, the Sry gene on the Y chromosome of the male triggers the synthesis of Sry protein and this protein causes the medulla of each primordial gonad to grow and to develop into a testis • If Sry protein is injected into a genetic female fetus 6 weeks after conception, the result is a genetic female with testes; if drugs clock the effects of Sry protein are injected into a male fetus, the result is a genetic male with ovaries • Internal Reproductive Ducts • Six weeks after fertilization both males and females have two complete sets of reproductive ducts • They have a male Wolffian system, which has the capacity to develop into the male reproductive ducts • And they have a female Mullerian system, which has the capacity to develop into the female ducts • In the third month of male feta development, the testes secrete testosterone an Mullerian-inhibiting substance • The testosterone stimulates the development of the Wolffian system, and the Mullerian-inhibiting substance causes the Mullerian system to degenerate and the testes to descend into the scrotum - the sac that hold the testes outside the body cavity Chapter 13: Hormones and Sex • The development of the Mullerian system occurs in any fetus that is not exposed to testicular hormones during the critical fetal period • Ovariectomy is the removal of the ovaries, and orchidectomy is the removal of the testes • Gonadectomy or castration is the surgical removal of gonads - either ovaries or testes • External Reproductive Organs • Every normal fetus develops separate precursors for the male and female gonads and for the male reproductive ducts; then, only one set, male or female, develops • Both male and female genitals - external reproductive organs - develop from the same precursor • In the second month of pregnancy, the bipotential precursor of the external reproductive organs consists of four parts: the glans, the urethral folds, the lateral bodies, and the labioscrotal swellings • Then it begins to differentiate • The glans grow into the head of the penis in the male or clitoris in the female • The urethral folds fuse in the male or enlarge to become the labia minora in the female • Lateral bodies form the shaft of the penis in the male or the hood of the clitoris in the female • Labioscrotal swellings form the scrotum in the male or the labia majora in the female • The development of external genitals is controlled by the presence or absence of testosterone • Puberty: Hormones and Development of Secondary Sex Characteristics • During childhood, levels of circulating gonadal hormones are low, reproductive organs are immature, and males and females differ little in general appearance • Secondary sec characteristics are those features other than the reproductive organs that distinguish sexually mature men and women •
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