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Chapter 1

Chapter 1 Reading Notes.odt

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Diane Glebe

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READING NOTES CHAPTER 1: Biological Transitions Introduction • Puberty is affected by the context in which it occurs • Physical development is influenced by a host of environmental factors, and the timing and rate of pubertal growth varies across regions of the world ◦ Today, in contemporaryAmerica, the average girl reaches menarche 0 the time of first menstruation – at about age 12.5 ◦ In New Guinea, the typical girl does not reach menarche until after 18 years of age • In sum, the most universal aspect of adolescent – puberty – is hardly universal in its impact on the young person Puberty: An Overview • pubertas = “adult” • Puberty has three chief physical manifestations: ◦ a rapid acceleration in growth, resulting in dramatic increases in both height and weight ◦ the development of primary sex characteristics, including the further development of the gonads (sex glands) ▪ which results in hormonal changes that ultimately enable reproduction ◦ the development of secondary sex characteristics, including changes in the genitals and breasts, and the growth of pubic, facial, and body hair • Each of these sets of changes is the result of developments in the endocrine and central nervous system The Endocrine System • Terminology: ◦ The endocrine system produces, circulates, and regulates levels of hormones in the body ◦ Hormones are highly specialized substances that are secreted by one or more endocrine glands and then enter the bloodstream and travel throughout the body ◦ Glands are organs that stimulate particular parts of the body to respond in specific ways ▪ Just as specialized hormones carry “messages” to particular cells in the body, so are the body's cells designed to receive hormonal messages selectively ◦ Gonadotropin-releasing hormone (GnRH) neurons are specialized neurons that are activated by certain pubertal hormones ◦ Pituitary gland is responsible for regulating levels of hormones in the body ◦ Hypothalamus is a part of the brain that controls the functioning of the pituitary gland ◦ Gonads are the glands that secrete sex hormones ▪ Testes in males, ovaries in females • The Hormonal Feedback Loop ◦ The endocrine receives its instructions to increase or decrease circulating levels of particular hormones from the CNS, mainly through the firing of GnRH neurons ◦ Hormonal levels are “set” at a certain point (like a thermostat) ▪ Similarly, when a particular hormonal level in your body dips below the endocrine's set point for that hormone, secretion of the hormone increases • When the level reaches the set point, secretion temporarily stops • Set point is a physiological level or setting that the body attempts to maintain through a self-regulating system ◦ Such a feedback loop becomes important at the onset of puberty ▪ Feedback loop is a cycle through which two or more bodily functions respond to and regulate each other, such as that formed by the hypothalamus, the pituitary gland, and the gonads ◦ Long before adolescence, before birth, a feedback loop develops involving the pituitary gland, the hypothalamus, and the gonads, which release the “sex” hormones – androgens and estrogens ▪ This feedback loop is known as the HPG axis ◦ Androgens and estrogens are hormones produced by both sexes ◦ Your HPG axis is set to maintain certain levels of androgens and estrogens ▪ When these levels fall below the set points, the hypothalamus no longer inhibits the pituitary, thus permitting it to stimulate the release of sex hormones by the gonads ▪ When the sex hormone levels reach the set point, the hypothalamus responds by inhibiting its stimulation of the pituitary gland • Adrenarche ◦ Adrenarche is the maturation of the adrenal glands that takes place during adolescence ▪ May be responsible for first sexual attraction that takes place at the “magical age of 10” ◦ Changes at puberty in the brain system that regulates the adrenal gland are important because this is the brain system that controls how we respond to stress ◦ Adolescence is a period of great vulnerability for the onset of many serious mental disorders ▪ This is due to hormonal changes of puberty that make us more responsive to stress • This leads to excessive secretion of the stress hormone cortisol • Cortisol at high and chronic levels can cause brain cells to die What Triggers Puberty? • HPG axis is awakened and signals the body for puberty • Some of this is due to a clock whose “puberty alarm” is set very early by information coded in the genes • Some reawakening of the HPG axis at puberty is due to multiple signals ◦ Signal indicate whether there are sexually mature mating partners in the environment ◦ Whether there are sufficient nutritional resources to support a pregnancy ◦ Whether the individual is physically mature and healthy enough to begin reproducing • Evidence suggests that rising levels of a protein produced by fat cells, leptin, may be the most important signal in females ◦ The signal carried by rising levels of leptin instructs the hypothalamus to stop doing things that have been inhibiting puberty ▪ As a result, this signalling process, the GnRH neurons are excited, and the hypothalamus initiates a cascade of hormonal events that ultimately results in sexual maturation ◦ This idea is consistent with observations that individuals may not go through puberty until they have accumulated a certain amount of body fat • The delay of the onset of puberty can be due to: ◦ stress ◦ illness ◦ nutritional deficiencies ◦ excessive exercise ◦ excessive thinness How Hormones Influence Adolescent Development • Hormones play two different roles in adolescent development ◦ An organizational role (of hormones): The process through which early exposure to hormones, especially prenatally, organizes the brain or other organs in anticipation of later changes in behaviour or patterns of growth ◦ An activational role (of hormones): The process though which changes in hormone levels, especially at puberty, stimulate changes in the adolescent's behaviour, appearance, or growth • Until about 8 weeks after conception, the human brain is “feminine” unless exposed to “masculinizing” hormones (i.e. Testosterone) • Differences in adolescence are likely an impact of prenatal hormones, rather than from hormonal changes are puberty ◦ Hormonal changes are said to activate changes in behaviour (not cause them) ◦ Changes in puberty are likely to be results of an interaction between organization and activational effects of hormones Recap • Puberty has three main physical manifestations: ◦ Arapid acceleration in growth ◦ The development of primary sex characteristics ◦ The development of secondary sex characteristics • Puberty is regulated by a hormonal feedback loop that includes the hypothalamus, the pituitary gland, and the gonads • Many important changes at puberty involves a process called adrenarche, which involves the regulation of the adrenal gland rather than the gonads • The onset of puberty is triggered by several factors, including a biological “alarm” that is coded in the individual's genes and various environmental signals, which let the body know that its ready to begin reproduction • Physical and behavioural changes at puberty result from two different hormonal processes: ◦ Organizational, which take long before adolescence ◦ Activational, which result form changes in hormones when puberty takes place Somatic Development Changes in Stature and The Dimensions of the Body • TheAdolescent Growth Spurt ◦ The dramatic increase in height and weight that occurs during puberty is referred to as the adolescent growth spurt ◦ At the time of peak height velocity – the time at which the adolescent is growing most rapidly – he or she is growing at the same rate as a toddler ◦ One marker of the conclusion of puberty is the closing of the ends of the long bones in the body, a process called epiphysis, which terminates growth in height ▪ Most of the height gain results from an increase in torso length, not leg length ▪ The head, hands, and feet are the first to accelerate in growth, then the arms, legs, torso and shoulders • Sex Differences in Muscle and Fat ◦ Muscle tissue grows faster in boys than girls ◦ Body fat increases more so for females than males just before puberty ▪ For boys, there is actually a slight decline in body fat just before puberty ◦ Adolescent girls' diets, especially those of Black girls, are generally less adequate nutritionally than the diets of boys • Body Dissatisfaction AmongAdolescent Girls ◦ Black adolescents seem less vulnerable to feelings of body dissatisfaction than other girls ▪ Consequently, they are less likely to diet Sexual Maturation (see table 1.1, 1.4, 1.5 on p. # 31-33) • Secondary sex characteristics: The manifestations of sexual maturity at puberty, including the development of breasts, the growth of facial and body hair, and changes in the voice ◦ The development of secondary sex characteristics is typically divided into five stages, often called Tanner stages Recap • The adolescent growth spurt, which takes place about two years earlier among girls than boys, involves a rapid increase in height and weight • Puberty brings with it changes in ratio of muscle to fat, marked with gender differences in the way in which body composition changes ◦ The increase in fat, which is greater among females, is a source of concern to many girls • Sexual maturation in both sexes is measured according to Tanner stages The Timing and Tempo of Puberty Variations in the Timing and Tempo of Puberty • The onset of puberty can occur as early as age 7 in girls and 9.5 in boys, or as late as age 13 in girls and 13.5 in boys • The interval between the first sign of puberty and complete physical maturation can be as short as a year and a half or as long as six years • There is no relation between the age at which puberty begins and the rate at which pubertal development proceeds • The timing of puberty may have a small effect on one's ultimate height or weight ◦ However, with late maturers, being taller than early maturers as adults and early maturers, being somewhat heaviest – at least among females Genetic and Environmental Influences on Pubertal Timing • Individual Differences in Pubertal Maturation ◦ Differences in the timing and rate of puberty among individuals growing up in the same general environment may be due to genetic factors ◦ Every individual inherits a predisposition to develop at a certain rate and to begin pubertal maturation at a certain time but it is subject to the influence of many external factors ▪ It is the result of the interaction between nature and nurture • The most important environmental influences are nutrition and health • Puberty occurs earlier among individuals who are better nourished throughout their prenatal, infant, and childhood years • Delayed puberty is likely to occur among individuals with a history of protein/ caloric deficiency ◦ As well as chronic illness during childhood and adolescence and excessive exercise • Familial Influences on Pubertal Timing ◦ Social factors in the home environment may influence the onset of maturation, especially in girls ◦ Puberty occurs somewhat earlier among girls who grew up in father-absent families, in less cohesive
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