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Chapter 5

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Western University
Psychology 2040A/B
Jackie Sullivan

Chapter Five: Physical Growth The Course of Physical Growth - Humans experience a prolonged period of physical growth Changes in Body Size - Infancy body size change us the most rapid o Growth slows down by early to middle childhood - Then puberty accelerates growth - Two types of growth curves used to track overall changes in body size o Distance Curve: plots the average size of a sample of children at each age, indication typical yearly progress toward maturity  Shows that weight & height follow a similar trend (increase linearly)  Notice that boys & girls are similar but then girls become taller (age 10-11) but then boys later o Velocity Curve: plots the average amount of growth at each yearly interval, revealing the exact timing of growth spurts  Rapid but decelerating growth in infancy; slower, constant during early-middle childhood & sharp increase during adolescents; followed by decrease in adulthood Changes in Body Proportions - As overall size increases, parts of the body grow at different rates - Two growth patterns describe these changes o Cephalocaudal Trend – from the Latin for “head to tail”, during the prenatal period, the head develops first from the primitive embryonic disk, followed by the lower part of the body (head & neck develop faster than trunk & legs) o Proximodistal Trend – growth proceeds “near to far” from the center of the body outward; in the prenatal period, the head, chest & trunk grow first then the arms & legs, finally the hands & feet; during childhood, the arms & legs continue to grow somewhat ahead of the hands & feet o In puberty it is in reverse, the limbs grow faster, followed by torso Changes in Muscle-Fat Makeup - Body fat increases in the last few weeks of prenatal life & continues to do so after birth (peaking at 9 months) o Helps the baby regulate temperature o Around age 8, women gain more fat & continue to do so throughout puberty, while men slim down - Muscle accumulates slowly throughout infancy & childhood with a dramatic rise at adolescence o Both sexes gain muscle mass at puberty but greater in boys Skeletal Growth - The best estimate of a child’s physical maturity is skeletal age – a measure of development of the bones of the body o Before birth, bone is more cartilage-like, but just before birth, special growth centers called epiphyses, appear at the 2 extreme ends of each of the long bones in the body o Cartilage cells continue to be produced at the growth plates, which increase in number throughout childhood & as growth continue, get thinner & disappear (after than no further growth in bones)  Girls are considerably ahead of boys in skeletal age and same with organs, maybe that is why women have less developmental problems Gains in Gross-Motor Skills - Changes in size, proportions & muscle strength support an explosion of new gross-motor skills Advances in Early & Middle Childhood - At age 2, gait becomes smooth & rhythmic (running & jumping) – throw is rigidity - At age 3-6, more advanced (hopping, galloping & skipping) – throw is more smooth - During the school years, improved balance, strength, agility & flexibility support refinements in running, jumping & ball skills (Milestone pg. 179) Organized Youth Sports - Physical activity is declining but organized sports have increased - Organized sports benefit children in multiple ways, but may overemphasize competition and coaches & parents sometimes criticize instead of encouraging kids & negative attitudes cause kids not to want to continue to play Hormonal Influences on Physical Growth - The most important hormones for growth are released by the pituitary gland, located at the base of the brain near the hypothalamus, a structure that initiates & regulates pituitary secretions - Once pituitary hormones enter the bloodstream, they act directly on body tissues to induce growth, or stimulate the release of other hormones - The hypothalamus contains special receptors that detect hormone levels in the bloodstream and help in the feedback loop, telling the pituitary to increase or decrease its secretions o Growth Hormone (GH), the only pituitary secretion produced continuously throughout life, affects development of all tissues except the CNS & the genitals  GH production doubled during puberty, contributing to the increase in body size & then decreases  GH also acts directly on the body to stimulate liver & epiphyses of the skeleton to release the hormone insulin-like growth factor (IGF-1), which triggers cell duplication throughout the body; however it does not seem to affect prenatal growth  Some suffer from a GH deficiency & without shots, they do not grow o Another pituitary hormone, thyroid-stimulating hormone (TSH), prompts the thyroid gland in the neck to release thyroxine, which is necessary for brain development & for GH to have its full impact on body size  Infants with a deficiency of thyroxine must receive it at once, or they will be mentally retarded; but once the most rapid period of brain development is complete, thyroxine deficiency no longer affects the CNS o Sex maturation is controlled by pituitary secretions that stimulate the release of sex hormones; although we think of estrogen as female hormones & androgens as male hormones, both types are present in each sex, but in different amounts  A lot of testosterone is released from the testes & causes a lot of male growth; also a but of estrogen is released, estrogen increases GH adding to the growth spurt & estrogen is released by the ovaries in girls for them to grow Worldwide Variations in Body Size - Ethnic variations in growth rate are common & same with different countries - Both heredity & environment contribute to these differences o Ex. Those in hot climate might have long, lean physiques; those where food is plentiful might grow the tallest & vice versa with countries with less food Secular Trends - Secular trends in physical growth – changes in the body size from one generation to the next – have occurred in industrialized nations o In Canada, US, Australia, Japan & nearly all European countries, most children are taller & heavier than their parents & grandparents were o Larger size of today’s children is mostly due to faster rate of physical development; consistent with this, age of first menstruation has declined steadily o Improved health & nutrition are also responsible for increase in secular gains Brain Development - Brain reaches its adult size earlier than any other organ Development of Neurons - Human brain has 100-200 billion neurons, or nerve cells that store & transmit information, many of them having connections with other neurons o Although, not packed together – they have tiny gaps, synapses, where fibers from different neurons come close together but do not touch – neurons send messages to one another by releasing chemicals called NT, which cross the synapses o Once neurons are in place, they differentiate, establishing unique functions - A surprising aspect of brain growth is programmed cell death, which makes space for these connective structures: As synapses form, many surrounding neurons die – 20-80%, depending on brain region – but have way too many neurons, so will be fine - As neurons form connections, stimulation becomes vital to their survival, allows an elaborate system of communication o Stimulation result in massive overabundance of synapses, many of which serve identical functions, thereby ensuring that the child will acquire the motor, cognitive & social skills that our species needs to survive o Neurons that are seldom stimulated soon lose their synapses, in a process called synaptic pruning, that returns neurons not needed at the moment to an uncommitted state so they can support further development - Since only a few neurons are produced after the prenatal period, we know that half of the brain’s volume comes from glial cells, which are responsible for myelination, the coating of neural fibers with an insulating fatty sheath (myelin) that improves efficiency of message transfer (develop from pregnancy to age 2 & then slowly through life) Development of the Cerebral Cortex - Cerebral Cortex: surrounds the rest of the brain, resembling half of a shelled walnut, it is the largest brain structure – accounting for 85% of the brain’s weight & containing the greatest number of neurons & synapses - Since it is the last brain structure to stop growing, it is sensitive to environmental influences for a much longer period Regions of the Cerebral Cortex - The cortical regions with the most extended period of development are the frontal lobes - The prefrontal cortex, lying in front of areas controlling body movement is responsible for thought – in particular, consciousness, attention, inhibition of impulses, integration of info & use of memory, reasoning, planning & problem- solving strategies Lateralization & Plasticity of the Cerebral Cortex - Lateralization: the cortex has 2 hemispheres o Left side: verbal abilities & positive emotion o Right side: spatial abilities & negative emotion o Lateralization permits a wider array of functions to be carried out effectively than if both sides processed information exactly the same way - Brain Plasticity: a highly plastic cortex, in which many areas are not yet committed to specific functions, has a high capacity for learning; and if a part of the cortex is damaged, other parts can take over the tasks it would have handled o At birth, hemispheres have already begun to specialize, most infants favour the right side of their body for head movements & reactions, but also show greater activation on the left side while listening to speech or displaying a positive state & in contrast, right side reacts to non-speech sounds & negative emotions o The brain is more plastic during the first few years than it will ever be again, an overabundance of synaptic connections support brain plasticity, ensuring that young children will acquire certain capacities even if some areas are damaged - Lateralization & Handedness: o Handedness reflects the greater capacity of one side of the brain – the individual’s dominant cerebral hemisphere – to carry out skilled motor action (language & hand-control in left hemi if right-handed) o One genetic theory proposes that most children inherit a gene that biases them for right-handedness & left- dominant cerebral hemisphere o Left-handedness is more common with mentally retarded people & associated with prenatal & birth difficulties but most left-handers have no problems Advances in Other Brain Structures - At the rear & base of the brain is the cerebellum, a structure that aids the balance & control of body movement o Controls motor coordination & cognitive aspects (memory, planning & language) - The reticular formation, a structure in the brainstem that maintains alertness & consciousness, generates synapses & myelinates from early childhood to adolescence (also helps sustained & controlled attention) - An inner-brain structure called the hippocampus, which plays a vital role in memory & in images of space that help us find our way, undergo rapid synapse formation & myelination in the second half of the first year, when recall memory & independent movement emerge - Located in the inner brain, adjacent to the hippocampus, is the amygdala, a structure that plays a central role in processing emotional information - The corpus callosum is a large bundle of fibers connecting the 2 cerebral hemispheres, it supports smooth coordination of movements on both sides of the body & also integration of perception, attention, memory, language & problem-solving (essential for communication between hemispheres) Brain Development in Adolescence - Adolescents recruit prefrontal cortex less effectively than adults because the prefrontal cognitive-control network still requires fine-tuning - Adding to self-regulation difficulties are changes in the brain’s emotional/social network the cognitive control network is not functioning optimally so adolescents react more strongly to stimuli (ex. Drug-taking, reckless driving) - In addition, the surge of sex hormones heightens the sensitivity of the prefrontal cortex Sensitive Periods in Brain Development - Much evidence confirms that the brain is particularly sponge-like during the 1 few years - 2 types of brain development o Experience-Expectant Brain Growth  Occurs early & naturally; refers to the young brain’s rapidly developing organization, which depends on ordinary experiences – opportunities to interact with people, hear language & other sounds, see & touch objects and move about & explore the environment o Experience-Dependent Brain Growth  Occurs throughout our lives; it consists of additional growth & refinement of established brain structures as a result of specific learning experiences that vary widely across individuals & cultures Factors Affecting Physical Growth Heredity - When diet & health are adequate, height & rate of physical growth are largely determined by heredity - As long as negative environmental influences are not severe, children typically show catch-up growth – a return to a genetically influenced growth path once conditions improve (but brain, heart & other organs may have damage) - Genes influence growth by controlling the body’s production of & sensitivity to hormones; mutations can disrupt this Nutrition - Is important at any time of development but crucial during the first 2 years Breastfeeding Versus Bottle-Feeding - In early infancy, breast milk is ideally suited to their needs - Many benefits from breast milk: o Provides the correct balance of fat & protein o Ensures nutritional completeness o Helps ensure healthy physical growth o Protects against many diseases o Protects against faulty jaw development & tooth decay o Ensures digestibility o Smooths the transition to solid foods - Women in industrialized countries breastfeed more than those in developing countries, but the benefits of breast milk has been promoted - Women who breastfeed are not more attached to their babies though Nutrition in Childhood & Adolescence - Around 1 year, diets should include all the basic food groups o Many become picky eaters as they get older & do not always eat enough - Kids admire what other eat, what they eat also promotes what they want (serve veggies, more likely to like healthy foods), if parents offer bribes kids might stop liking other food (don’t say things like eat your veggies or no cookie) - During puberty, rapid growth leads to a dramatic rise in food intake, also comes at a time when eating habits are bad - Frequency of family meals is a powerful indicator of healthy eating – greater intake of healthy foods Malnutrition - Maramus is a wasted condition of the body caused by a diet low in the essential nutrients; it usually appears in the first year of life when a baby’s mother is too malnourished to produce enough breast milk & bottle-feeding is also inadequate - Kwashiorkor is caused by an unbalanced diet very low in protein; the disease usually strikes after weaning, between year 1-3 o Children who survive these forms of malnutrition grow up smaller & suffer from lasting brain damage - Learning & behaviour can also be affected (intelligence, eating habits, motor coordination, stress response) - They also have iron-deficiency anemia, a condition affecting 25% of infants & children worldwide which interferes with the CNS processes - Inadequate nutrition is not just in developing countries, 21% of U.S children suffer from food security (high among l
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