PSYC 2500H Chapter Notes - Chapter 6: Primary Sensory Areas, Pubic Hair, Reticular Formation
26 Jun 2018
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Chapter 6 - pages 164 - 180 (Physical Development: the brain, body, motor skills and
sexual development)
An Overview of Maturation and Growth:
Changes in Height and Weight:
oBabies double birth weight by 4 - 6 months and tripling it by the end of the first
year
oMay not show any growth and then spurts of more than a cm in one day
oBy age 2 toddlers are already half their eventual adult height and have
quadrupled their birth weight
oAge 2 to puberty children gain 5 - 8 cm in height and about 3 kg in weight each
year
oPhysical growth and development very evident in puberty (3 or 2 year growth
spurt) having an annual gain of about 4.5 to 7 kg and 5 to 10 cm in height
Changes in Body Proportions:
oCephalocaudal Development: a sequence of physical maturation and growth that
proceeds from the head (cephalic region) to the tail (caudal region)
oNewborn’s head is already 70% of eventual adult head size and represents one-
quarter of total body length - the same fraction as the legs
oDevelopment proceeds in a cephalocaudal direction (head downward)
oAt 1 year child’s head no accounts only 20% of total body length
oLegs grow rapidly accounting for more than 60% of increase in height from
child’s first birthday to adolescent growth spurt
oDuring adolescence the trunk becomes the fastest growing segment of the body -
legs are growing rapidly at the same time
oAs adults legs will account for 50% of total height and our heads account for only
12%
oProximodistal development: a sequence of physical maturation and growth that
proceeds from the centre of the body (the proximal region) ti the extremities
(distal regions)
oChildren grow upwards and outwards
In prenatal development the chest and internal organs form first followed
by arms and legs and then hands and feet
oCentre-outward growth pattern reverses just before puberty and hands and feet
begin to grow rapidly and become first body parts to reach adult proportions
Reason why teenagers seem clumsy because hard to navigate large
hands and feet
Skeletal Development:
oPrenatal skeleton is made of soft cartilage that will gradually ossify (harden) into
bony material - nearly all the bones are source of blood cells
oPostnatal development the production of blood cells is limited to a few specific
bones
oAt birth bones are soft, pliable, and difficult to break
oNeonates skull consists of several soft bones - compressible to pass through the
cervix and birth canal - separated through 6 soft spots (fontanelles) that gradually
filled in by minerals to form a single skull by age 2 with pliable points at the
seams where skull bones join - these seams or sutures allow for the skull to
expand as the brain grows larger
oOther parts of the body - ankles, wrists, feet, hands - develop more bones as the
child matures
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Method of estimating age is x-raying hands, feet, ankles, wrists - number
of bones and the extent of their ossification
oSkeletal age: a measure of physical maturation based on the child’s level of
skeletal development
oGirls often faster than boys - girls generally 4 - 6 weeks ahead at birth but about
2 full years by the age of 12
oSkull and hands mature first - leg bones continue to develop into late teens
oSkeletal development is complete by age 18 - although widths/thickness of skull,
hands and legs continue to increase throughout life
Muscular Development:
oNeonates born with all muscle fibers they will ever have
oAt birth muscles are 35% water – accounts for no more than 18 – 24% of baby’s
body weight
oMuscle fibers grow as cellular fluid accumulates as proteins and salts
oMuscular development proceeds in cephalocaudal and Proximodistal directions
oHead and neck muscles mature before trunk and limb muscles
oMuscle growth happens slowly during childhood and then accelerates during
early adolescence
Muscular growth increases in muscle mass and physical strength largely
for both girls and boys but more dramatic for boys
oSkeletal muscle accounts for about 40% of body weight of an average male and
24% od body weight of an average female by mid 20s
Variations in Physical Development:
oBrain and head grow faster to reach adult proportions than the rest of the body
oIndividual Variations:
Development of body systems is an uneven/asynchronous process
Individual differences in the rate of maturation not only result in visible
differences in physical features (height) – but also contribute to
differences in other areas of development (cognition)
oCultural Variations
People from Asia, South America, Africa tend to be smaller than North
Americans, Northern Europeans, Australians
Asian and African American children tend to mature faster than European
American and European children
Asynchronies in the maturation of different body systems are built into
species’ heredity – common maturational program that all humans share
Significant variations in the rates at which individuals grow can be from
environmental factors such as:
Food people eat
Diseases they may encounter
Emotional climate in which they live
Development of the brain:
oBrain growth spurt: the period between the seventh prenatal month and 2 years
of age when more than half of the child’s eventual brain weight it added
oAt birth brain is 25% of eventual adult weight – at 2 years brain is 75% of adult
weight
oBrain increases in weight by about 1.7g a day or more than a mg/min
oBrain weight does not equal brain maturation or other aspects of development
oNeural Development and Plasticity:
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Synapses: the connective space (juncture) between one nerve cell
(neuron) and another
Neurons: nerve cells that receive and transmit neural impulses
Basic units of the brain and nervous system
Neurons are produced in the neural tube of the developing embryo –
migrate along pathways laid down by a network of guiding cells to form
the major parts of the brain
Majority of neurons have already formed by the end of the second
trimester – before the brain growth spurt has even begun
Formation of new neurons in the hippocampus occurs throughout life
Glia: nerve cells that nourish neurons and encase them in insulating
sheaths of myelin
Major contributor to development
Far more numerous than neurons – continue to form throughout
life
oCell Differentiation and Synaptogenesis:
Individual neurons have the potential to serve any neural function, and
the function each serves depends on where it ends up
If a neuron that would normally migrate to the visual area of the brain was
transplanted to the area that controls hearing – it will change to become
an auditory neuron instead of visual neuron
Synaptogenesis: formation of connections (synapses) among neurons
Happens rapidly during the brain growth spurt
The average infant has far more neurons and neural connections than
adults do
About half the neurons produced early in life also die early in life
Surviving neurons form hundreds of synapses, many of which also
disappear if the neuron is not properly stimulated
Plasticity: capacity for change; a developmental state that has the
potential to be shaped by experience – the fact that its cells are highly
responsive to the effects of experience
Brain has evolved so that it produces an excess of neurons and synapses
in preparation for receiving any and all kinds of sensory and motor
stimulation that a human could conceivably experience
No brain had this broad a range of experiences – much of one’s neural
circuitry remains unused
Neurons and synapses that are most often stimulated continue to function
– other surviving neurons that are stimulated less often lose their
synapses (process called synaptic pruning) and stand in reserve to
compensate for brain injuries or to support new skills
Development of the brain early in life is not entirety to the unfolding of
maturational program – result of both a biological program and early
experience
oNeural Plasticity: Role of Experience
Neurons that are not properly stimulated degenerate – “use it or lose it”
Foster the neural development of an immature, malleable brains by
exposing participants to enriched environment that provide a wide variety
of stimulation
Environmental exposure to information may be necessary for maintaining
plasticity in the development of underlying perceptual and cognitive skills
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