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Chapter 5

Psychology 2040A/B Chapter Notes - Chapter 5: Encephalization, Muscular Development, Birth Weight

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Psych 2040A
Chapter 5: Physical Development: The brain, body, motor skills, and sexual
An Overview of Maturation and Growth
growth and physical development includes external and internal changes
Changes in Height and Weight
babies grow rapidly in the first 2 years, often doubling their birth weight by 4-6
months of age and tripling it by the end of first year
by age 2, toddlers are already half their adult height and have quadrupled their
birth weight
physical growth and development are most obvious during puberty, where people
go through a 2-3 year growth spurt
Changes in Body Proportions
the newbornʼs head is already 70% of its eventual adult size and represents one
quarter of the total body length, the same fraction as legs
development proceeds in a cephalocaudal fashion - a sequence of physical
maturation and growth that proceeds from the head (cephalic region) to the tail
(caudal region) - development proceeding head down
trunk and legs growth the fastest for children
when children grow upward, they are also growing outward - proximodistal
development - a sequence of physical maturation and growth that proceeds from
the centre of the body (the proximal region) to the extremities (distal regions)
internal organs form first in a fetus before the extremities
Skeletal Development
skeletal structures that form during prenatal development are soft cartilage that will
gradually ossify (harden) into bone
newbornʼs bones are soft, pliable, and difficult to break
the neonateʼs skull consists of several soft bones that can be compressed to allow
the child to pass through the cervix and the birth canal
one method of estimating a childʼs level of physical maturation is to x-ray the wrist
and hand
skeletal age: a measure of physical maturation based on the childʼs level of
skeletal development
x-ray shows number of bones and extent of ossification
girls mature faster than boys - at birth, girl are only 4 to 6 weeks ahead in
skeletal development, but by age 12, the gender difference has widened to 2
full years.
skeletal development is usually completed by age 18
Muscular Development
neonates are born will all the muscle fibres they will ever have
at birth, muscle tissue is 35% water and it accounts for no more than 18 to 24
percent of a babyʼs body weight
muscular development proceeds in cephacaudal and proximodistal directions, with
muscles in the head and neck maturing before those in the trunk and limbs

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increase in muscle mass are more dramatic for boys
Variations in Physical Development
physical development is a very uneven process in which different bodily systems
display unique growth patterns - brain and head actually grow much faster and are
quicker to reach adult proportions than the rest of the body
Individual Variations
individual differences in the rate of maturation not only result in visible
differences in physical features such as height, but also contribute to
differences in other areas of development, such as cognition
faster processing speed was found in early maturers, especially boys
Cultural Variations
people from Asia, South America, and Africa tend to be smaller than North
there are cultural differences in the rate of physical growth
Asian ad African-American children mature faster than European or
European-American children
Development of the Brain
the brain grows very early in life, reaching 75% of its weight by age 2
brain growth spurt: the period between the 7th prenatal month and 2 years of age
when more than half of the childʼs eventual brain weight is added
brain increases about 1.7g per day
Neural Development and Plasticity
synapses: the connective space (juncture) between one neuron and another
neurons: nerve cells that receive and transmit neural impulses
basic units of the nervous system
neurons are produced in the neural tube of the embryo
the majority of the neurons a person will ever have are developed by the end of the
2nd trimester of pregnancy, before the brain growth spurt
however, scientists have established that formation of new neurons in the
hippocampus occurs throughout life
glia: nerve cells that nourish neurons and encase them in insulating sheaths of
they continue to form throughout life, unlike neurons
Cell Differentiation and Synaptogenesis
neurons assume specialized functions (ie. hearing, vision, smell, etc) once they
migrate to a specific area of the brain
individual neurons have the potential to serve any neural function, and the
function each serves depends on where it ends up
synaptogenesis: formation of connections, synapses, among neurons
this proceeds rapidly during the brain growth spurt
the average infant has far more neurons and neural connections than do
reason is the neurons that successfully interconnect with other neurons
crowd out those that donʼt, so that about half of the neurons produced in
early life also die in early life.

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plasticity: capacity or change; a developmental state that has the potential to
be shaped by experience
brain has produced excess neurons and synapses in preparation for
receiving any and all kinds of sensory and motor information a human being
can experience
synaptic pruning: surviving neurons that are stimulated less often loose their
synapses and stand in reserve to compensate for brain injuries or to support
new skills
The Role of Experience
Austin Reisen and his colleages did a study on chimpanzees - infant
chimpanzees that were reared in the dark for periods ranging up to 16 months
dark-reared chimpanzees experienced atrophy of the retina and the neurons
that make up the optic nerve
this atrophy is reversible if the animalʼs visual deprivation did not exceed 7
animals raised with lots of companions and many toys to play with have brains
that are heavier and display more extensive networks of neural connections
Brain Differentiation and Growth
at birth, the most highly developed brain areas are the subcortical (lower) brain
first areas of the cerebrum to mature are the primary motor areas and the primary
sensory areas
myelin sheath acts like an insulator to speed the transmission of neural
impulses, thus allowing the brain to communicate more efficiently with different
parts of the body - myelin can be found around the axon of a neuron
myelinization: the process by which neurons are enclosed in waxy myelin
sheaths that will facilitate the transmission of neural impulses.
myelination can take place up until puberty
Cerebral Lateralization
cerebrum: the highest brain centre; includes both hemispheres of the brain
and the fibres that connect them
connected by the corpus callosum - a bundle of neural fibres separating
the two hemispheres of the cerebrum
cerebral cortex: the outer layer of the brainʼs cerebrum that is involved in
voluntary body movements, perception, and higher intellectual functions such
as learning, thinking, and speaking.
left side of the body is controlled by the right brain and the right side of the body
is controlled by the left brain
cerebral lateralization: the specialization of brain functions in the left and the
right cerebral hemispheres
brain is a lateralized organ - involves a preference for using one hand or
one side of the body more than other.
two hemispheres are not independent of one another (this is why we have a
corpus callosum)
brain lateralization originates as early as the prenatal period
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