PSYC 2450 Chapter Notes - Chapter 6: Pubic Hair, Anorexia Nervosa, Exosphere
27 Jun 2018
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Chapter 6 Physical Development: The Brain, Body, Motor Skills, and Sexual Development 1
Chapter 6
Physical Development: The Brain, Body,
Motor Skills, and Sexual Development
Chapter 6 Outline and Summary
An Overview of Maturation and Growth
1. Changes in Height and Weight
The growth of the child in early development is more rapid than at any other time period
than adolescence. By 6 months of age most infants will weigh twice as much as they did at
birth, by the end of the first year they will weigh three times what they did at birth, and by
the age of 2 they will weigh four times their birth weight. Length or height of the child also
increases rapidly, and by the age of 2 an infant’s height is approximately one-half of their
final adult height.
From age 2 until puberty children grow approximately 5 centimetres per year and gain
approximately 3 kilograms per year. This slow, steady growth becomes a “growth spurt”
when the child reaches puberty. During each year of the two- to three-year adolescent
growth spurt the child will gain between about 4.5 and 7 kilograms and grow 5 to 10
centimetres in height.
2. Changes in Body Proportions
During childhood physical development follows two general trends: cephalocaudal and
proximodistal. Cephalocaudal means that growth proceeds from the head down, with the top
of the body initially growing more quickly than the legs. During the prenatal period the head
grew most rapidly; over the first year the trunk will show the most rapid growth, and from
the first birthday until adolescence the legs will grow most rapidly. Proximodistal means that
growth proceeds from the centre of the body outward toward the extremities. Throughout
infancy and childhood the arms and legs continue to grow faster than the hands and feet.
Both these growth trends reverse just before puberty. During adolescence the trunk again
grows faster than the rest of the body, and the hands and feet reach adult proportions before
the arms and legs or trunk reach their adult proportions.
3. Skeletal Development
At birth, most of the bones are a source of blood cells. As well, most of the infant’s bones
are soft, pliable, and difficult to break. There are fewer bones in the feet and hands and the
bones are less well connected than they will be in adolescence. Skeletal maturation shows
the same cephalocaudal trend that was evident in general growth. The skull and hands
mature before the legs and feet.
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Chapter 6 Physical Development: The Brain, Body, Motor Skills, and Sexual Development 2
4. Muscular Development
Infants are born with all the muscle fibres they will ever have, but the muscle fibres are
immature and account for only 18 to 24 percent of the newborn’s body weight. Muscular
development also follows the cephalocaudal and proximodistal pattern of growth, and by the
time individuals reach their mid-20s muscle tissue will account for 40 percent of total body
weight in males and 24 percent of total body weight in females.
5. Variations in Physical Development
There are significant individual differences in the rate of growth and there are also
meaningful cultural and subcultural variations in physical growth and development.
Generally, individuals from Asia, South America, and Africa tend to be smaller than North
Americans, Northern Europeans, and Australians. Also, Asian-American and African-
American children tend to mature faster than European-American and European children.
Development of the Brain
In newborns the brain weighs approximately 450 grams, 25 percent of its final adult weight;
by the age of 2 the brain has increased to nearly 1350 grams, or 75 percent of its final adult
weight.
1. Neural Development and Plasticity
Neurons are the basic cells that make up the brain and spinal cord. By the end of the second
trimester of pregnancy almost all of the neurons that a person will ever have are already
formed. The brain growth spurt that occurs during the last trimester of pregnancy and the
first two years of life occurs because glial cells are developing and neurons are maturing, not
because new neurons are being created.
a. Cell Differentiation and Synaptogenesis
In young infants the brain shows a remarkable level of plasticity, meaning that the brain is
responsive to the effects of experience. During the first year of life the infant has more
neurons and forms more neural connections than are found in the adult brain. Neurons that
do not successfully connect to other neurons die, and synaptic connections that are not
stimulated disappear through synaptic pruning. Differences in synaptic organization can be
seen as a function of age. Taken together, these processes produce a more efficient
information-processing system.
2. Brain Differentiation and Growth
At birth the most highly developed areas of the brain are the brain stem and midbrain, which
control states of consciousness, reflexes, and vital biological functions. The first areas of the
cerebrum to mature are the primary motor and sensory areas. By 6 months of age, the
primary motor areas of the cerebral cortex have developed sufficiently to direct most of the
infant’s movements. Inborn reflexes such as the palmar grasp and the Babinski reflex
disappear as the motor cortex develops and voluntary control of motor responses develops.
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Chapter 6 Physical Development: The Brain, Body, Motor Skills, and Sexual Development 3
a. Myelinization
Myelinization involves the formation of a myelin sheath around axons. This waxy coating
insulates the neuron and helps to speed the transmission of neural impulses. The pathways
between the brain and the skeletal muscles are myelinated in a cephalocaudal and
proximodistal pattern. As the neurons are myelinated the child will become capable of
increasingly complex motor activities. Myelinization proceeds rapidly in the first few years
of life but some areas of the brain, such as the reticular formation and frontal cortex, are not
fully myelinated until late adolescence or early adulthood. The outcome of greater
myelinization is better communication between more primitive, subcortical parts of the
brain, and prefrontal areas associated with regulating behaviour. As myelinization improves
then, so does the child’s ability to monitor his or her own emotional reactions.
b. Cerebral Lateralization
The cerebrum consists of two hemispheres that are connected by the corpus callosum. The
two hemispheres appear identical, but each hemisphere controls different specialized
functions. This means that functions such as language and spatial processing are lateralized
within the cortex. Cerebral lateralization also involves a preference for using one side of the
body more than the other. The corpus callosum plays an important role in integrating these
lateralized functions.
c. Development of the Brain During Adolescence
During adolescence myelinization of higher brain centres continues, which may increase
attentionial control, working memory, and speed of neural processing. In addition,
reorganizations of neural circuits in the prefrontal cortex continue until at least the early
adult years, bringing about increases in the ability to engage in strategic planning (see
Chapter 9).
Motor Development
Motor skills over the first two years evolve in a definite sequence. Although there is wide
variability among children, the majority of children (more than 50 percent) can roll over at
about 3 months of age, can sit without support at about 6 months of age, and walk without
assistance just past their first birthday.
1. Basic Trends in Locomotor Development
Motor skill development follows cephalocaudal and proximodistal trends similar to those
observed during growth and myelinization. For example, children use their entire hand for
grasping before they learn to use their fingers for grasping. However, although it has long
been argued that children gain control of their arms before they have good control of their
legs—this fact has recently been called into question. It seems that the stability of the hip
joint makes children more likely to touch a novel toy with their feet before they are able to
make the same attempt with their hands.
There are three possible explanations for the sequencing and timing of early motor
development. The maturational viewpoint suggests that motor development is the unfolding
of a genetically programmed sequence of events. One supporting piece of evidence comes
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Document Summary
An overview of maturation and growth: changes in height and weight. The growth of the child in early development is more rapid than at any other time period than adolescence. Length or height of the child also increases rapidly, and by the age of 2 an infant"s height is approximately one-half of their final adult height. From age 2 until puberty children grow approximately 5 centimetres per year and gain approximately 3 kilograms per year. This slow, steady growth becomes a growth spurt when the child reaches puberty. During each year of the two- to three-year adolescent growth spurt the child will gain between about 4. 5 and 7 kilograms and grow 5 to 10 centimetres in height: changes in body proportions. During childhood physical development follows two general trends: cephalocaudal and proximodistal. Cephalocaudal means that growth proceeds from the head down, with the top of the body initially growing more quickly than the legs.
