Page 160 - 86 human physiology part-2

Development: Birth Through Death

Changes in body Proportions

Cephalocaudal trend means that growth occurs from head to tail. The head develops more rapidly
than the lower part of the body. At birth the head takes up to one fourth of the total body length and
legs only one third. The lower body catches up by age 2 and the head accounts for only one fifth and
legs for nearly one half of the body length.

Proximodistal trend means that head growth proceeds literally form near to far or from center of
the body outward.

At birth the brain is nearer it's adult shape and size than any other physical structure. The brain
continues to develop at an astounding pace throughout infancy and toddlerhood.

The Brain Development

The neurons of infants and adults differ in 2 significant ways: Growth of neural fibers and
synapses increases connective structures. When synapses are formed, many surrounding neurons die.
This occurs in 20 to 80 percent of the brain region.

Dendrites synapses: Synapses are tiny gaps between neurons where fiber from different neurons
come close together but do not touch. Neurons release chemicals that cross the synapses sending
messages to one another. During the prenatal period the neural tube produces far more neurons than the
brain will ever need. Myelinization: The coating of neural fibers with a fatty sheath called myelin that
improves the efficiency of message transfer. Multi-layered lipid cholesterol and protein covering
produced by neuralgia cause a rapid gain in overall size of brain due to neural fibers and myelination.

Synaptic pruning: Neurons seldom stimulated soon loose their synapses. Neurons not needed at
the moment return to an uncommitted state so they can support future development. However, if
synaptic pruning occurs in old age neurons do lose their synapses. If neurons are stimulated at young
age even though neurons where pruning they will be stimulated again.

Cerebral Cortex: Surrounding the brain, it is the largest most complex brain structure. The cortex
is divided into four major lobes: occipital lobe, parietal lobe, temporal lobe, and frontal lobe which is
the last to develop.

Brain plasticity: The brain is highly plastic. Many areas are not yet committed to specific
functions. If a part of the brain is damaged, other parts can take over tasks that they would not normally
have handled.

Changing states of Arousal

How children develop more regular “sleep patterns” around 4 to 6 months of age: Sleep patterns
are more developed as the brain develops. It is not until the first year of life that the secretion of
melatonin, a hormone produced in the brain, affects more drowsiness in the night than in the day. In
addition, REM is decreased.

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