Chapter 13: Neural Development and Developmental Disorders
The brain has not completed its developmental journey at birth, and there is long
period of extended development that may be completed only sometime in
Many of the striking behavioural changes that occur childhood correspond w/
periods of brain growth or reorganization.
Early in embryonic life (approximately three weeks after conception), neural plate
(patch of cells that on the dorsal surface of embryo, which eventually becomes
nervous system) forms from ectoderm of embryo. Cells of the dorsal ectoderm in
neural plate are stem cells (embryonic cells that develop into any type of cell in
body) that are pluripotent (cells that can develop into different types of cells),
meaning have potential to develop into different types of nervous system cells.
As development progresses, neural plate start to form groove, which by embryonic
day 24 fuses to form neural tube. Different sections of neural tube become
different parts of nervous system, w/ interior surface of neural tube becoming
ventricles and central canal of spinal cord.
Between third and fifth months of gestation, rapid cell proliferation and neural
migration are dominant events.
Cells of neural tube w/in ventricular zone are rapidly dividing through process
Embryonic day 40, there are three prominent bumps on anterior portion of neural
tube. Bumps eventually form forebrain, midbrain and hindbrain of central nervous
Cells migrate from interior ventricular zone to final location by following certain
types of glia.
Beginning second month of gestation, telencephalon undergoes tremendous growth,
developing from cortical plate.
Deepest layer of neurons develop first and subsequent layers of neurons migrate
through already established neurons to reach their destination.
Once neurons migrate, begin grow axons and dendrites and differentiate into final
form. Development of axons and dendrites occurs both prenatally and postnatally,
cell differentiation essentially complete at birth. Exact mechanism underlying how
axons grow toward their target not known, tips of growing axons and dendrites
follow chemicals, arranging themselves in orderly fashion in position relative to
initial place on cortical plate.
Problems w/ any phase of development can lead to significant abnormalities in
CNS. Particularly vulnerable during last four to five months of gestation, failures
at any point in CNS development have significant impact on final form of brain.
Adverse events can originate from problems w/in neurons themselves, or can be
introduced by external factors. External events include intrauternine trauma or exposure to toxins. Effects of these
disruptions often depend on nature, duration and extent of disruption. Type of
CNS malformation give us clue as to when disruption occurred.
One of more common congenital malformations of CNS occurs when neural tube
fails to close. Complete failure of closure of neural tube is fatal, resulting in
condition known as craniorachischisis, characterized by CNS appearing as groove
in top of head and body. Syndrome ranging spina bifida to anencephaly result
from partial closure of neural tube. Anencephaly occurs when rostral part of
neural plate does not fuse and is characterized by general absence of cerebral
hemispheres. Anencephaly is generally fatal, not all neural tube defects are fatal.
Spina Bifida is disorder that caused by failure of neural tube to close completely.
Number of diff. subtypes of spina bifida, symptoms of which depend on part of
neural tube that did not close.
By seventh month of prenatal development, most neurons migrated and
differentiated into their final forms. Neurons undergo long period of synaptogensis
and dendritic branching, producing more synapsis and dendrites than needed in
Synaptogensis and dendrite branching occur after birth, some cases into adulthood.
Dendritic branching occurs slowly, w/ initial dendrites appearing as simple
extensions exiting cell body of neuron. W/ time, dendrities become more complex,
adding branches and spines. It is on dendritic spines that most dendritic synapses
Early embryonic life, synaptogenesis, or synapse formation, relatively sparse and
occurs relatively independent of experience. Period extending from shortly before
birth to about 2 years of age, synapse formation enter period of rapid growth.
Thereafter, period of synapse reduction begins occur.
Appears that about 50% more neurons produced in developing brain required in
adult brain. Death actually normal and critical feature of development, which
occurs predictable waves. Much neural death is apoptotic, or planned, programmed
Unclear what triggers apoptosis, clear that apoptotic changes controlled by genes.
Synapses that do not make functional connections or make incorrect connections
are especially likely to die, leaving room for other synapses to sprout.
Most striking features of any babys early development is emergence of behaviors
such as sitting, walking, or speaking. These behaviors are correlated w/ extensive
growth in cortical areas of brain, which increase its volumes by four times between
birth and adulthood.
Possible to approach question from number of different perspectives.
Plastic change is ability of CNS to alter itself in response to environmental stimuli.
There are critical periods of plastic change in which environment can have
maximal effect on CNS. Duration and timing of these critical periods vary by
species, although appear that longer-lived animals exhibit prolonged critical periods
that often occur later in life, well beyond prenatal period. Plasticity during critical periods occur response to experience-expectant or
experience-dependent plasticity phenomena.
Experience-expectant plastic changes are those CNS changes that dependent on
experience(s) during critical period for specific synapses to develop as they should.
Much of sensory cortex appears have these experience-expectant critical periods,
and studies demonstrated organism does not experience sensory stimulation during
critical period, long-lasting impairments in sensory modality occur.
Experience-dependent plastic changes are idiosyncratic experiences that occur
during critical periods that also affect brain development. Musical training in
childhood have long-lasting changes on size of auditory cortex in adulthood BUT
these changes were most profound for individuals who began practicing their
instrument of choice before they were 9 years old.
Critical periods during which musical training can have maximal effects on cortex.
These changes are not random.
Volumes of cortical grey matter increase until 4 years of age w/ much of postnatal
growth and plastic change that occurs in brain resulting from synaptogensis,
myelination of axons and dendritic branching.
Synaptogensis and dendrtic branching maximal during critical periods. Brains
ability to engage in plastic change often reduced as we age, especially once critical
period has passed.
Plastic areas change in response to significant environmental events.
Factors underlying plastic change not completely understood, appear