CHAPTER 13.docx

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CHAPTER 13
NEURAL DEVELOPMENT
Early Development
Neural plate forms from the ectoderm of embryo a patch of cells on the dorsal surface of
embryo which eventually becomes the nervous system
Cells of the dorsal ectoderm in the neural plate are stem cells that are pluripotent: have the
potential to develop into different types of nervous system cells
The neural plate starts to form a groove, which fuses to form the neural tube
Different sections of neural tube become different parts of nervous system, with the interior
surface becoming the ventricles and the central canal of spinal cord
Telencephalon develops from the cortical plate
As migration occurs from the inside out, the deepest layer of neurons develop first
Although development of axons and dendrites occurs both prenatally and postnatally, cell
differentiation is essentially complete at birth
Problems with any phase of development can lead to significant abnormalities in the CNS
Complete failure of the closure of the neural tube is fatal, resulting in a condition known as
cranioachischisis: characterized by the CNS appearing as a groove in the top of the head and
body --- however is rare
Anencephaly: partial closure of the rostral neural tube, and is characterized by a general
absence of the cerebral hemispheres
Spina Bifida: partial closure of neural tube (posterior?) results in neurological difficulties that
are associated with locomotion rather than cognitive difficulties effects spinal cord
Neurons undergo a long period of synaptogenesis and dendritic branching, producing far more
synapses and dendrites than are needed in the adult brain
Dendritic branching occurs slowly it is on the dendritic spines that most synapses occur
Synaptogenesis or synapse formation, is relatively sparse and occurs relatively independent of
experience shortly after birth to about 2 years, undergoes a period of rapid growth followed
by synapse reduction
Prenatal Development
Prenatal environment can have significant effects on CNS
Plastic change: the ability of the CNS to alter itself in response to environmental stimuli
Critical Periods: of plastic change in which the environment can have a maximal effect on the
CNS
Duration and timing of these critical periods vary by species longer lived animals i.e. humans
have a prolonged critical period that often occurs later in life
Plasticity during critical periods occurs in response to specific experiences -- can be referred
to as either :
a) Experience expectant plastic changes
- Those CNS changes that are dependent on experiences during the critical period for
specific synapses to develop as they should i.e. sensory cortex has these if an organism
doe not experience sensory stimulation during the critical period, long-lasting
impairments in the sensory modality occur
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b) Experience-Dependent Plastic Changes
- Those idiosyncratic experiences that occur during critical periods that also affect brain
development i.e. musical training in childhood can have long-lasting changes on the size
of the auditory cortex in adulthood however these changes were most profound for
those who started before age of 9
Although these changes occur in tandem with individual experience, these changes are not
random
Volumes of cortical gray matter increase until about 4 years of age
Synaptogenesis and dendritic branching both occur prenatally and postnatally and are maximal
during critical periods and are both sensitive to experience expectant and dependent plasticity
Unlike cortical volume changes, white matter volumes increase steadily until about age 20
Myelination is not uniform throughout the cortex, i.e. primary motor and sensory areas have
relatively complete myelination by age 4, whereas frontal cortex does not exhibit complete
myelination until late teens
Parietal Lobe Development
Has inconsistent levels of development at birth; some are more mature than others
Myelination in spinal cord and thalamic centers begins prenatally and is relatively complete by
age 1
Babies 2-3, exhibit large increases in glucose utilization in parietal lobes corresponding to
improvements in visuospatial and visuosensorimotor skill
Under age 6 have difficulty localizing points on their hands
Basic tactile sensations mature early, complex tactile discriminations require more time to
develop
Degree to which sensory systems integrate with other functions, such as motor, depends on the
maturation of other neural systems
Parietal lobe is also a component of the dorsal visual stream one important component of this
is the processing of motion infants first show sensitivity to directional motion sometime during
the 12th week of life, adult levels of global motion processing occurs sometime after age 4
Individuals with Williams syndrome provide an important insight into the development of the
parietal lobe and role that the connections between the parietal lobe and others have in
mediating spatial behaviour
Williams syndrome is a genetic condition in which some of chromosome 7 has been deleted
indiv. Have mild to moderate cognitive impairments, no affect on verbal ability, but significant
difficulties with tasks of visuospatial ability
Smaller brain volume not throughout the CNS, smaller parietal and occipital lobes, as well as
less white matter
Occipital Lobe Development
Development is incomplete at birth
Newborns have sophisticated visual systems, capable of distinguishing 2D and 3D stimuli
Also have rudimentary form perception, become very competent with more complex stimuli,
which may depend on the myelination of the optic tract and requires the optic radiations to
become functionally connected with the sensory organs with other areas of the brain
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By 3 months, optic tract, and optic radiation shows heavy myelination
Amblyopiathe “normal” eye has only poor visual acuity after the cataract is removed
Similar effects are observed when the eyes are misaligned as in strabismus those who
experience corrective measures after the critical period often have impaired depth perception
For humans, critical period for binocular vision begins sometime within in the first few month of
life and peaks between 1 and 3 years of age, critical period ends sometime between age 3&4
Simple face and emotion recognition at adult level doesn’t develop until ages 6-8
More complex tasks requiring the participation of the frontal lobes may not develop until the
teens
Temporal Lobe Development
Function divided into two main types: linguistic ability and hipocampally dependent memory
function
Development of temporal lobe depends on specific structure and function being studied
Linguistic ability is multifaceted development of speech production and comprehension is the
result of the cooperation and development of the fronta and temporal lobes
Infants exhibit stereotypical stages of linguistic development that involve botht he
comprehension and production of speech therefore development of linguistic competence
involves the development of both the frontal and temporal lobes as well as the myelination of
the connections among the lobes
Between ages 1-2, a number of important commissural systems are undergoing myelination
these areas are involved in connecting the right and left hemispheres failure to experience
linguistic stimulation at this time will result in permanent deficits in attaining adultlike linguistic
skill, suggesting that this is the beginning of a critical period for language
2-12, mark changes in dendritic arborisation occurring in speech areas in the brain
Correlation between neural maturation and linguistic ability may reflect idiosyncratic patterns
of maturation of the speech areas that may have a large environmental component
Hippocampus attains adult volumes around 7-10 months and shows high levels of glucose
utilization from birth unlike rest of temporal lobe structures
Other structures in the limbic system exhibit a major growth spurt between 1-2 however,
hippocampally dependent memory processes are not mature at birth
Hippocampus is one of the sites in adulthood that exhibits neurogenesis therefore it may be
that the memory functions of the hippocampus result from its ability to extend its
developmental period throughout the life span
Memory functions of the hippocampus rely on functional connections with the parietal and
front lobes
Ex. Although visual memory is functional at birth in primates, it does not reach adult levels of
performance until about 5 years of age in humans
Autobiographical memories are subserved by a diverse number of areas, including cingulated,
parietal, temporal, and prefrontal areas
Frontal lobes are among the last to mature
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