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Summary Anderson: Developmental Neuropsychology - A clinical approach
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PEDIATRISCHE NEUROPSYCHOLOGIE – SAMENVATTINGChapter 2: Cerebral Development
Introduction
- The development of the human CNS is a protracted process that commences early in
gestation and continues into adulthood, following a series of precise and genetically
predetermined stages. It involved a sequence of complicated and overlapping
processes, the outcome of which is partially determined by the outcome of each of the
previous stages of development. Much early research was based on:
• Animal models
• Brain-damaged subjects.
- New techniques:
• Electro microscopy
• Insertion of chemical markers
• Neuronal and tissue transplantation.
- A number of aspects of CNS development are of relevance to professionals interested in
both normal and aberrant patterns of cognitive development.
1. Knowledge of the biological processes and timing of CNS maturation may lead to the
identification of parallels between specific stages of CNS development and
associated cognitive progress.
2. An understanding of the sequence of events occurring within the CNS and their
timing may enhance current knowledge of the nature and possible recovery of
cognitive deficits exhibited by children sustaining pre- en postnatal CNS insult. Already
research described defined periods of neuronal redundancy within the CNS had led to
hypothesis linking these periods with increased cerebral plasticity.
- Increased awareness of the interplay between genetically predetermined factors and
experiential/environmental influences in both normal and aberrant development has
emphasizes the potential for optimizing development and improving recovery following
CNS insult via enriched environment experiences and more traditional rehabilitation
methods.
- Aim of this chapter: highlight the ongoing and dynamic interplay between biological,
cognitive, and psychosocial factors in the process of development. Some say this
interplay is least relevant in the early stages of CNV development: “genetically
predetermined” or “hard-wired”. But even during the initial stage of development,
there is a continuous dialogue between biological and environmental influences.
Brain development: general principles
- Structural morphology of the brain is mature by birth, but growth continues during the
postnatal period. The fastest rate of brain growth occurs prenatally. Between birth and
adulthood the human brain quadruples in size, increasing from around 400g at birth to
1500g at maturity in early adulthood, peaking between 18 and 30 years en then
commencing a gradual decline.
- Postnatal increase in brain weight is largely due to:
Differentiation
Growth
Maturation of existing neurons, including elaboration of dendrites and synapses
Ongoing myelination
- Cerebral Development: There are 2 qualitative distinct stages of CSN development
(with birth providing a rough marker for the transition):
1. Prenatal
o Primary concerned with the structural formation of CNS.
o Is largely genetically determined
o Interruptions to development during this period, via genetic mechanisms or
interuterine trauma or infections significant impadt on cerebral structure
abnormal brain morphology
2. Postnatal:
o Functional elaboration of the CNS:
o Dendritic arborisation
o Myelination
, o Synaptogenesis
o Although still largely genetically regulated, these processes are thought to be
more susceptible to the impact of neuronal activity and thus to environmental
and experiential influences.
o Interruptions: will have less impact on gross brain morphology, but may interfere
with ongoing CNS elaboration and of the development of interconnections and
functional systems within the CNS.
- Around day 40 of embryonic life: CNS begins to develop. 100 days gestation: brain is
recognizable in its mature form. Not mature until early adolescence.
- Cerebral development has a range of developmental phenomena that have been
observed to occur simultaneously, reflecting differential developmental timing throughout
various areas of the brain, and varying developmental models for specific elements of the
cerebral system.
- Within the immature CNS, two major developmental processes have been described:
1. Simple additive development, where there is an ongoing accumulation or growth
process. It is generally agreed that several neuronal processes develop in this way.
a. Myelination of the nervous system is one such process development
progressing in a stage-like manner throughout the CNS during childhood and
early adolescence.
b. Similarly, there is a continuous increase in the formation and elaboration of
dendritic connections throughout the CNS.
2. Exhibiting periods of regression, characterized by an initial overproduction, and
following by an elimination of redundant elements.
a. Prenatally, the number of neurons generated is in excess of what is required in
the mature CNS. During the differentiation stage, a number of these redundant
neurons die off.
b. Same for synapses.
These regressive processes are not seen as detrimental (schadelijk) to the functioning
of the CNS, but rather represent finetuning, with elements of the system that are not
locked into functional networks dying off to ensure efficient, direct transmission.
- Series of growth spurts, rather than a gradual, continuous developmental progression,
with the anterior regions the last to reach maturity:
1. 24-25 weeks of gestation: earliest spurts. Coinciding (samenvallen) with the
completion of neuronal generation.
2. First year of life: due to dendritic and synaptic development and myelination
3. 7-9 years
4. Final spurt: 16-19 years
- A disruption to development during a growth spurt may be particularly detrimental
(schadelijk) to ongoing development, causing a cessation (ophouden) of development or
altering the course of development. Associated with these growth spurts is the concept
of critical or sensitive periods.
- The critical period is a stage in the developmental sequence during which an aspect of
behavioral function may experience a major progression. If this progression does not occur
appropriately, then it may never occur (example: visual deprivation during the visual
critical period has been shown to have irreversible effects on ongoing maturation of
particular visual processes). Critical periods are seen as a window of opportunity during
which skills need to be consolidated so that the system involved can then establish
interconnections with other systems. Some researchers have extrapolated such
observations to suggest that intensive learning should be practiced during these growth
spurts/critical periods.
- Influences on brain development: genes, intrauterine trauma, maternal nutrition, maternal
alcohol/drugs/stress, and exposure to toxins.
- Sequence in which the areas of the brain mature
1. Hierarchical progression within the CNS: cerebellar/brain stem areas maturing first,
followed by posterior areas, and lastly anterior regions, particularly the frontal cortex.
a. Progress in spurts, representing the ongoing elaboration of the system.
b. Supported by research
c. Arguments against it: not all CNS development conforms to this hierarchical
model. I.e. synaptogenesis appears to be simultaneous in multiple areas and
layers of the cortex with neurotransmitter receptors throughout the brain
reported to mature at the same time.
, 2. Concurrent development: where posterior and anterior structures develop along
approximately the same time line.
a. Researchers say this may be present in non-human species, but not in humans.
Conclusion: Clearly CNS development is complex, with a range of developmental
mechanisms occurring both in sequence and simultaneously. Any disruption or trauma will
result in irreversible change to these processes and to the final outcome. Effects of more
subtle interruptions is difficult to predict.
Influences on brain development
- Risk factors for anomalies in the CNS development:
o Prenatal factors: maternal stress and age, health (history of infection, rubella, AIDS,
herpes simplex), nutrition (diet, malnutrition), drug and alcohol addiction,
environmental toxins.
o Postnatal factors: birth complications, nutrition, environmental toxins (lead,
radiation, trauma), cerebral infection, environment/experience.
May lead to dramatic structural malformations and cerebral reorganization.
o Psychosocial factors: quality of mother-child relationship, level of stimulation
available to the child, social support structures, access to resources.
Thus: children with severe CNS lesions or insults who come from disadvantaged
environments will show particularly poor development, children with less severe insults
from well-resourced families show best outcome.
Prenatal CNS development
- Structural features of CNS development
Fertilized cell experiences rapid cell division quickly becomes the embryonic disc.
Embryonic disc exists of three layers, which later form specific organic systems within
the human body.
Second week of gestation: the nervous system emerges via a process of neurolation
from the outer layer (ectoderm) of the embryonic disc, which forms in on itself and
forms a tube
Third week of gestation: a neural plate becomes visible as a thickened area of the
ectoderm. Gradually a longitudinal neural groove begins to form, and is flanked by
two edges or neural folds.
Fourth week of gestation: Neural folds deepen and fold until they fuse, creating the
neural tube.
- Disruptions may lead to:
Myelomeningocele: incomplete closure of the spinal cord, resulting in spina
bifida.
Anencephaly: incomplete closure of the neural tube, leading to an absent skull
vault, and usually incompatible with life.
Arnold-Chiari malformation: structural abnormality of the CNS often associated
with hydrocephalus.
- The neural tube develop along three different dimensions: length, circumference and
radius. With each dimension having relevance for specific features of the CNS.
Length: important for the major structural aspects of the CNS: the forebrain,
midbrain and spinal cord bulges or vesicles which are recognizable as mature
CNS in the 5th week. Disruption during this phase leads to a failure in the formations
of these structural divisions:
o Holoprosencephaly: failure to form two cerebral hemi’s.
o Craniosynostosis: incomplete fusion of the skull.
Dimension: is responsible for differentiation between sensory (dorsal) and motor
(ventral) systems.
Radial: leads to the layers and cell types observed within the brain.
Cellular basis of development
- The nervous system consists of two main classes of cells:
Neurons develop from neuroblasts. Active role: they are the basic functional
units of the CNS, and transmits impulses within a complex network of
interconnecting brain cells.
Glial cells develop from glioblasts. Have a supportive and nutrient role within
the nervous system, supporting the neurons, enabling regeneration of damaged
neurons, producing scar tissue to occupy damaged sites and transporting nutrients
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