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Brain plasticity and phantom pains
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This case consists of brain plasticity, what happens when a brain is damaged and phantom limb pains.
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Task 3:Brain plastic and phantom sensations
1. What is brain plasticity?
Brain plasticity (neuroplasticity): the brain’s ability to change and adapt as a result of
experience.
Modern research has demonstrated that the brain continues to create new neural pathways and alter
existing ones in order to adapt to new experiences, learn new information and create new memories.
There are two types of neuroplasticity, including:
- Functional plasticity: the brain’s ability to move functions from a damaged area of the brain to
other undamaged areas.
- Structural plasticity: the brain’s ability to actually change its physical structure as a result of
learning.
Developmental plasticity occurs when neurons in the young brain rapidly sprout branches and form
synapses. Then, as the brain begins to process sensory information, some of these synapses strengthen
and others weaken. Eventually, some unused synapses are eliminated completely (synaptic pruning)
which leaves behind efficient networks of neural connections.
Researchers have identified 4 other types of neuroplasticity:
- Homologous area adaptation: this occurs during the early period of development. If a particular
brain module becomes damaged in early life, its normal operations have the ability to shift to
brain areas that do not include the affected module. The function is often shifted to a module in
the homologous (matching) area of the opposite brain hemisphere.
- Compensatory masquerade: the brain figuring out an alternative strategy for carrying out a task
when the initial strategy cannot be followed due to impairment. The only change that occurs in
the brain is a reorganization of preexisting neuronal networks.
- Cross-modal reassignment: the introduction of new inputs into a brain area deprived of its main
inputs. This occurs because neurons communicate with one another in the same abstract. This is
a change in the actual functional assignment of a local brain region.
- Map expansion: the flexibility of local brain regions that are dedicated to performing one type
of function or storing a particular form of information. When one function is carried out
frequently enough through repeated behavior or stimulus, the region of the cortical map
dedicated to this function grows and shrinks as an individual exercises this function.
Development of the brain
After 14 days, the embryo consists of several sheets of cells with a raised area in the middle, the
primitive body. By 3 weeks after conception, it possesses a primitive brain (a sheet of cells at one end).
This sheet rolls up and forms the neural tube. By 7 weeks, the embryo begins to resemble a miniature
person, and about 100 days after conception, the brain looks distinctly human. However, it does not
begin to form gyri and sulci until 7 months. There is thus a decrease in the plasticity of the brain as one
gets older.
In development, a series of changes take place in a fixed sequence. This program of development has
two extraordinary features. First, nervous-system subcomponents form from cells whose destination
and function are largely predetermined. Second, development is marked by an initial abundance of cells
and connections, by apoptosis.
,Stages of brain development:
1. Cell birth (neurogenesis, gliogenesis)
2. Cell migration
3. Cell differentiation
4. Cell maturation (dendrite and axon growth)
5. Synaptogenesis (formation of synapses)
6. Cell death and synaptic pruning
7. Myelogenesis (formation of myelin)
Deficits in genetic program, intrauterine trauma, the influence of toxic agents, or other factors may lead
to peculiarities or errors in development that contribute to obvious and severe deformities. Less-
pronounced deficits may lead to such problems as learning disabilities or may appear only as subtle
changes in behavior.
Types of abnormal development
Types Symptom
Anencephaly Cerebral hemispheres, diencephalon and midbrain are absent.
Holoprosencephaly Cortex forms as a single undifferentiated hemisphere
Lissencephaly Brain fails to form sulci and gyri
Micropolygyria Gyri are more numerous, smaller and more poorly developed
Macrogyria Gyri are broader and less numerous than typical
Microencephaly Development of brain is rudimentary, low intelligence
Porencephaly Cortex has symmetrical cavities where cortex/white matter should be
Heterotopia Displaced islands of gray matter, caused by aborted cell migration
Calllosal agenesis Entire corpus callosum or part is absent
Cerebellar agenisis Part cerebellum, basal ganglia or spinal cord are absent/ malformed
Neuron generation
The neural tube has multi potential stem cells, which have an extensive capacity for self-renewal. In an
adult, these neural stem cells line the ventricles, forming the
subventricular zone. Stem cells have, besides lining the ventricles,
another function; they give rise to progenitor (precursor) cells. The
progenitor cells also divide, but they eventually produce non-dividing
cells known as neuroblast and glioblasts that mature, respectively into
specialized neurons and glial cells.
Neural stem cells give rise to all of the many specialized brain and
spinal cord cells. Stem cells continue to produce neurons and glia (in
the hippocampus) not just in the early adulthood but also in an aging
brain. This means that when injury or disease causes neurons to die in
an adult, perhaps the brain can be induced to replace those neurons.
However, injury to central nervous system tissue usually is permanent.
The production of new neurons continuously suggests that old neurons
are dying.
, Cell migration and differentiation
The production of neuroblasts destined to form the cerebral cortex is largely complete by the middle of
gestation (4,5 months), whereas the cell migration to various regions continues for a number of months,
with some regions not completing migration until 8 months after birth. During the last 4,5 months of
gestation, the brain is most vulnerable to injury or trauma.
The brain can more easily cope with injury during neuron generation than during cell migration and
differentiation. This is because after neurogenesis has stopped, it does not naturally start again. If
neurogenesis is still progressing, the brain may be able to replace its own injured cells or perhaps
allocate existing healthy cells differently. At the completion of general neurogenesis, cell differentiation
begins, in which neuroblasts become specific types of neurons. Cell differentiation is complete at birth,
whereas neuron maturation can continue into adulthood.
The cortex is organized into various areas that differ from one another in their cellular makeup. The sub-
ventricular zone contains a primitive map of the cortex that predisposes cells born in a certain sub-
ventricular region to migrate to a certain cortical location.
Cells know where these different parts are located because they travel along roads made of radial
glacial cells, each of which has a fiber extending from the sub-ventricular zone to the cortical surface.
The cells from a given region of the sub-ventricular zone only have to follow the glacial rode, and they
end up in the right location. Most cortical neurons follow the radial glial fibers, but a small number
appear to migrate by following some type of chemical signal.
A distinctive feature of neuronal migration in the cerebral cortex is that its layers develop from the
inside out. The neurons of innermost layer V1 migrate to their locations first, followed by those destined
of V. in this way, successive waves of neurons pass earlier-arriving neurons to assume progressively
more exterior positions in the cortex.
Neural maturation
After neurons have migrated to their final destinations and differentiated into specific neuron types,
they begin the process of growing dendrites to provide the surface area for synapses with other cells.
They also extend their axons to appropriate targets to initiate the formation of other synapses. These
processes are part of neural maturation.
Two events take place in the development of a dendrite:
1. Dendritic arborization (branching)
2. The growth of dendritic spines
Dendrites begin as simple, individual processes protruding from the
cell body. Later, they develop increasingly complex extensions, which
is called aborization. The dendritic branches begin to form spines, on
which most dendritic synapses take place. Dendritic development
begins prenatally in humans, and continues long after birth.
In contrast with development of axons, dendritic growth proceeds at a relatively slow rate. The disparity
between the development rates is important; the faster-growing axon is able to contract its target cell
before that cell’s dendrites are completely formed, enabling the axon to influence dendritic
differentiation.
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