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Brain and Its Functions Part 3
Dr. Prithika Chary
Consultant Neurologist and Neurosurgeon
Adopted by Prof.K.Prabhakar,
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Can We change Brain?
Many a times we are under the mistaken notion thatbrain goes through an irreversible process. Once itis formed we cannot change it. In reality the latest
research show that Brain is Plastic. You can changeyour brain physiology by constant learning.Therefore I am a sixty year old man, I cannotchange my profession or learn new skills is notcorrect. In fact a person at any given point of a time
in his life time subjected to having healthy organscan CHANGE HIS Or HER BRAIN.
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Neuroplasticity
Is the lifelong ability of the brain to
reorganize neural pathways based on
new experiences
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Can the brain change ?
Is it nature or nurture?
Twenty years ago we thought that the structure of the brain
develops during childhood and once that organization in thebrain has been developed that there is very little room forchanges and for plastic alterations.
Now we know that there is enormous capacity.
In addition to genetic factors, the brain is shaped by the
characteristics of a person's environment and by the actions of that same person.
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PLASTIC – Ability to adapt Brain plasticity means the ability of the nervous
system to adapt to changed circumstances, tofind new ways of learning, sometimes after aninjury or a stroke, but more commonly when
you want to acquire a skill for, say, a hobby oreven a new job.
The ability of the brain to change withlearning is what is known as
neuroplasticity.
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NEUROPLASTICITY Involves many processes
Involves changes
occurring throughoutone’s life
Involves all parts of thenervous system, neurons,
glia and vascular cells Is of four different types
as we know it today
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NEUROPLASTICITYNeuroplasticity does not consist of a single type of
morphological change, but rather includes several
different processes that occur throughout an
individual’s lifetime.
Many types of brain cells are involved in
neuroplasticity, including neurons, glia, and
vascular cells.
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Periods of rapid change or plasticity occur in
the brain under four main conditions:
Developmental plasticity: when the immature brain first
begins to process sensory information Activity-dependent plasticity: when changes in the
body, like a problem with eyesight, alter the balance of sensory activity received by the brain
Plasticity of learning and memory: when we alter ourbehavior based on new sensory information
Injury-induced plasticity: following damage to thebrain
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While plasticity occurs over an individual’s
lifetime, different types of plasticity dominate
during certain periods of one’s life and are
less prevalent during other periods.During normal brain development when the
immature brain first begins to process sensoryinformation through adulthood (developmental
plasticity and plasticity of learning and memory).
As an adaptive mechanism to compensate for lostfunction and/or to maximize remaining functions
in the event of brain injury.
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What are the basic processes
involved in brain development? Neurogenesis is the formation of neurons in the brain
Neural migration is the movement of neurons to
different areas of the brain Myelination, the covering of the neuron's axon with a
fatty sheath, allows neurons to conduct signals moreefficiently and protects the axon
Synaptogenesis is the formation of synapses, orconnections between neurons
Synaptic Pruning is the selective elimination of synapses
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Developmental Plasticity: Synaptic
Pruning Over the first few years of life, the brain grows rapidly.
As each neuron matures, it sends out multiple branches
(axons, which send information out, and dendrites, whichtake in information), increasing the number of synaptic
contacts and laying the specific connections from neuron
to neuron.
At birth, each neuron in the cerebral cortex has
approximately 2,500 synapses.
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Developmental Plasticity: Synaptic
Pruning By the time an infant is two or
three years old, the number of
synapses is approximately15,000 synapses per neuron(Gopnick, et al., 1999).
This amount is about twice that
of the average adult brain. Aswe age, old connections aredeleted through a process calledsynaptic pruning.
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ACTIVITY DETERMINED
NEUROPLASTICITY Synaptic pruning eliminates weaker synaptic contacts
while stronger connections are kept and strengthened.
Experience determines which connections will bestrengthened and which will be pruned;
Connections that have been activated most frequently arepreserved.
Neurons must have a purpose to survive.
Without a purpose, neurons die through a process calledapoptosis in which neurons that do not receive or transmitinformation become damaged and die.
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APOPTOSISApoptosis is called “ programmed cell death “
It takes place to avoid redundancy in the nervous
system
For the right cells to die/or less cells to die
nurturing the child’s brain is necessary with
adequate stimulation, because neurons that have
nothing to do will just literally kill themselves
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APOPTOSIS The illustration shows a neuronundergoing a common form of apoptosis.
(A) The healthy neuron has adefined cell membrane and thecytoplasm and nucleus, which
contains DNA, are intact. (B) When apoptosis kicks in, the
cell contorts and the DNA breaksup.
(C) In the final stage of apoptosis,the cell is broken into membrane-bound pieces.
Specialized cells calledmacrophages or microglia removethe debris
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Cortical MapsThe cortex contains maps
These maps represent our skills and our knowledge
of the world.
And the brain's mapmakers are kept very busy,indeed.
When a skill develops or changes, the corticalmaps also change, and neuron populations may beannexed for specific purposes, later abandoned,and sometimes annexed again.
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The adult brain is driven by
behavioral experience We now know that the brain is plastic: it can and
does remodel itself, sometimes within a remarkablyshort period of time.
These biological changes in the adult brain aren'tdriven by developmental timelines or inherited traits.Instead, they are driven by behavioral experience
Just as the migratory behavior of residents can
change the map of a city, so can our learningbehavior change the maps in our brain, causingneurons populations to synchronize their actions,respond to new inputs, and support new skills.
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Practice makes perfect When we approach learning seriously,
however, something else happens: we attend
to a task, we practice it over and over again,and we become emotionally involved.
Under these conditions, brain plasticityhappens - the winemaker can sharpen her
taste buds, the blind person can learn to readBraille, the musician can perfect his pitch, andyou can become an honest-to-goodness guitarplayer.
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Selective attention Why are attention, repetition, and intensive
practice the prerequisites of brain plasticity?
Do we really have to listen to our teachers, goto class every day, and do homework everynight?
In 1890, philosopher and psychologist William
James wrote. My experience is what I agreeto attend to. Only those items which I noticeshape my mind - without selective interest,experience is an utter chaos."
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space and time The crucial role played by the dimensions of
space and time doesn't end with our
behavioral experience. Brain maps change spatially by taking over
neighboring neuronal populations on differentparts of the cortex.
But brain maps can also change in time, bysynchronizing the actions of neurons moretightly so that a specific group of neuronsmay provide near-simultaneous responses to
the same input.
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VersatilityThese timing relationships may actually
help support the plasticity of existing
cortical maps and the generation of newones, because a single neuron canparticipate in the representation of
several different sensory or motorrepresentations at different times.
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Keeping in touch If we take a closer look at a single neuron and
its synaptic connections, we see that timing is
everything. Suppose a neuron sends weak, sporadic
chemical messages to the another neuron.
This situation is a bit like receiving postcards
once every few years from a long-lostacquaintance - the messages aren't alwayseffective enough to cause a sustained reactionin the second neuron
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Weightlifting for the Mind: Enriched
Environments and Cortical Plasticity In the 1960's in Berkeley's biology labs,
Mark Rosenzweig and his colleagues
Edward Bennett, Marian Diamond, andDavid Krech made a proposition -- thatexperience can induce concrete andobservable changes in brain structure – This would profoundly influence ourunderstanding of education and thehuman mind for decades afterwards.
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If negative early experiences could impairbrain development, could positive
experiences enhance the brain?
At Harvard, David Hubel and Torsten Wieselstudied cats raised blind in one eye, and by1962 they had demonstrated that suchdeprivation caused profound structuralchanges in the cats' visual cortex.
This work made it clear that severedeprivation during critical developmentalperiods could have catastrophic effects on agrowing brain, but the question of whetherthe opposite was true remained.
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"cerebral exercise"
Rats raised in an "enriched" environment, with toysand social activities, were not only smarter than rats
raised in impoverished environments, but that theimprovement in performance correlated with anincrease in the weight of the rats' cerebral cortex.
The idea that the brain, like a muscle, might respondto "cerebral exercise" with physical growth was
surprising to many, and gave strength to anincreasingly powerful theory suggesting that allaspects of the mind - from memory, to dreams, toemotions - might have physical correlates.
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Enriched Environments The brain expects, and
perhaps even depends upon,interaction with the
environment in order todevelop and reachmaturation.
Babbling may assist in thedevelopment of languagecapabilities, just as playingwith objects assists indevelopment of motor skills.
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Enriched Environments
Children who are exposed to a rich andvaried education early in life develop a
great capacity for learning throughoutlife.
Real learning, not just rote exercise, can
have a dramatic influence on thephysical structure of the brain.
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The orange dots represent the multiple
synapses on a single neuron.
The extent of synaptic
interconnectivity as we agedetermines our functional abilityto use our brains
In spite of losing neurons as weage, the densitiy of
interconnectivity makes up for theloss
This depends on continuous newlearning & environmentalenrichment
USE IT OR LOSE IT
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Developmental Plasticity – synaptic
pruning Ineffective or weak connections are
"pruned" in much the same way a gardener
would prune a tree or bush, giving the plant
the desired shape.
It is plasticity that enables the process of
developing and pruning connections,allowing the brain to adapt itself to its
environment.
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Plasticity of Learning and Memory
It was once believed that as we aged, the brain’s networksbecame fixed.
In the past two decades, however, an enormous amount of research has revealed that the brain never stops changingand adjusting.
Learning, as defined by Tortora and Grabowski (1996), is
“the ability to acquire new knowledge or skills throughinstruction or experience.
Memory is the process by which that knowledge isretained over time.
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LEARNING The capacity of the brain to change with learning is
plasticity.
So how does the brain change with learning? According to Durbach (2000), there appear to be at least
two types of modifications that occur in the brain withlearning:
1. A change in the internal structure of the neurons, themost notable being in the area of synapses.
2. An increase in the number of synapses betweenneurons.
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Learning WindowsDuring a child's development, there are a series of
time periods, or "windows," in which a child can
best learn or refine a particular ability, such as
speech.
After this time period is over it becomes much
more difficult, sometimes impossible, for the childto learn the same thing.
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Myelination There are millions of neurons,
which form the electricalconnections that let us think.
These cells send their signalsthrough axons, some of which canreach a length of up to a meter inhumans.
Wrapped around many of the axons
are cells which form myelin sheaths,composed mainly of fat.
These sheaths serve to insulate theaxon, letting its signal travel about100 times faster than in anunmyelinated axon.
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Myelination Myelinization is the key to
learning windows
Myelination is the major cause of theincrease in a child's brain size.
At birth, the infant brain weighs 300-350 grams (2/3 to ¾ pound).
In the first four years of life, the
brain increases to 80% of the adultweight of 1200-1500 grams (2.6 - 3.3pounds).
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At birth… Few nerve centers are myelinated at birth.
In the beginning, only reflexes needed for
survival are completely myelinated
However, after birth the primary visual and
auditory cortex neurons rapidly receive their
myelination.
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In childhood…
Myelination continues. During the first year-and-
a-half of life, the corticospinal motor tract receives
its myelination enabling gross control over arms,torso, and legs.
The brain continues to change and mature
during adolescence
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During Adolescence…
Final myelination of the frontal lobes occurs inearly adolescence.
An adolescent's brain reaches the weight of anadult brain by about age fourteen due to myelinaccumulation and dendritic branching.
At this time the potential for contribution toinsight, judgment, inhibition, reasoning, and socialconscience are possible.
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During Adolescence…
The adolescent's frontal lobes are increasingly active, and
this ability enables the adolescent to consider several
things in the mind while comparing or interrelating them. The density of synapses declines during adolescence
due to selective pruning of redundant or unused
connections.
Synapse formation continues despite ongoing pruning
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Into Adulthood…
The brain continuously remodels itself-even into
adulthood.
Synapses continue to be formed in select areas
of the brain but growth of new neurons is
limited
Lifelong enrichment experiences are important
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Injury-induced Plasticity: Plasticity
and Brain Repair During brain repair following injury, plastic changes are
geared towards maximizing function in spite of the
damaged brain. In studies involving rats in which one area of the brain
was damaged, brain cells surrounding the damaged areaunderwent changes in their function and shape that
allowed them to take on the functions of the damagedcells.
Although this phenomenon has not been widely studied inhumans, data indicate that similar (though less effective)
changes occur in human brains following injury.
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Plasticity after amputation When nerve stimulation changes,
as with amputation, the brainreorganizes.
In one theory, signals from afinger and thumb of an uninjuredperson travel independantly toseparate regions in the brain's
thalamus (left). After amputation, however,
neurons that formerly respondedto signals from the finger respondto signals from the thumb (right).
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Brains of human & animals
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Babies start to babble around six to ten
months of age, and that not long afterwardthey say a few words like "no" or "uh-oh.“
At around two years, already more likechildren and less like babies, they beginspeaking grammatically correct sentences
and their vocabulary undergoes a growthspurt.
And by three years, most children canspeak in a manner that is essentially adult-
like.
LANGUAGE ACQUISITION
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LANGUAGE ACQUISITION
Research has identified certain areas of the adult brainthat are typically responsible for specific aspects of language, and these can serve as starting points for
understanding children's brains. The left hemisphere appears to be critical in most right
handers and many left handers
Lesions to the right hemisphere are not usually
associated with language loss, but there is evidence thatthe right hemisphere plays a role in emotion
The right hemisphere has the potential to assume some
language functions if the left hemisphere is damaged.
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BRAIN LATERALIZATION The term brain lateralization
refers to the fact that the twohalves of the human brain
are not exactly alike. Each hemisphere has
functional specializations:some function whose neuralmechanisms are localizedprimarily in one half of thebrain.
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WHAT DOES HANDEDNESS HAVE TO DO WITH
BRAIN LATERALIZATION?
Most humans (70% to 95%)(but not all) have left
hemisphere specialization for language abilities.
5% to 30% have anomalous patterns of
specialization. These might include:
(a) having a right-hemisphere language
specialization or
(b) having little lateralized specialization.
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