Download Development of the Brain

Chapter 5
Development and Plasticity
of the Brain
Development of the Brain.
• The human central nervous system begins to
form when the embryo is approximately 2
weeks old.
– The dorsal surface thickens forming a
neural tube surrounding a fluid filled cavity.
– The forward end enlarges and
differentiates into the hindbrain, midbrain
and forebrain.
– The rest of the neural tube becomes the
spinal cord.
Development of the Brain
• The fluid-filled cavity becomes the central
canal of the spinal cord and the four
ventricles of the brain.
– The fluid is the cerebrospinal fluid.
Development of the Brain
• At birth, the human brain weighs
approximately 350 grams.
• By the first year. the brain weighs
approximately 1000 grams.
• The adult brain weighs 1200-1400 grams.
Development of the Brain
•
The development of neurons in the brain
involves the following four processes:
1. Proliferation
2. Differentiation
3. Myelination
4. synaptogenesis
Development of the Brain
• Proliferation refers to the production of new
cells/ neurons in the brain primarily occurring
early in life.
• Early in development, the cells lining the
ventricles divide.
• Some cells become stem cells that continue
to divide
• Others remain where they are or become
neurons or glia that migrate to other
locations.
Development of the Brain
• Migration refers to the movement of the newly
formed neurons and glia to their eventual
locations.
• Migration occurs in a variety of directions
throughout the brain.
• Migration occurs via cells following chemical
paths in the brain of immunoglobins and
chemokines.
Development of the Brain
• Differentiation refers to the forming of the
axon and dendrite that gives the neuron its
distinctive shape.
• The axon grows first either during migration
or once it has reached its target and is
followed by the development of the dendrites.
• Neurons differ in their shape and chemical
component depending on their location in the
brain.
Development of the Brain
• Myelination refers to process by which glia
produce the fatty sheath that covers the
axons of some neurons.
• Myelin speeds up the transmission of neural
impulses.
• Myelination first occurs in the spinal cord and
then in the hindbrain, midbrain and forebrain.
• Myelination occurs gradually for decades.
Development of the Brain
• Synaptogenesis is the final stage of neural
development and refers to the formation of
the synapses between neurons.
• Occurs throughout the life as neurons are
constantly forming new connections and
discarding old ones.
• Synaptogenesis slows significantly later in the
lifetime.
Development of the Brain
•
Originally believed that no new neurons
were formed after early development.
• Recent research suggests otherwise:
1. Stem cells are undifferentiated cells found in
the interior of the brain that generate
“daughter cells” which can transform into
glia or neurons.
2. New olfactory receptors also continually
replace dying ones.
Development of the Brain
• Development of new neurons also occurs in
other brain regions.
– Example: songbirds have a steady
replacement of new neurons in the singing
area of the brain.
• Stem cells differentiate into new neurons in
the adult hippocampus of mammals and
facilitate learning.
Development of the Brain
• Axons must travel great distances across the
brain to form the correct connections.
• Sperry’s (1954) research with newts indicated
that axons follow a chemical trial to reach
their appropriate target.
• Growing axons reach their target area by
following a gradient of chemicals in which
they are attracted by some chemicals and
repelled by others.
Development of the Brain
• When axons initially reach their targets, they
form synapses with several cells.
• Postsynaptic cells strengthen connection with
some cells and eliminate connections with
others.
• The formation or elimination of these
connections depends upon input from
incoming of axons.
Development of the Brain
• Some theorists refer to the idea of the
selection process of neural connections as
neural Darwinism.
• In this competition amongst synaptic
connections, we initially form more
connections than we need.
• The most successful axon connections and
combinations survive while the others fail to
sustain active synapses.
Development of the Brain
• Nerve growth factor (NGF) is a type of
neurotrophin released by muscles that
promotes the survival and growth of axons.
• The brain’s system of overproducing neurons
and then applying apoptosis enables the
exact matching of the number of incoming
axons to the number of receiving cells.
Development of the Brain
• A neurotropin is a chemical that promotes the
survival and activity of neurons.
• Axons that are not exposed to neurotropins
after making connections undergo apoptosis,
a preprogrammed mechanism of cell death.
• Therefore, the healthy adult nervous system
contains no neurons that failed to make
appropriate connections.
Development of the Brain
• The elimination and period of massive cell
death is part of normal development and
maturation.
• After maturity, the apoptotic mechanisms
become dormant.
• Neurons no longer need neurotrophins for
survivals, but neurotrophins increase the
branching on axons and dendrites throughout
life.
Development of the Brain
• Early stages of brain development are critical
for normal development later in life.
• Chemical distortions in the brain during early
development can cause significant
impairment and developmental problems.
Development of the Brain
• Fetal alcohol syndrome is a condition that
children are born with if the mother drinks
heavily during pregnancy.
• The condition is marked by the following:
– Hyperactivity and impulsiveness
– difficulty maintaining attention
– varying degrees of mental retardation
– motor problems and heart defects
– facial abnormalities.
Development of the Brain
• The dendrites of children born with fetal
alcohol syndrome are short with few
branches.
• Exposure to alcohol in the fetus brain
suppresses glutamate and enhances the
release of GABA.
• Many neurons consequently receive less
excitation and exposure to neurotrophins than
usual and undergo apoptosis.
Development of the Brain
• Children of mothers who use cocaine during
pregnancy show a decrease in language
skills, a slight decrease in IQ scores and
impaired hearing.
• Children of mothers who smoked during
pregnancy are at increased risk for low birth
weight, sudden infant death syndrome,
ADHD, long term intellectual deficits and
impairments of the immune system.
Development of the Brain
• Neurons in different parts of the brain differ
from one another in their shape and chemical
components.
• Immature neurons transplanted to a
developing part of the cortex develop the
properties of the new location.
• Neurons transplanted at a later stage of
development develop some new properties
but retain some of the old properties.
• Example: Ferret experiment
Development of the Brain
• The brain has some limited ability to
reorganize itself in response to experience.
– Axons and dendrites continue to modify
their structure and connections throughout
the lifetime.
– Dendrites continually grow new spines.
• The gain and loss of spines indicates new
connections and potentially new information
processing.
Development of the Brain
• Rats raised in an enriched environment
develop a thicker cortex and increased
dendritic branching.
• Measurable expansion of neurons has also
been shown in humans as a function of
physical activity.
• The thickness of the cerebral cortex declines
in old age but much less in those that are
physically active.
Development of the Brain
• Neurons also become more finely tuned and
responsive to experiences that have been
important in the past.
• This may account for the fact that blind
people often have enhanced tactile senses
and increased verbal skills.
– The occipital lobe normally dedicated to
processing visual information adapts to
also process tactile and verbal information.
Development of the Brain
• Extensive practice of a skill changes the brain
in a way that improves the ability for that skill.
• For example, MRI studies reveal following:
– the temporal lobe of professional
musicians in the right hemisphere is 30%
larger than non-musicians.
– thicker gray matter in the part of the brain
responsible for hand control and vision of
professional keyboard players
Development of the Brain
• Practicing a skill reorganizes the brain to
maximize performance of that skill.
• Certain types of training may also exert a
bigger effect if it begins early in life.
Development of the Brain
• Focal hand dystonia or “musicians cramp”
refers to a condition where the reorganization
of the brain goes too far.
• The fingers of musicians who practice
extensively become clumsy, fatigue easily
and make involuntary movements.
• This condition is a result of extensive
reorganization of the sensory thalamus and
cortex so that touch responses to one finger
overlap those of another.
Plasticity After Brain Damage
• Survivors of brain damage show subtle to
significant behavioral recovery.
• Some of the mechanisms of recovery include
those similar to the mechanisms of brain
development such as the new branching of
axons and dendrites.
Plasticity After Brain Damage
• Possible causes of brain damage include:
– Tumors
– infections
– exposure to toxic substances
– degenerative diseases
– closed head injuries.
Plasticity After Brain Damage
• A closed head injury refers to trauma that
occurs when a sharp blow to the head drives
the brain tissue against the inside wall of the
skull.
– One of the main causes of brain injury in
young adults
• A stroke or cerebrovascular accident is
temporary loss of blood flow to the brain.
– A common cause of brain damage in the
elderly
Plasticity After Brain Damage
• Types of strokes include:
• Ischemia -the most common type of stroke,
resulting from a blood clot or obstruction of an
artery. Neurons lose their oxygen and glucose
supply.
• Hemorrhage -a less frequent type of stroke
resulting from a ruptured artery. Neurons are
flooded with excess blood, calcium, oxygen
and other chemicals.
Plasticity After Brain Damage
• Ischemia and hemorrhage also cause:
• Edema-the accumulation of fluid in the brain
resulting in increased pressure on the brain
and increasing the probability of further
strokes.
• Disruption of the sodium-potassium pump
leading to the accumulation of potassium ions
inside neurons.
Plasticity After Brain Damage
• Edema and excess potassium triggers the
release of the excitatory neurotransmitter
glutamate.
• The overstimulation of neurons leads to
sodium and other ions entering the neuron in
excessive amounts.
• Excess positve ions in the neuron block
metabolism in the mitochondria and kill the
neuron.
Plasticity After Brain Damage
• A drug called tissue plasminogen activator
(tPA) breaks up blood clots and can reduce
the effects of an ischemic strokes.
• Research has begun to attempt to save cells
in the penumbra or region that surrounds the
immediate damage by:
– blocking glutamate synapses
– opening potassium channels
Plasticity After Brain Damage
• One of the most effective laboratory methods
used to minimize damage caused by strokes
is to cool the brain.
• Mechanisms are uncertain but cooling
someone during the first three days is
beneficial.
Plasticity After Brain Damage
• Cannabanoids have also been shown to
potentially minimize cell loss after brain
stroke, closed head injury and other kinds of
brain damage.
• Benefits are most likely due to cannabinoids
antioxidant or anti-inflammatory actions.
• Application of omega-3 fatty acids, a major
component of cell membranes, may help to
block apoptosis and other neural damage.
Plasticity After Brain Damage
• Following brain damage, surviving brain
areas increase or reorganize their activity.
• Diaschisis -decreased activity of surviving
neurons after damage to other neurons.
• Because activity in one area stimulates other
areas, damage to the brain disrupts patterns
of normal stimulation.
• Use of drugs to stimulate activity in healthy
regions of the brain after a stroke may be a
mechanism of later recovery.
Plasticity After Brain Damage
• Destroyed cell bodies can not be replaced,
but damaged axons do grow back under
certain circumstances.
• If an axon in the peripheral nervous system is
crushed, it follows its myelin sheath back to
the target and grows back toward the
periphery at a rate of about 1 mm per day.
Plasticity After Brain Damage
• Damaged axons only regenerate 1 to 2
millimeters in mature mammals.
• Paralysis caused by spinal cord damage is
relatively permanent.
• Scar tissue makes a mechanical barrier to
axon growth.
• Myelin in the central nervous system also
releases proteins that inhibit axon growth.
Plasticity After Brain Damage
• Collateral sprouts are new branches formed
by other non-damaged axons that attach to
vacant receptors.
• Cells that have lost their source of innervation
release neurotrophins that induce axons to
form collateral sprouts.
• Over several months, the sprouts fill in most
vacated synapses and can be useful, neutral,
or harmful.
Plasticity After Brain Damage
• Postsynaptic cells deprived of synaptic inputs
develop increased sensitivity to the neurotransmitter
to compensate for decreased input.
• Denervation supersensitivity- the heightened
sensitivity to a neurotransmitter after the destruction
of an incoming axon
• Disuse supersensitivity- the hypersensitivity to a
neurotransmitter after a result of inactivity by an
incoming axon.
• Both due to increased number and effectiveness of
receptors.
Plasticity After Brain Damage
• Phantom limb refers to the continuation of
sensation of an amputated body part and
reflects this process.
• The cortex reorganizes itself after the
amputation of a body part by becoming
responsive to other parts of the body.
• Original axons degenerate leaving vacant
synapses into which others axons sprout.
Plasticity After Brain Damage
• Phantom limb can lead to the feeling of
sensations in the amputated part of the body
when other parts of the body are stimulated.
Plasticity After Brain Damage
• Deafferenated limbs are limbs that have lost
their afferent sensory input.
• Deafferented limbs can still be used but are
often not because use of other mechanisms
to carry out the behavior are easier.
• The study of the ability to use deafferented
limbs has led to the development of therapy
techniques to improve functioning of brain
damaged people.
– focus on what they are capable of doing.