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How the brain works
The normal brain
What happens in the brain during an epileptic
seizure?
Who can have seizures?
The epileptic brain
The normal brain
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The brain's structure
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The brain is built up of many
billions of nerve cells, the neurons. The function of the neurons
is to send out electrical impulses which pass from neuron to neuron
with the help of chemical "messengers", the neurotransmitters. When
the electrical impulse reaches the end of a neuron, a chemical substance
is released which sets off a reaction in the "receiver area", a
so-called synapsis in the next nerve cell. This enables the electrical
impulse to travel on through this cell to the next. In this way
electrical impulses are transmitted along the innumerable neuron
chains which are found in the brain. The final result of these impulse
messages depends on which of the neuron link ups were involved.
If it, for example, was impulses from the area of the brain which
is responsible for speech, the so-called speech center, the result
is that we are able to talk. When we use our eyes a lively traffic
of impulses results along the nerve networks which link the eye
with the area at the back of the brain, which is responsible for
us being able to understand what we see. The end result of the neurons
impulse transmission determines what function they have. The transmission
of impulses goes on in an orderly pattern, which ensures a normal
result. The build-up of millions of neuron networks enables the
brain to control the countless functions it is responsible for. |
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The brains braking systems
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Neurons can often be too " eager"
to send on impulses. Therefore the brain has a widely branched network
of braking functions. In fact it has been proved that each individual
neuron network can either have a "stimulating" or an "inhibiting"
effect. This is determined by which neurotransmitter the individual
network uses. Some neurotransmitters have an exitatory effect, for
example glutamate, which encourages further impulse transmission,
whereas inhibitory neurotransmitters, such as GABA (gamma-amino-butyric-acid),
will try to brake further impulse transmission. |
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What happens
in the brain during an epileptic seizure?
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Epileptic seizures, whatever the
seizure-type, always occur because of a too "lively" transmission
of impulses in the brain. What is characteristic of these abnormal
impulses is that they do not occur in the usual "normal" pattern.
A synchronization occurs. They can involve a larger or smaller
group of neurons, or even all the neurons in the entire brain.
The localization determines what symptoms the epileptic seizure
will have. Because they arise in this way, epileptic seizures
can be of many different types. The precise reason why these synchronized
epileptic impulses occur is still not known. In theory there are
two factors which could cause them.
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| GABA |
If the inhibitory neuron network
does not function just as it should, one can imagine that the other
neurons have "free play" and begin to transmit uncontrolled "epileptic"
impulses. This situation will occur if the concentration of the
inhibitory neurotransmitter GABA is not normal. In fact it has been
suggested that there is too little GABA present in the brains of
some people with epilepsy. This has led to the development of drugs
(vigabatrin (Sabril) and tiagabine (Gabitril)) which increase the
concentration of GABA in the brain. These drugs are effective in
the treatment of some types of epilepsy. |
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Glutamate
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The other possibility of an
epileptic seizure occurring is when the inhibitory neuron network
is functioning normally, but the stimulating system is too powerful.
This could be caused by the concentration of the stimulating neurotransmitter
glutamate in the brain being too high. To what extent this situation
exists in people with epilepsy is not fully clear. From tests on
animals with epileptic seizures one knows however, that substances
that counteract glutamate's function, the so-called glutamate antagonists,
can prevent seizures. Many drug companies are therefore in the process
of developing these substances for testing on people with epilepsy. |
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Who can have seizures?
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Seizure threshold
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It is important to understand that,
in principal, anyone can have a seizure. Many people (5% of the
population) will have a single seizure at some time of their life.
There will be a provoking factor, such as extreme stress, lack
of sleep, or an excessive intake of alcohol. Some people can tolerate
extreme stress without having a seizure, while others may have
a seizure when subjected to much less stress. One talks of the
person's "seizure threshold" being reached. This threshold is
arbitrary and can only with difficulties be measured. It is different
from person to person, presumably depending on hereditary factors,
amongst other things. The important thing is that it can be exceeded
in all people, if they are subjected to sufficient strain. Thus
even a "normal" brain can have seizures.
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Electroshock
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This fact is made use of in
psychiatry when, by the application of electrical stimulation of
the brain, a so-called "shock" is triggered off. This is in fact
a seizure. This treatment is given to cure depression. The thing
that is wrong with people with epilepsy is that their threshold
is abnormally low. These people have repeated seizures, often without
special provoking factors. Hereditary factors, the previously described
chemical changes in the brain's function and presumably other factors
which we do not yet understand, all contribute to the low seizure
threshold. |
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The epileptic
brain
| Definition |
It is important to note that
epilepsy is not an illness, but a collection of symptoms which make
themselves apparent by repeated seizures elicited from the nerve
cells. Just as a headache can have many different causes, epilepsy
is a symptom which can be the result of many different illnesses
in the brain. Epilepsy is not contagious. |
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Symptomatic epilepsy
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In about 70% of cases of epilepsy
we can find the cause. This group is called organic or symptomatic
epilepsy. The causes can be head trauma, a congenital malformation
of the brain, lack of oxygen during birth, a brain tumor, a stroke,
a cerebral hemorrhage, alcoholism or some other known factor such
as the aftermath of encephalitis or meningitis. Seizures as a result
of one of these causes often begin in a small group of nerve cells
around the affected area, before spreading to larger areas of the
brain. The seizure's appearance and development can often lead to
symptomatic epilepsy being suspected. |
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Idiopathic epilepsy
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The remaining 30% of persons
with epilepsy have the metabolic or idiopathic (unknown) form. We
do not know the cause of this type of epilepsy, but presume it to
be more widespread chemical changes in the brain. The nature of
these changes may be malformations of ion channels in the membranes
of the nerve cells. In about 30% of cases the idiopathic epilepsy
is inherited. Seizures may be focal or partial or generalized, originating
in the central area of the brain in an interplay with the cerebral
cortex. The focal seizures may spread to the brain and be generalized
whereas the generalized seizures do not spread slowly but involve
the whole brain immediately. |
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The brain's electrical impulses
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The electrical impulses which
originate in the nerve cells and are spread by the axons can be
registered from outside the head by an apparatus which amplifies
the impulses (electroencephalography = EEG). The impulses are recorded
and the curves resulting can be used in making the diagnosis epilepsy. |
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Brain damage as a result
of seizures
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Many epileptic seizures are in themselves
damaging to the brain. This is due to the after effects of the
seizure. Because of respiratory difficulties and even the cessation
of breathing during seizures, while the brain and the muscles
are at the same time consuming large amounts of oxygen, nerve
cells may be lost. Another cause of cell loss can be a too high
concentration of the neurotransmitter glutamate, which can cause
an imbalance in the brain's salt balance, where specially the
extra calcium present can damage the very active cells.
During a seizure the pulse speeds
up as a result of a faster heart beat, which can also be irregular,
thereby affecting the functioning of the brain. Small hemorrhages
can occur in the brain as a result of increased pressure in the
blood vessels which carry blood away from the brain.
Not only convulsive seizures are
thought to damage the brain. New investigations indicate that
the abnormal electrical activity accompanying even minor attacks
as partial seizures may also lead to cells dying.
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Dementia
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The results of such damage can
be a poorly functioning brain with all the symptoms of a reduction
of intellectual powers. This is manifested by bad memory, lack of
concentration, increased tiredness and slower reaction time, all
of which can be grouped together under the heading "dementia". As
a rule, it takes many seizures to bring about dementia. It is, amongst
other things, to avoid these irreversible changes that epilepsy
should always be treated, and in the correct way. |
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