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Epileptic seizures are caused by excessive
electrical activity of neurons in the
brain, and this electrical activity is generated
by the flow of sodium, potassium and calcium
ions into / out of the neuron. The ions
enter and leave via structures known as
ion channels (see figure 1), and once the
current reaches a certain level, the neuron
"fires", and a signal known as an action
potential travels down its long arm (axon)
to an axon terminal (see figure 2). At the
axon terminals are synapses.
Synapses are essentially narrow gaps between
the end (terminal) of one neuron and the
beginning of another, across which signals
are transported (the beginning of a neuron
is called a dendrite - see figure 2. A neuron
may have several dendrites or just one).
Signals do not cross synapses in an electrical
form, but rather a chemical called a neurotransmitter
is released from the end of the neuron (the
pre-synaptic membrane), and this
carries the information across the synapse
in a chemical from. The next neuron has
receptors
for the neurotransmitter on its surface
(the post-synaptic membrane), and
when the neurotransmitter binds to its receptor,
the signal is converted back to an electrical
form and continues its journey. (See figure
3)
Neurotransmitters can be either excitatory
or inhibitory, meaning that the next
neuron will be stimulated or suppressed
respectively. The main excitatory neurotransmitter
in the brain is called glutamate.
If too much glutamate is released
from the pre-synaptic membrane, the next
neuron may become hyperexcitable
and more prone to seizure activity.
The major inhibitory neurotransmitter in
the brain is called GABA. If there
is too little GABA released at the
pre-synaptic membrane, or if its receptors
are not functioning properly, this also
make neurons more active and susceptible
to seizures.
NOTE: Neurons that release glutamate
from their terminal are often called excitatory
neurons, whilst those that release GABA
are referred to as inhibitory neurons.
Figure 1: A diagram of
an ion channel
Figure 2: The structure
of a typical neuron
Figure 3: An axon terminal
/ synaptic terminal
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