MadSci Network: Neuroscience
Query:

Re: What causes seizures? Do seizures come from genetics or nerves?

Date: Fri Oct 19 09:57:51 2001
Posted By: Eric Tardif, Post-doc/Fellow, Institut de Physiologie, Université de Lausanne
Area of science: Neuroscience
ID: 1003332714.Ns
Message:

Dear friend,

The nervous system can be divided into two major systems: the central 
nervous system (brain and spinal cord) and the peripheral nervous system 
(which include peripheral nerves). The term seizure refers to a transient 
alteration of behavior due to abnormal synchronized and repetitive bursts 
of firing of neurons in the central nervous system. Neurons have the 
capacity to communicate with distant neurons by sending electrical 
messages. In other words, they send impulses. These impulses can be of 
various frequencies (ex. 100 impulses/s). Seizures occur when a population 
of neurons fire at abnormally excessive high firing rate (ex. 500 
impulse/s). Partial seizures begin in a localized brain region, whereas 
generalized seizures show widespread involvement of both hemispheres. 
Seizures are a characteristic symptom of epilepsy which is a collection of 
disorders.

It is very important to note that seizures can result from several 
neurological abnormalities (ex. head injury, stroke, metabolic 
disturbances, infections, tumors, toxic agents, degenerative disorders and 
many more). That is, there is not a single cause to seizures. Moreover, in 
many epileptic patients, the exact cause is not known. It is now accepted 
that some individuals may have a genetic predisposition to develop seizures.

One interesting fact about epilepsy is that more often than not, seizures 
may be avoided by taking antiepileptic medications. A look at what the 
medication does in the nervous system is helpful to a better understanding 
of mechanisms producing seizures although a lot remains unknown. As I said 
previously, neurons communicate together by sending electrical impulses. 
You may know that information is exchanged from a neuron to another via a 
chemical agent: the neurotransmitter. Some of these neurotransmitters are 
known to excite neurons (i.e. making them fire) while some others are known 
to inhibit neurons (i.e. making them more « silent »). One important 
typical inhibitory neurotransmitter is called g-amino butyric acid (GABA). 
Many antiepileptic drugs act on GABAergic neurons, enhancing their 
inhibitory activity through various ways. This and other evidence has 
led some scientists to postulate that some forms of epilepsy are related 
to a loss of GABA inhibition in some particular brain regions (often the 
temporal lobe and more particularly the hippocampus). In such cases, the 
excitatory neurons would lack of inhibition and lead to abnormally high 
electrical activity in the brain and to seizures. However, this may not be 
as simple as that. There is also some evidence that GABAergic neurons 
taken from surgically removed human epileptic tissue are not in so bad a
condition. Rather, it was some other kind of neurons that were severely 
damaged and these neurons were actually excitatory. Since the connectivity 
pattern of the subcortical temporal regions are relatively well known, it 
is known that these excitatory cells actually excite GABAergic neurons. 
That is, it may not be a loss of GABA cells that causes seizures but 
rather a loss of excitation of these GABA cells. As you can see, it is 
quite complicated and it should be remembered that although the GABA model 
can account for some kind of epilepsy, there are also several other 
possible causes!

Since I am not an expert in that domain, I recommend you to visit this 
site, where you can ask real experts all your questions about seizures.
http://www.epilepsyfoundation.org/answerplace/


See also:

Shin C, McNamara JO. 1994. Mechanism of epilepsy. Annu. Rev. Med.;45:379-89



Best regards,
Eric






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