MadSci Network: Neuroscience |
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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