Pathophysiology Of An Electrical Impulse Carrying A Neuronal Message

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Pathophysiology of Epilepsy
An electrical impulse carrying a neuronal message is called an action potential. During action potential, the neural membrane allows a net positive inflow of ions into the cell and negatively charged ions out of the cell. This causes a voltage change in the neuronal membrane which is also known as depolarization. Ions that participate in establishing an action potential are sodium, potassium, calcium and chloride. In a normal brain, hyper excitability of neurons is achieved by different inhibitory mechanisms.
There are two types of nerve impulses
1. Excitatory: Glutamate is the main excitatory neurotransmitter in the brain.
2. Inhibitory: Gamma amino Butyric acid (GABA) is
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Increased Na+conductance (B, upper panel) creates a situation in which a single action potential initiates sustained depolarization as a PDS
(B, lower panel). Decreased K+ conductance (C, upper panel) also can predispose to PDS. (Adapted with permission from Chang BS, Lowenstein DH. Epilepsy. N Engl J Med 2003;349:1261. Copyright © 2003 Massachusetts Medical Society.) by Henry R. Thomas. Epilepsy Board Review Manual

Mechanism of Seizure Formation
1. Excitation of a group of nerves: During a seizure, a small group of abnormal neurons repeatedly fire rapid action potentials. Therefore, there is no resting or refractory period for these neuron and they have a have prolonged depolarization. These neurons then transmit these impulses to adjacent neurons. A seizure occurs when a large number of neurons are involved and produce electrical discharges that cause a storm of electrical activity in the brain or to distant areas through established anatomic pathways. There is influx of Na+ and Ca++ ions and involves neurotransmitters like Glutamate and Aspartate.
2. Too Little Inhibition: Decreased inhibitory neurotransmission which is mainly brought about by Gamma amino butyric acid (GABA).
3. Hypersyncronization of a neuronal population: A single hyperexcitable neuron cannot generate a seizure. An adequate number of hyperexcitable, hypersynchronized neurons in a sustained depolarized state
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