Physiology of Seizures
In order to understand the process of electrical transmission within nerve cells, it is important to review the structure and function of a nerve. A nerve cell can be thought of as a tube, having an inner section, and bounded by a cell wall (membrane). Importantly, the chemical composition of the inside of the cell and the outside of the cell are very different. Specifically, there is a difference in the concentration of sodium and potassium salts, with sodium being much higher on the outside, and potassium being much higher on the inside. In a normal resting state, special pumps (called membrane pumps) are continually at work to maintain each salt in its proper location.
When a nerve is called upon to transmit an electrical signal, a sudden movement of these salts from one side of the cell's membrane to the other occurs. This movement spreads like a wave from one end of the nerve to the other, until it reaches the end. At this point, the nerve's signal may be transmitted to the next nerve cell either by a direct extension of this process, or, more commonly, by releasing a special chemical called a neurotransmitter.
Neurotransmitters generally have one of two special functions. One type is responsible for encouraging cell-to-cell communication and is referred to as an "excitatory" neurotransmitter. The second type is able to slow down, or even stop cell-to-cell communication and is called an "inhibitory" neurotransmitter. In some cases, overactivity of excitatory neurotransmitters or underactivity of inhibitory neurotransmitters may lead to seizure activity by allowing an uncoordinated flow of electrical activity in the brain.
Interestingly, certain areas of the brain are more likely than others to be the source of a seizure. These include the motor cortex (responsible for the initiation of body movement) and the temporal lobes (including a special deep area called the hippocampus, which is involved in memory). The reason for this likelihood may be that nerve cells in these areas are particularly sensitive to certain situations that can provoke abnormal electrical transmission. Examples include sensitivity to decreased oxygen levels, metabolic changes, and infection, any of which may lead to a seizure.
Many types of brain abnormalities can be responsible for producing seizure activity. Abnormal discharges may spread to other cells in a local area or to remote areas of the brain, resulting in intermittent disturbance in the brain's normal functions. Changes in brain biochemistry and communication between brain cells occur. These basic neurofunctional abnormalities that lead to epilepsy produce the clinical symptoms that are seen. In turn, recurrent seizures or prolonged seizures can cause injury to the brain. Seizures that last longer than 20 to 30 minutes can damage the brain's neurons.
A seizure is often divided into different parts. The aura is a period or warning prior to a seizure. Patients may experience unusual smells, visual symptoms, or feelings. The seizure itself is known as the ictus. The period of time after the seizure is called the postictal state.
Seizures were depicted by prehistoric man in cave paintings. Hippocrates wrote of epilepsy and of its relationship to the brain. Epilepsy also is described in the Bible. Writings from 4000 years ago depict epileptics as possessed by demons. Julius Caesar, the great Russian novelist Dostoyevsky, and King Charles II all are said to have had seizures.