Imagine you are looking down from the sky, watching a crowded business plaza in the middle of a busy city. Let's call it "Brain Plaza." People are moving every which way, each with an individual task to carry out. The result is something that looks something like random movement, but is actually a sign of business as usual.
Now, imagine that some noisy, crazy person somewhere in the plaza starts screaming. Everyone around that person panics, starts screaming themselves, and runs away from the crazy screamer. This causes more people to panic and run until the entire plaza is involved. Looking down from the top of the city plaza, the movement no longer looks random, but more organized as everyone runs away from the noise.
The movement of the people in the plaza is analogous to electrical activity in the brain. Usually, the electricity in our brain is flowing every which way in order to respond to various things in our environment. The sound of the air conditioner causes ripples of electricity in the part of the brain that deals with sound, idle thoughts in your mind cause irregular patterns in your frontal lobes, and the part of your brain that plans and initiates movement is constantly making dozens of small corrections and adjustments. The result is something that may look random, but is a sign of healthy activity.
In a seizure, an irritated piece of brain starts firing out regularly, like a person yelling in a crowded plaza. Because all pieces of the brain are connected, this abnormal electrical activity can spread to other parts of the brain, rippling out and away from the original source, like people moving away from the screamer.
As the electricity spreads, it causes more brain tissue in the cerebral cortex to abnormally activate and seize, like people in a plaza running not because they saw the original screamer, but because they noticed other people moving away in a panicked fashion. The result is a lot of very organized but unhelpful activity.
So in order to have a seizure, there needs to be an irritated bit of cortex to start trouble, which is called a seizure focus. This can be due to a stroke, trauma, tumor, infection, or even a malformation with which you were born.
But having such an abnormality is not quite enough to cause a seizure. The abnormal electrical activity has to spread.
We normally have protection against this sort of activity in our brains. For example, there are mechanisms that stop an electrical activity from building to unacceptable levels. In the plaza, this would be a security guard who would get any unruly people to calm down before trouble got out of hand.
Neurons also have to meet a certain electrical level of signal before firing. In the case of the plaza, this would be the amount of yelling the crowd was able to tolerate before they decided to panic and run.
The combination of natural defense systems and the nerve cells' propensity to seizure is known as a person's "seizure threshold." There are many things that can lower the seizure threshold. Imagine that Brain Plaza's "security guards" are busy with other problems, and just don't have the energy to pay attention to the noisy troublemaker. If your body doesn't have much energy due to fighting an infection, drinking too much alcohol, or too little sleep, you could be more prone to seizures. If neurons are jittery because they have drugs like cocaine on board or are withdrawing from alcohol, this can make them more prone to seizure, just like jumpy people may be more prone to run if someone is screaming at them.
The business plaza of the brain cannot get work done as usual when electrical activity is being otherwise engaged. It can only do whatever the abnormal electrical activity is telling it to do. Let's say a crazy screamer causes everyone in the Right Arm Building to run out and panic all over Brain Plaza before they run to hide in other buildings. This means that abnormal electrical activity will start in the right arm, with highly organized rhythmic jerking. That rhythmic jerking will then spread all over the rest of the body, causing a generalized seizure.
Of course, when everyone is running out of the plaza, nothing else will be able to get done. In fact, during a seizure there's not enough useful electrical activity in the brain to allow for conscious awareness, so the seizing person will black out and not remember what happened.
Afterwards, it takes some time for the system to return to normal. Like nervous people deciding if they can go back to work after a mass panic, neurons are fatigued after a seizure, leading to a period of unconsciousness or confusion known as the post-ictal period. If the seizure started from the region controlling the right arm, the limb can be paralyzed for a period of time after the initial seizure, a phenomenon known as Todd's paralysis.
Sometimes a seizure can start but not spread very far, either because the neurons have a high threshold for firing or because negative feedback quenches the abnormal activity. This is known as a focal seizure. Focal seizures can involve abnormal movements, or even be something as subtle as a funny smell or a sense of déjà vu. In any rate, consciousness is not lost unless the seizure spreads across the brain.
When neurologists perform an electroencephalogram (EEG), they are essentially looking down on Brain Plaza from very high up to get a sense of how electricity is moving through the brain. Even when the brain isn't seizing, they may appreciate a spiking seizure focus that is currently contained by a high seizure threshold. During a seizure, they may see highly organized electrical activity march across the brain, and get a sense of where the seizure started.
No allegory is perfect, but Brain Plaza may help clarify how seizures start and spread through the brain. Having a concept of how seizures work can help you better understand and participate in discussions about your seizures, and make decisions about how they can be best managed.
Leslie Rudzinski,Jerry Shih, The Classification Of Seizures And Epilepsy Syndromes. Continuum, Epilepsy, June 2010, Volume 16, Issue 3
Ropper AH, Samuels MA. Adams and Victor's Principles of Neurology, 9th ed: The McGraw-Hill Companies, Inc., 2009.