A seizure is the manifestation of abnormal electrical activity in the brain. A seizure is not a disease, but rather a sign of brain dysfunction, much like a “limp” is a sign of limb dysfunction. A seizure only tells us there is a problem with the brain. Neurons in the brain are essentially built to be active. It is their default but there are many mechanisms in place to “keep them quiet”. Think of the brain as a car at a stop light with the foot on the gas and the brakes at the same time. Too much gas and you lurch forward (seizure), not enough brakes and you go forward (seizure). So seizures typically result from either not enough inhibition of the nervous system (brakes) or too much excitement (gas). Seizures can also result when there is excessive neural synchronization. If large groups of neurons are “primed” to fire at the same time, a small trigger may initiate a large response. Kind of like a small group of people starting a “wave” in a stadium. The event starts small and gradually builds momentum until the entire stadium (i.e. brain) is active. At certain times the chances of an event occurring are greater because neurons are primed and synchronized (like fans might be right after a touchdown is scored).
We use medications to add more brakes to the nervous system or inhibit synchronization. Some medications are very specific and tend to target overly active neurons (the problem neurons). These medications naturally have less side effects as they tend to leave “normal” neurons alone. Other medications (typically the older ones) tend to be more broad spectrum, acting on all neurons to some degree, even the ones that are not a problem. They tend to cause more side effects, such as sedation or behavior changes. Fortunately, dogs and cats tolerate most medications very well and most side effects are short lived or mild.
To learn more about why seizures start and how they end, check out these books.