One of the ways in which events can go wrong is when a nerve cell loses some of its inputs from other cells because of damage to these other nerve cells. If inhibitory terminals are lost, then the cell will become over-excitable, and begin to switch on, or fire inappropriately, driving other nerve cells with which it is connected on the downstream side to similar activity. This may result in more and more nerve cells being incorporated into the abnormal pattern of discharge.
The biological background of an epileptic seizure is therefore an abnormal discharge of nerve cells in the cerebral hemispheres of the brain. The normal, quiet, and integrated function of nerve cells is interrupted as they are forced through the contacts they make with and receive from others into a paroxysmal discharge. Different types of seizure are a reflection of different patterns of paroxysmal discharge. If the seizure discharge spreads throughout large areas of the brain, then consciousness may be lost. If the discharge of nerve cells is confined to the temporal lobe of the brain (more or less above and in front of the ears), amongst those cells concerned with memory, the paroxysmal discharge may result only in a distortion of memory so that the sufferer perceives that he or she has experienced ongoing events before—the phenomenon of deja vu.