Seizures occur when neurons are activated synchronously. The kind of seizure depends on the location of the abnormal activity and the pattern of spread to different parts of the brain. The formation of a seizure focus in the brain may result from disruption of normal inhibitory circuits. This disruption may occur because of alterations in ion channels or from injury to inhibitory neurons and synapses. Alternatively, seizure focus may be formed when groups of neurons become synchronized by reorganization of neural networks after brain injury. After formation of a seizure focus, local discharge may then spread. This spread occurs by a combination of mechanisms. After synchronous depolarization of abnormally excitable neurons known as the paroxysmal depolarizing shift extracellular potassium accumulates, depolarizing nearby neurons. Increased frequency of depolarization then leads to increased calcium influx into nerve terminals. This increases neurotransmitter release at excitatory synapses by a process known as posttetanic potentiation, whereby normally quiescent voltage-gated and N-methyl-D-aspartate (NMDA) receptor-gated excitatory synaptic neurotransmission is increased and inhibitory synaptic neurotransmission is decreased. The net effect of these changes is recruitment of neighboring neurons into a synchronous discharge, causing a seizure.