After stopping the use of an ssri there can be symptoms called "brain zaps" what exactly is going on in the brain to cause this?

Brain "zaps" These "brain zaps" are a symptom of withdrawal from ssri's -- some call it a "discontinuation" syndrome. Your brain and body become habituated to the effects of the ssri, and react when the substance is no longer there (or is in reduced quantities). We're not sure of the exact mechanism in the brain. Could be a sudden relative deficiency of synaptic serotonin along w/other neuro changes.