A 70 kg body would have approximately 7*1027 atoms. That is, 7 followed by 27 zeros:
7,000,000,000,000,000,000,000,000,000
That's seven octillion, by the way. not 70 trillion.
The burning is just that the pathways are the proverbial 'paths of least resistance.' I assume in short-term memory areas of the brain it uses a chemical makeup that 'cleans' itself faster, like how some liquids reform faster (water) when disturbed and others hold it longer (like glass).
At least that is what I remember, maybe I am wrong.