What happens to the brain during oxygen deprivation?

The answer

Hypoxia (insufficient oxygen) affects the brain faster than any other organ because neurons have the highest metabolic rate of any cell type and virtually no energy reserves. Within 10 seconds of blood flow cessation, consciousness is lost. Within 20 seconds, electrical activity ceases. After 4 to 6 minutes, neurons begin to die through excitotoxicity, as depleted energy stores cannot maintain ion pumps, causing uncontrolled glutamate release that triggers a destructive calcium cascade. Paradoxically, hypoxia can produce vivid near-death experiences including tunnel vision (from peripheral retinal neurons failing before central ones), feelings of peace (from endorphin release), and life review (from disinhibited temporal lobe activity). Research on cardiac arrest survivors has shown that some brain activity persists for minutes after the heart stops, including gamma wave surges that may explain reported awareness during clinical death.