Question: Why are there long-term consequences to temporary deprivation of oxygen?
Not all cases of temporary deprivation of oxygen have long-term consequences. It depends on how long the O2 deprivation period is. If it’s just no more than a few minutes, like when you hold your breath, it’s ok. But if it’s longer than that, irreversible damage may occur:
“Approximately 20% of the oxygen consumed by the human body is used by the brain. The greater part of this oxygen is used to produce the ATP required to maintain the membrane potentials necessary for electrical signaling with synaptic and action potentials Harris et al., 2012. In many vertebrates, including adult humans, interruption of the oxygen supply to the brain for more than a few minutes leads to irreversible neurological damage, including neuronal death.Larson J, et al”
A lot of people who suffered cardiac arrests but had fast and effective CPRs and regained circulations and oxygenation in time recovered completely with no long-term consequences. Sadly, there are a lot of people, too, who were not revived in time and suffered long-term consequences (which can vary from mild to severe) or even brain death. It all depends on the duration of hypoxemia.
Edit: For the question of “How does oxygen deprivation affect neural activity?”
There is a recent comprehensive review here: This review aims at summarizing the molecular and cellular mechanisms underlying hypoxia and hyperoxia in brain and cerebral tissue.