system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do the different types of anesthesia work? How can you be knocked out through intense pain and woken back up at the anaesthesiologist's will?
gpt
General anesthesia uses a variety of drugs to cause unconscious. They all take advantage of receptors already in your brain (primarily GABA receptor) to achieve this. When GABA is activated it inhibits and slows brain transmission eventually resulting in unconciousness. Even with unconscious your body will still have a response to pain( increased hr, bp and reflex movement) so to controller these we give opioids such as fentanyl to block pain transmission, and muscle relaxants to prevent movement. 


Spinal anesthesia uses the injection of local anesthetic ( think novacaine for your teeth) near the spine to block transmission of pain signals. So even though your body is reporting pain the brain never receives it.


Local anesthesia is the same type of numbing medicine as the spinal, but is injected directly into the area that they are working on.