Hi there :)

Thanks for your comment and your question, it does indeed sound like your brother in law had a psychedelic experience!

Science doesn't know exactly yet what occurs in the brain to create the psychedelic experience, where a person feels they leave their body and go on a journey, but we do have an idea where in the brain these changes occur when under the influence of certain substances.

Firstly it depends on which drug we're talking about. Ketamine antagonises the NMDA receptor and affects the lateral habenula, but we don't know much else about its action. Marijuana stimulates cannabinoid receptors which are associated with a vast variety of functions, like pleasure, relaxation and hunger but also anxiety and paranoia.

Classic psychedelics like DMT, magic mushrooms and mescaline all affect the 2A serotonin receptor, which when stimulated seems to take the brain into a psychedelic/altered state of consciousness. It can be somewhat likened to the 'debug' mode in games and computer programmes, where a developer can poke around at the settings and have a look behind the scenes.

The cascade of psychedelic effects in the brain seems to be a combination of serotonin receptor stimulation and the temporary quietening down of the DMN. I have written about this in much more detail if you'd like to learn more, have a look at the links below :)

Therapeutic potential of illicit substances: https://medium.com/amalgamate/illegal-drugs-which-could-help-treat-mental-disorders-aca81875e4d1

DMT and the afterlife:
https://medium.com/amalgamate/the-afterlife-might-exist-but-entirely-within-your-head-fbbad90361a7

How magic mushrooms work in the brain:
https://medium.com/amalgamate/how-magic-mushrooms-work-in-your-brain-3c556e0c64d0

MDMA and the brain:
https://medium.com/amalgamate/how-mdma-works-in-the-brain-13792f2fceb5

Let me know if you have any more questions :)