Some of the Chinese word is influence by culture of Buddha, in order to explain 心，意，神. I think need the ancient Chinese wisdom to explain it...
I tried to explain it from what I know, here you can reference it
心：无形无相 = heart/core：(shapeless, formless)
神：let break this word and see, it's the combination of 示 and 申
示 means show
申 come from the basic word of 田(means farm or field)
When 田 become 申 it's show it connected or pass through to sky and ground or top or bottom, so this word 神 have the meaning of god.
When you are using 留神 this word it's have some meaning like stay focus.