The Astral Plane, Elder Things, Demons and Spirits

[b-dog]

I am wondering what would be the difference between the astral plane and rthe spirit realm. In myth the astral realm is supposed to be between the other planes and the mortal realm. In psionics it is a little different. The spirit realm seems similar to the astral plane of myth but are they distinct? Also, what about demons and Elder Things in the astral realm, do they have silver cords or do they have complete astral forms? Thanks.