Some personal thoughts which I do not claim are 100% correct but I think they are close:
The body is an expression of the spirit. Spirit manifests itself and expresses itself in the pattern that produces the life of the body, and directs its growth and actions.
In turn the body is a vehicle by which the spirit has expressed itself and in which a mind is formed which creates the bridge between earth and heaven. The brain is a part of and a function of the body and the Spirit acts upon the brain to create the mind which is the consciousness we experience in this time space world.
Of course the body that we have in this sin stained and sin darkened world is dead because of sin. It is dead because it has no life in itself and it is appointed to die. It has a date with destiny and it's destiny is within itself. Therefore it is defined as dead. It is approaching a terminal event.
Thus this body is in a way a prison of the Spirit which tends towards life and strives towards life and has been given life in Christ. We are imprisoned by our death, and our diseases, and our sin which dwells in our flesh.
But the spirit is alive because of Christ. And we know that it shall be clothed with a body that is prepared for it. We will not be found naked.
Though we are dead yet shall we live, and we who live shall never die.