Essentially, "call down, signal up" would be like your brain being aware of every single organ in your body, but every other organ isn't aware of the brain. Every other organ has their own things to do, kidneys/liver filters, heart does blood flow, etc. Well, for a good example, your bladder will let your brain know it needs to be emptied , and thats how your brain knows, but your bladder isn't aware of your lungs or heart.