Why is one side of the body controlled by the opposite cerebral hemisphere?
The decussation of the fibers happens at the level of medulla in the brain stem, but not 100% of fibers decussate. About 10% of all fibers coming down stay on the ipsilateral (same) side of the body. This is true for the cerebral hemisphere. For cerebellar hemispheres, the fibers decussate twice, meaning that the ascending fibers decussate once going into the cerebellum and then again as they descend back down. So that means that the left cerebellar hemisphere controls the left side of the body and same thing for the right hemisphere. There is some kind of adaptive/protective reason why 90% of fibers decussate and the other 10% don’t, but I have yet to find the exact reason for that.
