Surely going to church or going home and trying the doorknob aren't autonomic impulses! Jones' wife tried to open the door, and a walker busted down a glass door with a rock to get at Rick et al. in Atlanta. These zombies are tool users. The brain is reactivated by the infection. Granted, not necessarily all of it, but what's-his-name, the CDC doctor, killed TS-19 almost immediately after reanimation. It would have been useful if he had let the scanner run on an animated corpse longer. What if the infection reactivates more of the brain later? They might increase their intelligence over time.