SANTA CLARA -- Cornerback Chris Culliver returned to football activity on Thursday, leaving him one step away from passing the NFL's protocol on concussions to be cleared for the 49ers' game Sunday against the Chicago Bears.
Tramaine Brock, the starting cornerback on the other side, remained sidelined with a toe sprain. His status for Sunday appears to be doubtful. Perrish Cox is likely to start in place of Brock.
Right tackle Anthony Davis did not practice, and appears likely to be ruled out the second game to open the season. Jonathan Martin is in line to start Sunday against the Bears.
Defensive lineman Justin Smith returned to practice Thursday after the coaching staff gave him a day off.
Culliver sustained a concussion in the 49ers' season opener Sunday against the Dallas Cowboys. If he remains symptom-free through simulated contact any point before Sunday's game, he will receive medical clearance. During practice on Thursday, he wore a blue uniform to signify that he is off-limits for contact.