SAN FRANCISCO -- A second-quarter concussion forced Alex Smith out of Sunday's game with the 49ers trailing 14-7 to the visiting St. Louis Rams.

Before waking to the locker room with 2½ minutes remaining in the first half, Smith sustained three hard hits on a touchdown drive, which came on his third and final series.

The most likely culprit for Smith's concussion: linebacker Jo-Lonn Dunbar delivered a helmet-to-helmet shot Smith on that drive's first play. It was a 4-yard scramble in which Smith turned his back at the last second during an awkward slide.

Smith didn't come out of the game after that hit, and three plays later he got sacked for a 7-yard loss by Michael Brockers, with linebacker James Laurinaitis also in on the hit.

Another three snaps later, Smith ran a quarterback sneak up the middle for a fourth-and-1 conversion.

Smith's capacity to play didn't seem in doubt, as he completed his ensuing three passes, including a 14-yard strike to Michael Crabtree for a touchdown, capping the 12-play, 78-yard drive.

Smith last sustained a documented concussion in last season's home opener against the Dallas Cowboys, although he never left that game in which he got sacked six times.

Smith exited Sunday's game having completed 7 of 8 passes for 72 yards with a 143.8 rating. His only incompletion came when he overthrew Mario Manningham near the sideline. Smith was coming off an 18-of-19 passing display in a 24-3 win at Arizona on Oct. 29.

For more on the 49ers, see Cam Inman's Hot Read blog at blogs.mercurynews.com/49ers. Follow him on Twitter at twitter.com/CamInman.