After Lexine mentions her headache and Gabe comments on the locked doors, you'll arrive at a branching path. Your choice only determines what kind of enemies you will face in the circular room and the area immediately beyond. I choose the left path because I find it more interesting and because I think the spawned enemy types are a bit easier to deal with than the ones that spawn when choosing the other route.
Beware the Exploders. I got lucky with the one that detonated in the circular room, but usually the damage they do on the Impossible difficulty mode is nothing to scoff at.
In this Chapter, you'll be introduced to Lurkers. Many enemy types from Dead Space 1 return in Extraction, but Lurkers are one of two that don't function quite the same. In Dead Space: Extraction, Lurkers no longer fling sharp barbs straight at you. Instead, their barbs travel slower, have an explosive effect and can be grabbed (and launched back) with Kinesis.
This video ends shortly after Gabe tells the group to "man up" after being separated from Lexine.
This video begins shortly after Gabe tells the group to "man up" after being separated from Lexine.
The second Line Gun Weapon Upgrade is found behind a ceiling vent that is shown when Weller talks about his hallucination of hearing someone crying. (0:13)
The fourth and final P-Sec Pistol Weapon Upgrade is found behind the ceiling vent above the decapitated body on the railing. (0:32)
The first Force Gun Weapon Upgrade is found behind the ceiling vent above the Lurker that Nate hallucinates. (5:23)
Shortly after being attacked by a Grabber and cutting through a meaty wall, Nate will look down at the sewer fluid before looking up at a vent above a platform with nothing on it. (6:58) Behind the vent is the third Flamethrower Weapon Upgrade. This whole sequence occurs just before Weller says: "Lexine was right. Something's watching us."
This video ends just before the boss fight at the end of this Chapter.