Walkthrough: After reaching the door you opened, go inside and use the elevator to get to the next section. After saving at the Save Station, climb up the wall on the left. You'll quickly find the next Missile Expansion, as well as Red blocks that will prevent you from climbing any higher. Since you can't go any higher, enter the next area that is open to you. You should quickly come to an area with two doors. The top door is where you fight one of the Alpha Metroids. After defeating it, leave that room and go into the lower of the two doors. This door should lead to a large open room.
Once here, use your Spider Ball on the wall to the right, scaling up as far as you can. Next, run across to the right until you come to an opening. Since the door on the right cannot be opened, go into the one with the Red Barrier instead. It is here that you will battle Arachnus, one of the Bosses of sorts found within the game. By defeating him, you will acquire the Spring Ball for Samus, the same that will let you jump as the Morph Ball so you no longer Bomb to propel yourself.
Missile Expansion: After coming off the elevator in the beginning of this section, Spider Ball up the left wall. Continue climbing until you reach the point that needs to be Bombed. The Expansion will be on the other side of the wall.