borderlands skill tree help

I have looked everywhere online and I cant find the answer to my question. In order to progress down the skill tree for example I am the soldier in order for me to unlock shooting missles out of my turett, how do I make it to where I am able to move down the skill tiers. I have put 2 in impact, and 3 in sentry which are both in the top tiers of the infantry area. After I did that the next time I leveled up I had the next tier unlocked which were ,scatter shot, and metal storm now my question is why did it go to the next tier. Was it because I reached a certan level? Or wa it because I put in a certain amount in one of the higher skills like how I put 3 into sentry gun? Or none of the above? Basically lol I am asking how the heck do I go down into my tree do I have to put so many skill points into each level or is the lower levels unlocked the higher level u are. Right now I believe I am level 10. Please any info would be great cuz I dont wanna waste my skill points, and I want to get to the lower skill tree as fast as possible. ThanksP:S sorry if this is confusing I couldnt really think of the clearest way to ask this question.borderlands skill tree help
By adding to that section of the skill tree. Don't you notice the color going down everytime you add to that part of the tree?borderlands skill tree help
it takes 5 points in each tier to move to the next one. So you will need a combined 15 in that branch of the tree until you can put a point in the missle skill
ok 5 points in each tier makes sense cuz thats what I did and it moved down that was my 1st suspicion. Thanks alot guys.