I had a big surgery and had to stay in bed for months. Because of all this, I developed "hard" constipation. My doctor prescribed me Senna leaves. I have never before had problems with my stools, so I had no need for such medications. I am using this way of communication to find out more about senna leaves effect.
Senna leaves are used to treat constipation, as you already know. It is not supposed to be used longer than a week, unless your doctor told you to. Senna leaves are commonly used as tea. Main "action" is in your bowels. Some substances from the leaves are irritating your bowels inner face and make it push the contents out.