How do Senna laxatives work?

My frustration with my inability to find a good reference that explains how Senna based laxatives work (before I prescribed it to one of my patients with HD and encopresis) was relieved by one of our fine Pedi GI folks.  She promptly sent me this paper from the deep archives of an appropriately name journal.

Here's the answer.

Senna, an anthraquinone laxative, is a glycoside that is metabolized in the GI tract into oxy-methyl anthraquinone.  OMA's (just made that up; not the official name) soften the stool by decreasing water absorption from the colon and promote colonic contraction by stimulating the Auerbach plexus of nerves.

There you go.