laxatives

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>Senna derivatives are a type of stimulant laxative and are of the anthraquinone type.[1] While its mechanism of action is not entirely clear, senna is thought to act by increasing fluid secretion within and contraction of the large intestine.
>While its mechanism of action is not entirely clear.
>The breakdown products of senna act directly as irritants on the colonic wall to induce fluid secretion and colonic motility.
what do they mean by this

also what are some mechanisms we only half know?