What controls gut motility?
When discussing the control of gut motility there is more to mention than the use of serotonergics and opioids - a number of peptide and nonpeptide neurotransmitters are important. Somatostatin and nitric oxide are two examples which each happen be inhibitory of intestinal motor function, but by two completely different mechanisms. A bit of understanding in cell biology, specifically cell signalling would be helpful to fully grasp this, but the former acts by inhibiting the release of Acetylcholine and Substance P while the latter increases levels of cAMP/cGMP. And don't forget about the basics when it comes to motor control - the cholinergics and adrenergics (including dopamine) are logical opposites.

References
Leung, P.S. 2014, The Gastrointestinal System: Gastrointestinal, Nutritional and Hepatobiliary Physiology, Springer, 19/03/2018, http://www.springer.com/cda/content/document/cda_downloaddocument/9789401787703-c2.pdf?SGWID=0-0-45-1491980-p176563984